diff --git a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
index 187861ba127bd..87db08ef5b510 100644
--- a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
@@ -813,17 +813,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             if *outlived_f != ty::ReStatic {
                 return;
             }
+            let suitable_region = self.infcx.tcx.is_suitable_region(f);
+            let Some(suitable_region) = suitable_region else { return; };
 
-            let fn_returns = self
-                .infcx
-                .tcx
-                .is_suitable_region(f)
-                .map(|r| self.infcx.tcx.return_type_impl_or_dyn_traits(r.def_id))
-                .unwrap_or_default();
-
-            if fn_returns.is_empty() {
-                return;
-            }
+            let fn_returns = self.infcx.tcx.return_type_impl_or_dyn_traits(suitable_region.def_id);
 
             let param = if let Some(param) = find_param_with_region(self.infcx.tcx, f, outlived_f) {
                 param
@@ -839,15 +832,43 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             };
             let captures = format!("captures data from {arg}");
 
-            return nice_region_error::suggest_new_region_bound(
-                self.infcx.tcx,
-                diag,
-                fn_returns,
-                lifetime.to_string(),
-                Some(arg),
-                captures,
-                Some((param.param_ty_span, param.param_ty.to_string())),
-                self.infcx.tcx.is_suitable_region(f).map(|r| r.def_id),
+            if !fn_returns.is_empty() {
+                nice_region_error::suggest_new_region_bound(
+                    self.infcx.tcx,
+                    diag,
+                    fn_returns,
+                    lifetime.to_string(),
+                    Some(arg),
+                    captures,
+                    Some((param.param_ty_span, param.param_ty.to_string())),
+                    Some(suitable_region.def_id),
+                );
+                return;
+            }
+
+            let Some((alias_tys, alias_span)) = self
+                .infcx
+                .tcx
+                .return_type_impl_or_dyn_traits_with_type_alias(suitable_region.def_id) else { return; };
+
+            // in case the return type of the method is a type alias
+            let mut spans_suggs: Vec<_> = Vec::new();
+            for alias_ty in alias_tys {
+                if alias_ty.span.desugaring_kind().is_some() {
+                    // Skip `async` desugaring `impl Future`.
+                    ()
+                }
+                if let TyKind::TraitObject(_, lt, _) = alias_ty.kind {
+                    spans_suggs.push((lt.ident.span.shrink_to_hi(), " + 'a".to_string()));
+                }
+            }
+            spans_suggs.push((alias_span.shrink_to_hi(), "<'a>".to_string()));
+            diag.multipart_suggestion_verbose(
+                &format!(
+                    "to declare that the trait object {captures}, you can add a lifetime parameter `'a` in the type alias"
+                ),
+                spans_suggs,
+                Applicability::MaybeIncorrect,
             );
         }
     }
diff --git a/compiler/rustc_error_messages/locales/en-US/parse.ftl b/compiler/rustc_error_messages/locales/en-US/parse.ftl
index 8f063f5082c95..a3e2002da781c 100644
--- a/compiler/rustc_error_messages/locales/en-US/parse.ftl
+++ b/compiler/rustc_error_messages/locales/en-US/parse.ftl
@@ -238,6 +238,7 @@ parse_const_let_mutually_exclusive = `const` and `let` are mutually exclusive
 
 parse_invalid_expression_in_let_else = a `{$operator}` expression cannot be directly assigned in `let...else`
 parse_invalid_curly_in_let_else = right curly brace `{"}"}` before `else` in a `let...else` statement not allowed
+parse_extra_if_in_let_else = remove the `if` if you meant to write a `let...else` statement
 
 parse_compound_assignment_expression_in_let = can't reassign to an uninitialized variable
     .suggestion = initialize the variable
diff --git a/compiler/rustc_hir/src/hir.rs b/compiler/rustc_hir/src/hir.rs
index d6566860f8170..b456bd08048db 100644
--- a/compiler/rustc_hir/src/hir.rs
+++ b/compiler/rustc_hir/src/hir.rs
@@ -3524,6 +3524,13 @@ impl<'hir> Node<'hir> {
         }
     }
 
+    pub fn alias_ty(self) -> Option<&'hir Ty<'hir>> {
+        match self {
+            Node::Item(Item { kind: ItemKind::TyAlias(ty, ..), .. }) => Some(ty),
+            _ => None,
+        }
+    }
+
     pub fn body_id(&self) -> Option<BodyId> {
         match self {
             Node::TraitItem(TraitItem {
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index bbf7b81a2cc66..d5f37abb8b455 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -1619,6 +1619,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
             }
         }
     }
+
     fn note_unreachable_loop_return(
         &self,
         err: &mut Diagnostic,
diff --git a/compiler/rustc_hir_typeck/src/demand.rs b/compiler/rustc_hir_typeck/src/demand.rs
index bd1626dff7951..812dd4054b545 100644
--- a/compiler/rustc_hir_typeck/src/demand.rs
+++ b/compiler/rustc_hir_typeck/src/demand.rs
@@ -59,7 +59,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
             || self.suggest_clone_for_ref(err, expr, expr_ty, expected)
             || self.suggest_into(err, expr, expr_ty, expected)
-            || self.suggest_floating_point_literal(err, expr, expected);
+            || self.suggest_floating_point_literal(err, expr, expected)
+            || self.note_result_coercion(err, expr, expected, expr_ty);
         if !suggested {
             self.point_at_expr_source_of_inferred_type(err, expr, expr_ty, expected);
         }
@@ -697,6 +698,56 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         );
     }
 
+    pub(crate) fn note_result_coercion(
+        &self,
+        err: &mut Diagnostic,
+        expr: &hir::Expr<'tcx>,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+    ) -> bool {
+        let ty::Adt(e, substs_e) = expected.kind() else { return false; };
+        let ty::Adt(f, substs_f) = found.kind() else { return false; };
+        if e.did() != f.did() {
+            return false;
+        }
+        if Some(e.did()) != self.tcx.get_diagnostic_item(sym::Result) {
+            return false;
+        }
+        let map = self.tcx.hir();
+        if let Some(hir::Node::Expr(expr)) = map.find_parent(expr.hir_id)
+            && let hir::ExprKind::Ret(_) = expr.kind
+        {
+            // `return foo;`
+        } else if map.get_return_block(expr.hir_id).is_some() {
+            // Function's tail expression.
+        } else {
+            return false;
+        }
+        let e = substs_e.type_at(1);
+        let f = substs_f.type_at(1);
+        if self
+            .infcx
+            .type_implements_trait(
+                self.tcx.get_diagnostic_item(sym::Into).unwrap(),
+                [f, e],
+                self.param_env,
+            )
+            .must_apply_modulo_regions()
+        {
+            err.multipart_suggestion(
+                "use `?` to coerce and return an appropriate `Err`, and wrap the resulting value \
+                 in `Ok` so the expression remains of type `Result`",
+                vec![
+                    (expr.span.shrink_to_lo(), "Ok(".to_string()),
+                    (expr.span.shrink_to_hi(), "?)".to_string()),
+                ],
+                Applicability::MaybeIncorrect,
+            );
+            return true;
+        }
+        false
+    }
+
     /// If the expected type is an enum (Issue #55250) with any variants whose
     /// sole field is of the found type, suggest such variants. (Issue #42764)
     fn suggest_compatible_variants(
diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index 44da3fbe300ca..bb2dd290c6d5d 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -985,7 +985,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         let vis = self.get_visibility(id);
         let span = self.get_span(id, sess);
         let macro_rules = match kind {
-            DefKind::Macro(..) => self.root.tables.macro_rules.get(self, id).is_some(),
+            DefKind::Macro(..) => self.root.tables.is_macro_rules.get(self, id),
             _ => false,
         };
 
@@ -1283,7 +1283,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
     fn get_macro(self, id: DefIndex, sess: &Session) -> ast::MacroDef {
         match self.def_kind(id) {
             DefKind::Macro(_) => {
-                let macro_rules = self.root.tables.macro_rules.get(self, id).is_some();
+                let macro_rules = self.root.tables.is_macro_rules.get(self, id);
                 let body =
                     self.root.tables.macro_definition.get(self, id).unwrap().decode((self, sess));
                 ast::MacroDef { macro_rules, body: ast::ptr::P(body) }
@@ -1595,11 +1595,11 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
     }
 
     fn get_attr_flags(self, index: DefIndex) -> AttrFlags {
-        self.root.tables.attr_flags.get(self, index).unwrap_or(AttrFlags::empty())
+        self.root.tables.attr_flags.get(self, index)
     }
 
     fn get_is_intrinsic(self, index: DefIndex) -> bool {
-        self.root.tables.is_intrinsic.get(self, index).is_some()
+        self.root.tables.is_intrinsic.get(self, index)
     }
 }
 
diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index 2fa645cd9e33d..eebc2f21dfe4e 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -226,12 +226,7 @@ provide! { tcx, def_id, other, cdata,
     deduced_param_attrs => { table }
     is_type_alias_impl_trait => {
         debug_assert_eq!(tcx.def_kind(def_id), DefKind::OpaqueTy);
-        cdata
-            .root
-            .tables
-            .is_type_alias_impl_trait
-            .get(cdata, def_id.index)
-            .is_some()
+        cdata.root.tables.is_type_alias_impl_trait.get(cdata, def_id.index)
     }
     collect_return_position_impl_trait_in_trait_tys => {
         Ok(cdata
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 2ecaa33d4d315..97f0457ba7116 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -483,7 +483,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         self.lazy(DefPathHashMapRef::BorrowedFromTcx(self.tcx.def_path_hash_to_def_index_map()))
     }
 
-    fn encode_source_map(&mut self) -> LazyTable<u32, LazyValue<rustc_span::SourceFile>> {
+    fn encode_source_map(&mut self) -> LazyTable<u32, Option<LazyValue<rustc_span::SourceFile>>> {
         let source_map = self.tcx.sess.source_map();
         let all_source_files = source_map.files();
 
@@ -1130,7 +1130,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             attr_flags |= AttrFlags::IS_DOC_HIDDEN;
         }
         if !attr_flags.is_empty() {
-            self.tables.attr_flags.set(def_id.local_def_index, attr_flags);
+            self.tables.attr_flags.set_nullable(def_id.local_def_index, attr_flags);
         }
     }
 
@@ -1387,7 +1387,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         if impl_item.kind == ty::AssocKind::Fn {
             record!(self.tables.fn_sig[def_id] <- tcx.fn_sig(def_id));
             if tcx.is_intrinsic(def_id) {
-                self.tables.is_intrinsic.set(def_id.index, ());
+                self.tables.is_intrinsic.set_nullable(def_id.index, true);
             }
         }
     }
@@ -1519,7 +1519,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             }
             hir::ItemKind::Macro(ref macro_def, _) => {
                 if macro_def.macro_rules {
-                    self.tables.macro_rules.set(def_id.index, ());
+                    self.tables.is_macro_rules.set_nullable(def_id.index, true);
                 }
                 record!(self.tables.macro_definition[def_id] <- &*macro_def.body);
             }
@@ -1529,7 +1529,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             hir::ItemKind::OpaqueTy(ref opaque) => {
                 self.encode_explicit_item_bounds(def_id);
                 if matches!(opaque.origin, hir::OpaqueTyOrigin::TyAlias) {
-                    self.tables.is_type_alias_impl_trait.set(def_id.index, ());
+                    self.tables.is_type_alias_impl_trait.set_nullable(def_id.index, true);
                 }
             }
             hir::ItemKind::Enum(..) => {
@@ -1636,7 +1636,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         if let hir::ItemKind::Fn(..) = item.kind {
             record!(self.tables.fn_sig[def_id] <- tcx.fn_sig(def_id));
             if tcx.is_intrinsic(def_id) {
-                self.tables.is_intrinsic.set(def_id.index, ());
+                self.tables.is_intrinsic.set_nullable(def_id.index, true);
             }
         }
         if let hir::ItemKind::Impl { .. } = item.kind {
@@ -2038,7 +2038,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         }
         if let hir::ForeignItemKind::Fn(..) = nitem.kind {
             if tcx.is_intrinsic(def_id) {
-                self.tables.is_intrinsic.set(def_id.index, ());
+                self.tables.is_intrinsic.set_nullable(def_id.index, true);
             }
         }
     }
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 69690264ae4ea..698b2ebc4732a 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -185,9 +185,9 @@ enum LazyState {
     Previous(NonZeroUsize),
 }
 
-type SyntaxContextTable = LazyTable<u32, LazyValue<SyntaxContextData>>;
-type ExpnDataTable = LazyTable<ExpnIndex, LazyValue<ExpnData>>;
-type ExpnHashTable = LazyTable<ExpnIndex, LazyValue<ExpnHash>>;
+type SyntaxContextTable = LazyTable<u32, Option<LazyValue<SyntaxContextData>>>;
+type ExpnDataTable = LazyTable<ExpnIndex, Option<LazyValue<ExpnData>>>;
+type ExpnHashTable = LazyTable<ExpnIndex, Option<LazyValue<ExpnHash>>>;
 
 #[derive(MetadataEncodable, MetadataDecodable)]
 pub(crate) struct ProcMacroData {
@@ -253,7 +253,7 @@ pub(crate) struct CrateRoot {
 
     def_path_hash_map: LazyValue<DefPathHashMapRef<'static>>,
 
-    source_map: LazyTable<u32, LazyValue<rustc_span::SourceFile>>,
+    source_map: LazyTable<u32, Option<LazyValue<rustc_span::SourceFile>>>,
 
     compiler_builtins: bool,
     needs_allocator: bool,
@@ -315,21 +315,27 @@ pub(crate) struct IncoherentImpls {
 
 /// Define `LazyTables` and `TableBuilders` at the same time.
 macro_rules! define_tables {
-    ($($name:ident: Table<$IDX:ty, $T:ty>),+ $(,)?) => {
+    (
+        - nullable: $($name1:ident: Table<$IDX1:ty, $T1:ty>,)+
+        - optional: $($name2:ident: Table<$IDX2:ty, $T2:ty>,)+
+    ) => {
         #[derive(MetadataEncodable, MetadataDecodable)]
         pub(crate) struct LazyTables {
-            $($name: LazyTable<$IDX, $T>),+
+            $($name1: LazyTable<$IDX1, $T1>,)+
+            $($name2: LazyTable<$IDX2, Option<$T2>>,)+
         }
 
         #[derive(Default)]
         struct TableBuilders {
-            $($name: TableBuilder<$IDX, $T>),+
+            $($name1: TableBuilder<$IDX1, $T1>,)+
+            $($name2: TableBuilder<$IDX2, Option<$T2>>,)+
         }
 
         impl TableBuilders {
             fn encode(&self, buf: &mut FileEncoder) -> LazyTables {
                 LazyTables {
-                    $($name: self.$name.encode(buf)),+
+                    $($name1: self.$name1.encode(buf),)+
+                    $($name2: self.$name2.encode(buf),)+
                 }
             }
         }
@@ -337,9 +343,15 @@ macro_rules! define_tables {
 }
 
 define_tables! {
+- nullable:
+    is_intrinsic: Table<DefIndex, bool>,
+    is_macro_rules: Table<DefIndex, bool>,
+    is_type_alias_impl_trait: Table<DefIndex, bool>,
+    attr_flags: Table<DefIndex, AttrFlags>,
+
+- optional:
     attributes: Table<DefIndex, LazyArray<ast::Attribute>>,
     children: Table<DefIndex, LazyArray<DefIndex>>,
-
     opt_def_kind: Table<DefIndex, DefKind>,
     visibility: Table<DefIndex, LazyValue<ty::Visibility<DefIndex>>>,
     def_span: Table<DefIndex, LazyValue<Span>>,
@@ -370,7 +382,6 @@ define_tables! {
     impl_parent: Table<DefIndex, RawDefId>,
     impl_polarity: Table<DefIndex, ty::ImplPolarity>,
     constness: Table<DefIndex, hir::Constness>,
-    is_intrinsic: Table<DefIndex, ()>,
     impl_defaultness: Table<DefIndex, hir::Defaultness>,
     // FIXME(eddyb) perhaps compute this on the fly if cheap enough?
     coerce_unsized_info: Table<DefIndex, LazyValue<ty::adjustment::CoerceUnsizedInfo>>,
@@ -380,7 +391,6 @@ define_tables! {
     fn_arg_names: Table<DefIndex, LazyArray<Ident>>,
     generator_kind: Table<DefIndex, LazyValue<hir::GeneratorKind>>,
     trait_def: Table<DefIndex, LazyValue<ty::TraitDef>>,
-
     trait_item_def_id: Table<DefIndex, RawDefId>,
     inherent_impls: Table<DefIndex, LazyArray<DefIndex>>,
     expn_that_defined: Table<DefIndex, LazyValue<ExpnId>>,
@@ -395,18 +405,12 @@ define_tables! {
     def_path_hashes: Table<DefIndex, DefPathHash>,
     proc_macro_quoted_spans: Table<usize, LazyValue<Span>>,
     generator_diagnostic_data: Table<DefIndex, LazyValue<GeneratorDiagnosticData<'static>>>,
-    attr_flags: Table<DefIndex, AttrFlags>,
     variant_data: Table<DefIndex, LazyValue<VariantData>>,
     assoc_container: Table<DefIndex, ty::AssocItemContainer>,
-    // Slot is full when macro is macro_rules.
-    macro_rules: Table<DefIndex, ()>,
     macro_definition: Table<DefIndex, LazyValue<ast::DelimArgs>>,
     proc_macro: Table<DefIndex, MacroKind>,
     module_reexports: Table<DefIndex, LazyArray<ModChild>>,
     deduced_param_attrs: Table<DefIndex, LazyArray<DeducedParamAttrs>>,
-    // Slot is full when opaque is TAIT.
-    is_type_alias_impl_trait: Table<DefIndex, ()>,
-
     trait_impl_trait_tys: Table<DefIndex, LazyValue<FxHashMap<DefId, Ty<'static>>>>,
 }
 
@@ -419,6 +423,7 @@ struct VariantData {
 }
 
 bitflags::bitflags! {
+    #[derive(Default)]
     pub struct AttrFlags: u8 {
         const MAY_HAVE_DOC_LINKS = 1 << 0;
         const IS_DOC_HIDDEN      = 1 << 1;
diff --git a/compiler/rustc_metadata/src/rmeta/table.rs b/compiler/rustc_metadata/src/rmeta/table.rs
index dc003227d40bd..70dbf6476e2fa 100644
--- a/compiler/rustc_metadata/src/rmeta/table.rs
+++ b/compiler/rustc_metadata/src/rmeta/table.rs
@@ -16,6 +16,7 @@ use std::num::NonZeroUsize;
 /// but this has no impact on safety.
 pub(super) trait FixedSizeEncoding: Default {
     /// This should be `[u8; BYTE_LEN]`;
+    /// Cannot use an associated `const BYTE_LEN: usize` instead due to const eval limitations.
     type ByteArray;
 
     fn from_bytes(b: &Self::ByteArray) -> Self;
@@ -199,31 +200,31 @@ impl FixedSizeEncoding for Option<RawDefId> {
     }
 }
 
-impl FixedSizeEncoding for Option<AttrFlags> {
+impl FixedSizeEncoding for AttrFlags {
     type ByteArray = [u8; 1];
 
     #[inline]
     fn from_bytes(b: &[u8; 1]) -> Self {
-        (b[0] != 0).then(|| AttrFlags::from_bits_truncate(b[0]))
+        AttrFlags::from_bits_truncate(b[0])
     }
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 1]) {
-        b[0] = self.map_or(0, |flags| flags.bits())
+        b[0] = self.bits();
     }
 }
 
-impl FixedSizeEncoding for Option<()> {
+impl FixedSizeEncoding for bool {
     type ByteArray = [u8; 1];
 
     #[inline]
     fn from_bytes(b: &[u8; 1]) -> Self {
-        (b[0] != 0).then(|| ())
+        b[0] != 0
     }
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 1]) {
-        b[0] = self.is_some() as u8
+        b[0] = self as u8
     }
 }
 
@@ -273,44 +274,38 @@ impl<T> FixedSizeEncoding for Option<LazyArray<T>> {
 }
 
 /// Helper for constructing a table's serialization (also see `Table`).
-pub(super) struct TableBuilder<I: Idx, T>
-where
-    Option<T>: FixedSizeEncoding,
-{
-    blocks: IndexVec<I, <Option<T> as FixedSizeEncoding>::ByteArray>,
+pub(super) struct TableBuilder<I: Idx, T: FixedSizeEncoding> {
+    blocks: IndexVec<I, T::ByteArray>,
     _marker: PhantomData<T>,
 }
 
-impl<I: Idx, T> Default for TableBuilder<I, T>
-where
-    Option<T>: FixedSizeEncoding,
-{
+impl<I: Idx, T: FixedSizeEncoding> Default for TableBuilder<I, T> {
     fn default() -> Self {
         TableBuilder { blocks: Default::default(), _marker: PhantomData }
     }
 }
 
-impl<I: Idx, T> TableBuilder<I, T>
+impl<I: Idx, const N: usize, T> TableBuilder<I, Option<T>>
 where
-    Option<T>: FixedSizeEncoding,
+    Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
 {
-    pub(crate) fn set<const N: usize>(&mut self, i: I, value: T)
-    where
-        Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
-    {
+    pub(crate) fn set(&mut self, i: I, value: T) {
+        self.set_nullable(i, Some(value))
+    }
+}
+
+impl<I: Idx, const N: usize, T: FixedSizeEncoding<ByteArray = [u8; N]>> TableBuilder<I, T> {
+    pub(crate) fn set_nullable(&mut self, i: I, value: T) {
         // FIXME(eddyb) investigate more compact encodings for sparse tables.
         // On the PR @michaelwoerister mentioned:
         // > Space requirements could perhaps be optimized by using the HAMT `popcnt`
         // > trick (i.e. divide things into buckets of 32 or 64 items and then
         // > store bit-masks of which item in each bucket is actually serialized).
         self.blocks.ensure_contains_elem(i, || [0; N]);
-        Some(value).write_to_bytes(&mut self.blocks[i]);
+        value.write_to_bytes(&mut self.blocks[i]);
     }
 
-    pub(crate) fn encode<const N: usize>(&self, buf: &mut FileEncoder) -> LazyTable<I, T>
-    where
-        Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
-    {
+    pub(crate) fn encode(&self, buf: &mut FileEncoder) -> LazyTable<I, T> {
         let pos = buf.position();
         for block in &self.blocks {
             buf.emit_raw_bytes(block);
@@ -323,34 +318,27 @@ where
     }
 }
 
-impl<I: Idx, T: ParameterizedOverTcx> LazyTable<I, T>
+impl<I: Idx, const N: usize, T: FixedSizeEncoding<ByteArray = [u8; N]> + ParameterizedOverTcx>
+    LazyTable<I, T>
 where
-    Option<T>: FixedSizeEncoding,
+    for<'tcx> T::Value<'tcx>: FixedSizeEncoding<ByteArray = [u8; N]>,
 {
     /// Given the metadata, extract out the value at a particular index (if any).
     #[inline(never)]
-    pub(super) fn get<'a, 'tcx, M: Metadata<'a, 'tcx>, const N: usize>(
-        &self,
-        metadata: M,
-        i: I,
-    ) -> Option<T::Value<'tcx>>
-    where
-        Option<T::Value<'tcx>>: FixedSizeEncoding<ByteArray = [u8; N]>,
-    {
+    pub(super) fn get<'a, 'tcx, M: Metadata<'a, 'tcx>>(&self, metadata: M, i: I) -> T::Value<'tcx> {
         debug!("LazyTable::lookup: index={:?} len={:?}", i, self.encoded_size);
 
         let start = self.position.get();
         let bytes = &metadata.blob()[start..start + self.encoded_size];
         let (bytes, []) = bytes.as_chunks::<N>() else { panic!() };
-        let bytes = bytes.get(i.index())?;
-        FixedSizeEncoding::from_bytes(bytes)
+        match bytes.get(i.index()) {
+            Some(bytes) => FixedSizeEncoding::from_bytes(bytes),
+            None => FixedSizeEncoding::from_bytes(&[0; N]),
+        }
     }
 
     /// Size of the table in entries, including possible gaps.
-    pub(super) fn size<const N: usize>(&self) -> usize
-    where
-        for<'tcx> Option<T::Value<'tcx>>: FixedSizeEncoding<ByteArray = [u8; N]>,
-    {
+    pub(super) fn size(&self) -> usize {
         self.encoded_size / N
     }
 }
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index ce04d8d21f4cd..0b16270ea9874 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -997,6 +997,30 @@ impl<'tcx> TyCtxt<'tcx> {
         v.0
     }
 
+    /// Given a `DefId` for an `fn`, return all the `dyn` and `impl` traits in its return type and associated alias span when type alias is used
+    pub fn return_type_impl_or_dyn_traits_with_type_alias(
+        self,
+        scope_def_id: LocalDefId,
+    ) -> Option<(Vec<&'tcx hir::Ty<'tcx>>, Span)> {
+        let hir_id = self.hir().local_def_id_to_hir_id(scope_def_id);
+        let mut v = TraitObjectVisitor(vec![], self.hir());
+        // when the return type is a type alias
+        if let Some(hir::FnDecl { output: hir::FnRetTy::Return(hir_output), .. }) = self.hir().fn_decl_by_hir_id(hir_id)
+            && let hir::TyKind::Path(hir::QPath::Resolved(
+                None,
+                hir::Path { res: hir::def::Res::Def(DefKind::TyAlias, def_id), .. }, )) = hir_output.kind
+            && let Some(local_id) = def_id.as_local()
+            && let Some(alias_ty) = self.hir().get_by_def_id(local_id).alias_ty() // it is type alias
+            && let Some(alias_generics) = self.hir().get_by_def_id(local_id).generics()
+        {
+            v.visit_ty(alias_ty);
+            if !v.0.is_empty() {
+                return Some((v.0, alias_generics.span));
+            }
+        }
+        return None;
+    }
+
     pub fn return_type_impl_trait(self, scope_def_id: LocalDefId) -> Option<(Ty<'tcx>, Span)> {
         // `type_of()` will fail on these (#55796, #86483), so only allow `fn`s or closures.
         match self.hir().get_by_def_id(scope_def_id) {
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 06b970ad97977..40763da0bb547 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -337,7 +337,9 @@ pub(crate) struct IfExpressionMissingThenBlock {
     #[primary_span]
     pub if_span: Span,
     #[subdiagnostic]
-    pub sub: IfExpressionMissingThenBlockSub,
+    pub missing_then_block_sub: IfExpressionMissingThenBlockSub,
+    #[subdiagnostic]
+    pub let_else_sub: Option<IfExpressionLetSomeSub>,
 }
 
 #[derive(Subdiagnostic)]
@@ -348,6 +350,13 @@ pub(crate) enum IfExpressionMissingThenBlockSub {
     AddThenBlock(#[primary_span] Span),
 }
 
+#[derive(Subdiagnostic)]
+#[help(parse_extra_if_in_let_else)]
+pub(crate) struct IfExpressionLetSomeSub {
+    #[primary_span]
+    pub if_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_if_expression_missing_condition)]
 pub(crate) struct IfExpressionMissingCondition {
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index bf93a89f06555..3225a309a319b 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -11,15 +11,15 @@ use crate::errors::{
     ComparisonOrShiftInterpretedAsGenericSugg, DoCatchSyntaxRemoved, DotDotDot, EqFieldInit,
     ExpectedElseBlock, ExpectedEqForLetExpr, ExpectedExpressionFoundLet,
     FieldExpressionWithGeneric, FloatLiteralRequiresIntegerPart, FoundExprWouldBeStmt,
-    IfExpressionMissingCondition, IfExpressionMissingThenBlock, IfExpressionMissingThenBlockSub,
-    InvalidBlockMacroSegment, InvalidComparisonOperator, InvalidComparisonOperatorSub,
-    InvalidInterpolatedExpression, InvalidLiteralSuffixOnTupleIndex, InvalidLogicalOperator,
-    InvalidLogicalOperatorSub, LabeledLoopInBreak, LeadingPlusNotSupported, LeftArrowOperator,
-    LifetimeInBorrowExpression, MacroInvocationWithQualifiedPath, MalformedLoopLabel,
-    MatchArmBodyWithoutBraces, MatchArmBodyWithoutBracesSugg, MissingCommaAfterMatchArm,
-    MissingDotDot, MissingInInForLoop, MissingInInForLoopSub, MissingSemicolonBeforeArray,
-    NoFieldsForFnCall, NotAsNegationOperator, NotAsNegationOperatorSub,
-    OuterAttributeNotAllowedOnIfElse, ParenthesesWithStructFields,
+    IfExpressionLetSomeSub, IfExpressionMissingCondition, IfExpressionMissingThenBlock,
+    IfExpressionMissingThenBlockSub, InvalidBlockMacroSegment, InvalidComparisonOperator,
+    InvalidComparisonOperatorSub, InvalidInterpolatedExpression, InvalidLiteralSuffixOnTupleIndex,
+    InvalidLogicalOperator, InvalidLogicalOperatorSub, LabeledLoopInBreak, LeadingPlusNotSupported,
+    LeftArrowOperator, LifetimeInBorrowExpression, MacroInvocationWithQualifiedPath,
+    MalformedLoopLabel, MatchArmBodyWithoutBraces, MatchArmBodyWithoutBracesSugg,
+    MissingCommaAfterMatchArm, MissingDotDot, MissingInInForLoop, MissingInInForLoopSub,
+    MissingSemicolonBeforeArray, NoFieldsForFnCall, NotAsNegationOperator,
+    NotAsNegationOperatorSub, OuterAttributeNotAllowedOnIfElse, ParenthesesWithStructFields,
     RequireColonAfterLabeledExpression, ShiftInterpretedAsGeneric, StructLiteralNotAllowedHere,
     StructLiteralNotAllowedHereSugg, TildeAsUnaryOperator, UnexpectedIfWithIf,
     UnexpectedTokenAfterLabel, UnexpectedTokenAfterLabelSugg, WrapExpressionInParentheses,
@@ -2251,9 +2251,10 @@ impl<'a> Parser<'a> {
                     if let ExprKind::Block(_, None) = right.kind => {
                         self.sess.emit_err(IfExpressionMissingThenBlock {
                             if_span: lo,
-                            sub: IfExpressionMissingThenBlockSub::UnfinishedCondition(
-                                cond_span.shrink_to_lo().to(*binop_span)
-                            ),
+                            missing_then_block_sub:
+                                IfExpressionMissingThenBlockSub::UnfinishedCondition(cond_span.shrink_to_lo().to(*binop_span)),
+                                let_else_sub: None,
+
                         });
                         std::mem::replace(right, this.mk_expr_err(binop_span.shrink_to_hi()))
                     },
@@ -2279,9 +2280,15 @@ impl<'a> Parser<'a> {
             if let Some(block) = recover_block_from_condition(self) {
                 block
             } else {
+                let let_else_sub = matches!(cond.kind, ExprKind::Let(..))
+                    .then(|| IfExpressionLetSomeSub { if_span: lo });
+
                 self.sess.emit_err(IfExpressionMissingThenBlock {
                     if_span: lo,
-                    sub: IfExpressionMissingThenBlockSub::AddThenBlock(cond_span.shrink_to_hi()),
+                    missing_then_block_sub: IfExpressionMissingThenBlockSub::AddThenBlock(
+                        cond_span.shrink_to_hi(),
+                    ),
+                    let_else_sub,
                 });
                 self.mk_block_err(cond_span.shrink_to_hi())
             }
diff --git a/compiler/rustc_trait_selection/src/solve/fulfill.rs b/compiler/rustc_trait_selection/src/solve/fulfill.rs
index 40b9bedc84fd3..d59fa71406c31 100644
--- a/compiler/rustc_trait_selection/src/solve/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/solve/fulfill.rs
@@ -1,5 +1,6 @@
 use std::mem;
 
+use super::{Certainty, InferCtxtEvalExt};
 use rustc_infer::{
     infer::InferCtxt,
     traits::{
@@ -8,8 +9,6 @@ use rustc_infer::{
     },
 };
 
-use super::{search_graph, Certainty, EvalCtxt};
-
 /// A trait engine using the new trait solver.
 ///
 /// This is mostly identical to how `evaluate_all` works inside of the
@@ -66,9 +65,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
             let mut has_changed = false;
             for obligation in mem::take(&mut self.obligations) {
                 let goal = obligation.clone().into();
-                let search_graph = &mut search_graph::SearchGraph::new(infcx.tcx);
-                let mut ecx = EvalCtxt::new_outside_solver(infcx, search_graph);
-                let (changed, certainty) = match ecx.evaluate_goal(goal) {
+                let (changed, certainty) = match infcx.evaluate_root_goal(goal) {
                     Ok(result) => result,
                     Err(NoSolution) => {
                         errors.push(FulfillmentError {
diff --git a/compiler/rustc_trait_selection/src/solve/mod.rs b/compiler/rustc_trait_selection/src/solve/mod.rs
index da2a1a19957e1..70f094014453e 100644
--- a/compiler/rustc_trait_selection/src/solve/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/mod.rs
@@ -152,6 +152,36 @@ impl<'tcx> TyCtxtExt<'tcx> for TyCtxt<'tcx> {
     }
 }
 
+pub trait InferCtxtEvalExt<'tcx> {
+    /// Evaluates a goal from **outside** of the trait solver.
+    ///
+    /// Using this while inside of the solver is wrong as it uses a new
+    /// search graph which would break cycle detection.
+    fn evaluate_root_goal(
+        &self,
+        goal: Goal<'tcx, ty::Predicate<'tcx>>,
+    ) -> Result<(bool, Certainty), NoSolution>;
+}
+
+impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
+    fn evaluate_root_goal(
+        &self,
+        goal: Goal<'tcx, ty::Predicate<'tcx>>,
+    ) -> Result<(bool, Certainty), NoSolution> {
+        let mut search_graph = search_graph::SearchGraph::new(self.tcx);
+
+        let result = EvalCtxt {
+            search_graph: &mut search_graph,
+            infcx: self,
+            var_values: CanonicalVarValues::dummy(),
+        }
+        .evaluate_goal(goal);
+
+        assert!(search_graph.is_empty());
+        result
+    }
+}
+
 struct EvalCtxt<'a, 'tcx> {
     infcx: &'a InferCtxt<'tcx>,
     var_values: CanonicalVarValues<'tcx>,
@@ -164,18 +194,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         self.infcx.tcx
     }
 
-    /// Creates a new evaluation context outside of the trait solver.
-    ///
-    /// With this solver making a canonical response doesn't make much sense.
-    /// The `search_graph` for this solver has to be completely empty.
-    fn new_outside_solver(
-        infcx: &'a InferCtxt<'tcx>,
-        search_graph: &'a mut search_graph::SearchGraph<'tcx>,
-    ) -> EvalCtxt<'a, 'tcx> {
-        assert!(search_graph.is_empty());
-        EvalCtxt { infcx, var_values: CanonicalVarValues::dummy(), search_graph }
-    }
-
     #[instrument(level = "debug", skip(tcx, search_graph), ret)]
     fn evaluate_canonical_goal(
         tcx: TyCtxt<'tcx>,
diff --git a/library/core/src/cell/once.rs b/library/core/src/cell/once.rs
index 7757068a4f2b9..f74e563f1b9c0 100644
--- a/library/core/src/cell/once.rs
+++ b/library/core/src/cell/once.rs
@@ -298,3 +298,7 @@ impl<T> const From<T> for OnceCell<T> {
         OnceCell { inner: UnsafeCell::new(Some(value)) }
     }
 }
+
+// Just like for `Cell<T>` this isn't needed, but results in nicer error messages.
+#[unstable(feature = "once_cell", issue = "74465")]
+impl<T> !Sync for OnceCell<T> {}
diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs
index 2a7ec544f9e2e..0145ba5e3cde6 100644
--- a/library/core/src/fmt/mod.rs
+++ b/library/core/src/fmt/mod.rs
@@ -489,9 +489,26 @@ pub struct Arguments<'a> {
 }
 
 impl<'a> Arguments<'a> {
-    /// Get the formatted string, if it has no arguments to be formatted.
+    /// Get the formatted string, if it has no arguments to be formatted at runtime.
     ///
-    /// This can be used to avoid allocations in the most trivial case.
+    /// This can be used to avoid allocations in some cases.
+    ///
+    /// # Guarantees
+    ///
+    /// For `format_args!("just a literal")`, this function is guaranteed to
+    /// return `Some("just a literal")`.
+    ///
+    /// For most cases with placeholders, this function will return `None`.
+    ///
+    /// However, the compiler may perform optimizations that can cause this
+    /// function to return `Some(_)` even if the format string contains
+    /// placeholders. For example, `format_args!("Hello, {}!", "world")` may be
+    /// optimized to `format_args!("Hello, world!")`, such that `as_str()`
+    /// returns `Some("Hello, world!")`.
+    ///
+    /// The behavior for anything but the trivial case (without placeholders)
+    /// is not guaranteed, and should not be relied upon for anything other
+    /// than optimization.
     ///
     /// # Examples
     ///
@@ -512,7 +529,7 @@ impl<'a> Arguments<'a> {
     /// ```rust
     /// assert_eq!(format_args!("hello").as_str(), Some("hello"));
     /// assert_eq!(format_args!("").as_str(), Some(""));
-    /// assert_eq!(format_args!("{}", 1).as_str(), None);
+    /// assert_eq!(format_args!("{:?}", std::env::current_dir()).as_str(), None);
     /// ```
     #[stable(feature = "fmt_as_str", since = "1.52.0")]
     #[rustc_const_unstable(feature = "const_arguments_as_str", issue = "103900")]
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 1326fc9ab096f..74055602ec2e6 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -469,6 +469,62 @@ pub macro Copy($item:item) {
 #[cfg_attr(not(test), rustc_diagnostic_item = "Sync")]
 #[lang = "sync"]
 #[rustc_on_unimplemented(
+    on(
+        _Self = "std::cell::OnceCell<T>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::OnceLock` instead"
+    ),
+    on(
+        _Self = "std::cell::Cell<u8>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU8` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<u16>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU16` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<u32>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU32` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<u64>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU64` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<usize>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicUsize` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<i8>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI8` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<i16>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI16` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<i32>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<i64>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI64` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<isize>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicIsize` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<bool>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicBool` instead",
+    ),
+    on(
+        _Self = "std::cell::Cell<T>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`",
+    ),
+    on(
+        _Self = "std::cell::RefCell<T>",
+        note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead",
+    ),
     message = "`{Self}` cannot be shared between threads safely",
     label = "`{Self}` cannot be shared between threads safely"
 )]
diff --git a/tests/ui/async-await/issue-68112.drop_tracking.stderr b/tests/ui/async-await/issue-68112.drop_tracking.stderr
index f2802698fd5b6..bd648de30672d 100644
--- a/tests/ui/async-await/issue-68112.drop_tracking.stderr
+++ b/tests/ui/async-await/issue-68112.drop_tracking.stderr
@@ -5,6 +5,7 @@ LL |     require_send(send_fut);
    |                  ^^^^^^^^ future created by async block is not `Send`
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
 note: future is not `Send` as it awaits another future which is not `Send`
   --> $DIR/issue-68112.rs:34:17
    |
@@ -23,6 +24,7 @@ LL |     require_send(send_fut);
    |                  ^^^^^^^^ future created by async block is not `Send`
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
 note: future is not `Send` as it awaits another future which is not `Send`
   --> $DIR/issue-68112.rs:43:17
    |
@@ -43,6 +45,7 @@ LL |     require_send(send_fut);
    |     required by a bound introduced by this call
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
    = note: required for `Arc<RefCell<i32>>` to implement `Send`
 note: required because it's used within this `async fn` body
   --> $DIR/issue-68112.rs:50:31
diff --git a/tests/ui/async-await/issue-68112.no_drop_tracking.stderr b/tests/ui/async-await/issue-68112.no_drop_tracking.stderr
index 38eb85b302fd5..35b7341f63a4d 100644
--- a/tests/ui/async-await/issue-68112.no_drop_tracking.stderr
+++ b/tests/ui/async-await/issue-68112.no_drop_tracking.stderr
@@ -5,6 +5,7 @@ LL |     require_send(send_fut);
    |                  ^^^^^^^^ future created by async block is not `Send`
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
 note: future is not `Send` as it awaits another future which is not `Send`
   --> $DIR/issue-68112.rs:34:17
    |
@@ -23,6 +24,7 @@ LL |     require_send(send_fut);
    |                  ^^^^^^^^ future created by async block is not `Send`
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
 note: future is not `Send` as it awaits another future which is not `Send`
   --> $DIR/issue-68112.rs:43:17
    |
@@ -43,6 +45,7 @@ LL |     require_send(send_fut);
    |     required by a bound introduced by this call
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
    = note: required for `Arc<RefCell<i32>>` to implement `Send`
 note: required because it's used within this `async fn` body
   --> $DIR/issue-68112.rs:50:31
diff --git a/tests/ui/generator/issue-68112.rs b/tests/ui/generator/issue-68112.rs
index 21026f45cb823..9def544e3d25c 100644
--- a/tests/ui/generator/issue-68112.rs
+++ b/tests/ui/generator/issue-68112.rs
@@ -40,6 +40,7 @@ fn test1() {
     require_send(send_gen);
     //~^ ERROR generator cannot be sent between threads
     //~| NOTE not `Send`
+    //~| NOTE use `std::sync::RwLock` instead
 }
 
 pub fn make_gen2<T>(t: T) -> impl Generator<Return = T> {
@@ -66,6 +67,7 @@ fn test2() {
     //~| NOTE required for
     //~| NOTE required by a bound introduced by this call
     //~| NOTE captures the following types
+    //~| NOTE use `std::sync::RwLock` instead
 }
 
 fn main() {}
diff --git a/tests/ui/generator/issue-68112.stderr b/tests/ui/generator/issue-68112.stderr
index eb99d42c92068..b42bc93d01f66 100644
--- a/tests/ui/generator/issue-68112.stderr
+++ b/tests/ui/generator/issue-68112.stderr
@@ -5,6 +5,7 @@ LL |     require_send(send_gen);
    |                  ^^^^^^^^ generator is not `Send`
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
 note: generator is not `Send` as this value is used across a yield
   --> $DIR/issue-68112.rs:36:9
    |
@@ -23,7 +24,7 @@ LL | fn require_send(_: impl Send) {}
    |                         ^^^^ required by this bound in `require_send`
 
 error[E0277]: `RefCell<i32>` cannot be shared between threads safely
-  --> $DIR/issue-68112.rs:63:18
+  --> $DIR/issue-68112.rs:64:18
    |
 LL |     require_send(send_gen);
    |     ------------ ^^^^^^^^ `RefCell<i32>` cannot be shared between threads safely
@@ -31,25 +32,26 @@ LL |     require_send(send_gen);
    |     required by a bound introduced by this call
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
    = note: required for `Arc<RefCell<i32>>` to implement `Send`
 note: required because it's used within this generator
-  --> $DIR/issue-68112.rs:48:5
+  --> $DIR/issue-68112.rs:49:5
    |
 LL |     || {
    |     ^^
 note: required because it appears within the type `impl Generator<Return = Arc<RefCell<i32>>>`
-  --> $DIR/issue-68112.rs:45:30
+  --> $DIR/issue-68112.rs:46:30
    |
 LL | pub fn make_gen2<T>(t: T) -> impl Generator<Return = T> {
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: required because it appears within the type `impl Generator<Return = Arc<RefCell<i32>>>`
-  --> $DIR/issue-68112.rs:53:34
+  --> $DIR/issue-68112.rs:54:34
    |
 LL | fn make_non_send_generator2() -> impl Generator<Return = Arc<RefCell<i32>>> {
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: required because it captures the following types: `impl Generator<Return = Arc<RefCell<i32>>>`, `()`
 note: required because it's used within this generator
-  --> $DIR/issue-68112.rs:59:20
+  --> $DIR/issue-68112.rs:60:20
    |
 LL |     let send_gen = || {
    |                    ^^
diff --git a/tests/ui/generator/not-send-sync.stderr b/tests/ui/generator/not-send-sync.stderr
index a821c57b923a0..1711df729b8c0 100644
--- a/tests/ui/generator/not-send-sync.stderr
+++ b/tests/ui/generator/not-send-sync.stderr
@@ -12,6 +12,7 @@ LL | |     });
    | |_____^ `Cell<i32>` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
    = note: required for `&Cell<i32>` to implement `Send`
 note: required because it's used within this generator
   --> $DIR/not-send-sync.rs:16:17
@@ -36,6 +37,7 @@ LL | |     });
    | |_____^ generator is not `Sync`
    |
    = help: within `[generator@$DIR/not-send-sync.rs:9:17: 9:19]`, the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
 note: generator is not `Sync` as this value is used across a yield
   --> $DIR/not-send-sync.rs:12:9
    |
diff --git a/tests/ui/generator/print/generator-print-verbose-1.stderr b/tests/ui/generator/print/generator-print-verbose-1.stderr
index ebf35be581c60..45d018b8ebad5 100644
--- a/tests/ui/generator/print/generator-print-verbose-1.stderr
+++ b/tests/ui/generator/print/generator-print-verbose-1.stderr
@@ -5,6 +5,7 @@ LL |     require_send(send_gen);
    |                  ^^^^^^^^ generator is not `Send`
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
 note: generator is not `Send` as this value is used across a yield
   --> $DIR/generator-print-verbose-1.rs:35:9
    |
@@ -29,6 +30,7 @@ LL |     require_send(send_gen);
    |     required by a bound introduced by this call
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
    = note: required for `Arc<RefCell<i32>>` to implement `Send`
 note: required because it's used within this generator
   --> $DIR/generator-print-verbose-1.rs:42:5
diff --git a/tests/ui/generator/print/generator-print-verbose-2.stderr b/tests/ui/generator/print/generator-print-verbose-2.stderr
index 909e49c38b8d1..59112ce0a79e6 100644
--- a/tests/ui/generator/print/generator-print-verbose-2.stderr
+++ b/tests/ui/generator/print/generator-print-verbose-2.stderr
@@ -12,6 +12,7 @@ LL | |     });
    | |_____^ `Cell<i32>` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
    = note: required for `&'_#4r Cell<i32>` to implement `Send`
 note: required because it's used within this generator
   --> $DIR/generator-print-verbose-2.rs:19:17
@@ -36,6 +37,7 @@ LL | |     });
    | |_____^ generator is not `Sync`
    |
    = help: within `[main::{closure#0} upvar_tys=() {Cell<i32>, ()}]`, the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
 note: generator is not `Sync` as this value is used across a yield
   --> $DIR/generator-print-verbose-2.rs:15:9
    |
diff --git a/tests/ui/issues/issue-7364.stderr b/tests/ui/issues/issue-7364.stderr
index 5dc8c2b607e61..aee73380f15e7 100644
--- a/tests/ui/issues/issue-7364.stderr
+++ b/tests/ui/issues/issue-7364.stderr
@@ -5,6 +5,7 @@ LL | static boxed: Box<RefCell<isize>> = Box::new(RefCell::new(0));
    |               ^^^^^^^^^^^^^^^^^^^ `RefCell<isize>` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `RefCell<isize>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
    = note: required for `Unique<RefCell<isize>>` to implement `Sync`
    = note: required because it appears within the type `Box<RefCell<isize>>`
    = note: shared static variables must have a type that implements `Sync`
diff --git a/tests/ui/let-else/accidental-if.rs b/tests/ui/let-else/accidental-if.rs
new file mode 100644
index 0000000000000..3fba630435c71
--- /dev/null
+++ b/tests/ui/let-else/accidental-if.rs
@@ -0,0 +1,6 @@
+fn main() {
+    let x = Some(123);
+    if let Some(y) = x else { //~ ERROR this `if` expression is missing a block
+        return;
+    };
+}
diff --git a/tests/ui/let-else/accidental-if.stderr b/tests/ui/let-else/accidental-if.stderr
new file mode 100644
index 0000000000000..5474a67aac45a
--- /dev/null
+++ b/tests/ui/let-else/accidental-if.stderr
@@ -0,0 +1,19 @@
+error: this `if` expression is missing a block after the condition
+  --> $DIR/accidental-if.rs:3:5
+   |
+LL |     if let Some(y) = x else {
+   |     ^^
+   |
+help: add a block here
+  --> $DIR/accidental-if.rs:3:23
+   |
+LL |     if let Some(y) = x else {
+   |                       ^
+help: remove the `if` if you meant to write a `let...else` statement
+  --> $DIR/accidental-if.rs:3:5
+   |
+LL |     if let Some(y) = x else {
+   |     ^^
+
+error: aborting due to previous error
+
diff --git a/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.fixed b/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.fixed
new file mode 100644
index 0000000000000..aa3bce2945b68
--- /dev/null
+++ b/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.fixed
@@ -0,0 +1,45 @@
+// run-rustfix
+
+trait Greeter0 {
+    fn greet(&self);
+}
+
+trait Greeter1 {
+    fn greet(&self);
+}
+
+type BoxedGreeter<'a> = (Box<dyn Greeter0 + 'a>, Box<dyn Greeter1 + 'a>);
+//~^ HELP to declare that the trait object captures data from argument `self`, you can add a lifetime parameter `'a` in the type alias
+
+struct FixedGreeter<'a>(pub &'a str);
+
+impl Greeter0 for FixedGreeter<'_> {
+    fn greet(&self) {
+        println!("0 {}", self.0)
+    }
+}
+
+impl Greeter1 for FixedGreeter<'_> {
+    fn greet(&self) {
+        println!("1 {}", self.0)
+    }
+}
+
+struct Greetings(pub Vec<String>);
+
+impl Greetings {
+    pub fn get(&self, i: usize) -> BoxedGreeter {
+        (Box::new(FixedGreeter(&self.0[i])), Box::new(FixedGreeter(&self.0[i])))
+        //~^ ERROR lifetime may not live long enough
+    }
+}
+
+fn main() {
+    let mut g = Greetings {0 : vec!()};
+    g.0.push("a".to_string());
+    g.0.push("b".to_string());
+    g.get(0).0.greet();
+    g.get(0).1.greet();
+    g.get(1).0.greet();
+    g.get(1).1.greet();
+}
diff --git a/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.rs b/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.rs
new file mode 100644
index 0000000000000..20c88ec69813a
--- /dev/null
+++ b/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.rs
@@ -0,0 +1,45 @@
+// run-rustfix
+
+trait Greeter0 {
+    fn greet(&self);
+}
+
+trait Greeter1 {
+    fn greet(&self);
+}
+
+type BoxedGreeter = (Box<dyn Greeter0>, Box<dyn Greeter1>);
+//~^ HELP to declare that the trait object captures data from argument `self`, you can add a lifetime parameter `'a` in the type alias
+
+struct FixedGreeter<'a>(pub &'a str);
+
+impl Greeter0 for FixedGreeter<'_> {
+    fn greet(&self) {
+        println!("0 {}", self.0)
+    }
+}
+
+impl Greeter1 for FixedGreeter<'_> {
+    fn greet(&self) {
+        println!("1 {}", self.0)
+    }
+}
+
+struct Greetings(pub Vec<String>);
+
+impl Greetings {
+    pub fn get(&self, i: usize) -> BoxedGreeter {
+        (Box::new(FixedGreeter(&self.0[i])), Box::new(FixedGreeter(&self.0[i])))
+        //~^ ERROR lifetime may not live long enough
+    }
+}
+
+fn main() {
+    let mut g = Greetings {0 : vec!()};
+    g.0.push("a".to_string());
+    g.0.push("b".to_string());
+    g.get(0).0.greet();
+    g.get(0).1.greet();
+    g.get(1).0.greet();
+    g.get(1).1.greet();
+}
diff --git a/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.stderr b/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.stderr
new file mode 100644
index 0000000000000..808d8bb905885
--- /dev/null
+++ b/tests/ui/lifetimes/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.stderr
@@ -0,0 +1,15 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-103582-hint-for-missing-lifetime-bound-on-trait-object-using-type-alias.rs:32:9
+   |
+LL |     pub fn get(&self, i: usize) -> BoxedGreeter {
+   |                - let's call the lifetime of this reference `'1`
+LL |         (Box::new(FixedGreeter(&self.0[i])), Box::new(FixedGreeter(&self.0[i])))
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`
+   |
+help: to declare that the trait object captures data from argument `self`, you can add a lifetime parameter `'a` in the type alias
+   |
+LL | type BoxedGreeter<'a> = (Box<dyn Greeter0 + 'a>, Box<dyn Greeter1 + 'a>);
+   |                  ++++                     ++++                    ++++
+
+error: aborting due to previous error
+
diff --git a/tests/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr b/tests/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr
index 498a112fa9bb3..f34ccecdd45e6 100644
--- a/tests/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr
+++ b/tests/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr
@@ -37,6 +37,11 @@ help: add a block here
    |
 LL |     if let Some(n) = opt else {
    |                         ^
+help: remove the `if` if you meant to write a `let...else` statement
+  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:24:5
+   |
+LL |     if let Some(n) = opt else {
+   |     ^^
 
 error: this `if` expression is missing a block after the condition
   --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:28:5
diff --git a/tests/ui/stdlib-unit-tests/not-sync.stderr b/tests/ui/stdlib-unit-tests/not-sync.stderr
index 1ee358ba8368e..4e34e10e37789 100644
--- a/tests/ui/stdlib-unit-tests/not-sync.stderr
+++ b/tests/ui/stdlib-unit-tests/not-sync.stderr
@@ -5,6 +5,7 @@ LL |     test::<Cell<i32>>();
    |            ^^^^^^^^^ `Cell<i32>` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
 note: required by a bound in `test`
   --> $DIR/not-sync.rs:5:12
    |
@@ -18,6 +19,7 @@ LL |     test::<RefCell<i32>>();
    |            ^^^^^^^^^^^^ `RefCell<i32>` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
 note: required by a bound in `test`
   --> $DIR/not-sync.rs:5:12
    |
diff --git a/tests/ui/mutexguard-sync.rs b/tests/ui/sync/mutexguard-sync.rs
similarity index 100%
rename from tests/ui/mutexguard-sync.rs
rename to tests/ui/sync/mutexguard-sync.rs
diff --git a/tests/ui/mutexguard-sync.stderr b/tests/ui/sync/mutexguard-sync.stderr
similarity index 83%
rename from tests/ui/mutexguard-sync.stderr
rename to tests/ui/sync/mutexguard-sync.stderr
index 3fbb2ddf183d3..4dc5571196c14 100644
--- a/tests/ui/mutexguard-sync.stderr
+++ b/tests/ui/sync/mutexguard-sync.stderr
@@ -7,6 +7,7 @@ LL |     test_sync(guard);
    |     required by a bound introduced by this call
    |
    = help: the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
    = note: required for `MutexGuard<'_, Cell<i32>>` to implement `Sync`
 note: required by a bound in `test_sync`
   --> $DIR/mutexguard-sync.rs:5:17
diff --git a/tests/ui/sync/suggest-cell.rs b/tests/ui/sync/suggest-cell.rs
new file mode 100644
index 0000000000000..3284eae7be181
--- /dev/null
+++ b/tests/ui/sync/suggest-cell.rs
@@ -0,0 +1,31 @@
+fn require_sync<T: Sync>() {}
+//~^ NOTE required by this bound in `require_sync`
+//~| NOTE required by this bound in `require_sync`
+//~| NOTE required by this bound in `require_sync`
+//~| NOTE required by this bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+
+fn main() {
+    require_sync::<std::cell::Cell<()>>();
+    //~^ ERROR `Cell<()>` cannot be shared between threads safely
+    //~| NOTE `Cell<()>` cannot be shared between threads safely
+    //~| NOTE if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
+
+    require_sync::<std::cell::Cell<u8>>();
+    //~^ ERROR `Cell<u8>` cannot be shared between threads safely
+    //~| NOTE `Cell<u8>` cannot be shared between threads safely
+    //~| NOTE if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU8` instead
+
+    require_sync::<std::cell::Cell<i32>>();
+    //~^ ERROR `Cell<i32>` cannot be shared between threads safely
+    //~| NOTE `Cell<i32>` cannot be shared between threads safely
+    //~| NOTE if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
+
+    require_sync::<std::cell::Cell<bool>>();
+    //~^ ERROR `Cell<bool>` cannot be shared between threads safely
+    //~| NOTE `Cell<bool>` cannot be shared between threads safely
+    //~| NOTE if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicBool` instead
+}
diff --git a/tests/ui/sync/suggest-cell.stderr b/tests/ui/sync/suggest-cell.stderr
new file mode 100644
index 0000000000000..c232c1ccd53dd
--- /dev/null
+++ b/tests/ui/sync/suggest-cell.stderr
@@ -0,0 +1,59 @@
+error[E0277]: `Cell<()>` cannot be shared between threads safely
+  --> $DIR/suggest-cell.rs:12:20
+   |
+LL |     require_sync::<std::cell::Cell<()>>();
+   |                    ^^^^^^^^^^^^^^^^^^^ `Cell<()>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `Cell<()>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-cell.rs:1:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error[E0277]: `Cell<u8>` cannot be shared between threads safely
+  --> $DIR/suggest-cell.rs:17:20
+   |
+LL |     require_sync::<std::cell::Cell<u8>>();
+   |                    ^^^^^^^^^^^^^^^^^^^ `Cell<u8>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `Cell<u8>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU8` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-cell.rs:1:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error[E0277]: `Cell<i32>` cannot be shared between threads safely
+  --> $DIR/suggest-cell.rs:22:20
+   |
+LL |     require_sync::<std::cell::Cell<i32>>();
+   |                    ^^^^^^^^^^^^^^^^^^^^ `Cell<i32>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `Cell<i32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicI32` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-cell.rs:1:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error[E0277]: `Cell<bool>` cannot be shared between threads safely
+  --> $DIR/suggest-cell.rs:27:20
+   |
+LL |     require_sync::<std::cell::Cell<bool>>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^ `Cell<bool>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `Cell<bool>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicBool` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-cell.rs:1:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/sync/suggest-once-cell.rs b/tests/ui/sync/suggest-once-cell.rs
new file mode 100644
index 0000000000000..82fca45b1a47f
--- /dev/null
+++ b/tests/ui/sync/suggest-once-cell.rs
@@ -0,0 +1,12 @@
+#![feature(once_cell)]
+
+fn require_sync<T: Sync>() {}
+//~^ NOTE required by this bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+
+fn main() {
+    require_sync::<std::cell::OnceCell<()>>();
+    //~^ ERROR `OnceCell<()>` cannot be shared between threads safely
+    //~| NOTE `OnceCell<()>` cannot be shared between threads safely
+    //~| NOTE use `std::sync::OnceLock` instead
+}
diff --git a/tests/ui/sync/suggest-once-cell.stderr b/tests/ui/sync/suggest-once-cell.stderr
new file mode 100644
index 0000000000000..fadf05374d8ca
--- /dev/null
+++ b/tests/ui/sync/suggest-once-cell.stderr
@@ -0,0 +1,17 @@
+error[E0277]: `OnceCell<()>` cannot be shared between threads safely
+  --> $DIR/suggest-once-cell.rs:8:20
+   |
+LL |     require_sync::<std::cell::OnceCell<()>>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^ `OnceCell<()>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `OnceCell<()>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::OnceLock` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-once-cell.rs:3:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/sync/suggest-ref-cell.rs b/tests/ui/sync/suggest-ref-cell.rs
new file mode 100644
index 0000000000000..6b972ae09622e
--- /dev/null
+++ b/tests/ui/sync/suggest-ref-cell.rs
@@ -0,0 +1,12 @@
+#![feature(once_cell)]
+
+fn require_sync<T: Sync>() {}
+//~^ NOTE required by this bound in `require_sync`
+//~| NOTE required by a bound in `require_sync`
+
+fn main() {
+    require_sync::<std::cell::RefCell<()>>();
+    //~^ ERROR `RefCell<()>` cannot be shared between threads safely
+    //~| NOTE `RefCell<()>` cannot be shared between threads safely
+    //~| NOTE use `std::sync::RwLock` instead
+}
diff --git a/tests/ui/sync/suggest-ref-cell.stderr b/tests/ui/sync/suggest-ref-cell.stderr
new file mode 100644
index 0000000000000..9e8b8fcb42ed0
--- /dev/null
+++ b/tests/ui/sync/suggest-ref-cell.stderr
@@ -0,0 +1,17 @@
+error[E0277]: `RefCell<()>` cannot be shared between threads safely
+  --> $DIR/suggest-ref-cell.rs:8:20
+   |
+LL |     require_sync::<std::cell::RefCell<()>>();
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ `RefCell<()>` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `RefCell<()>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
+note: required by a bound in `require_sync`
+  --> $DIR/suggest-ref-cell.rs:3:20
+   |
+LL | fn require_sync<T: Sync>() {}
+   |                    ^^^^ required by this bound in `require_sync`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/type/type-check/coerce-result-return-value-2.rs b/tests/ui/type/type-check/coerce-result-return-value-2.rs
new file mode 100644
index 0000000000000..23bafa6c5c94c
--- /dev/null
+++ b/tests/ui/type/type-check/coerce-result-return-value-2.rs
@@ -0,0 +1,24 @@
+struct A;
+struct B;
+impl From<A> for B {
+    fn from(_: A) -> Self { B }
+}
+fn foo4(x: Result<(), A>) -> Result<(), B> {
+    match true {
+        true => x, //~ ERROR mismatched types
+        false => x,
+    }
+}
+fn foo5(x: Result<(), A>) -> Result<(), B> {
+    match true {
+        true => return x, //~ ERROR mismatched types
+        false => return x,
+    }
+}
+fn main() {
+    let _ = foo4(Ok(()));
+    let _ = foo5(Ok(()));
+    let _: Result<(), B> = { //~ ERROR mismatched types
+        Err(A);
+    };
+}
diff --git a/tests/ui/type/type-check/coerce-result-return-value-2.stderr b/tests/ui/type/type-check/coerce-result-return-value-2.stderr
new file mode 100644
index 0000000000000..5992162341e6e
--- /dev/null
+++ b/tests/ui/type/type-check/coerce-result-return-value-2.stderr
@@ -0,0 +1,47 @@
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value-2.rs:8:17
+   |
+LL | fn foo4(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     match true {
+LL |         true => x,
+   |                 ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |         true => Ok(x?),
+   |                 +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value-2.rs:14:24
+   |
+LL | fn foo5(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     match true {
+LL |         true => return x,
+   |                        ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |         true => return Ok(x?),
+   |                        +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value-2.rs:21:28
+   |
+LL |       let _: Result<(), B> = {
+   |  ____________________________^
+LL | |         Err(A);
+LL | |     };
+   | |_____^ expected enum `Result`, found `()`
+   |
+   = note:   expected enum `Result<(), B>`
+           found unit type `()`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/tests/ui/type/type-check/coerce-result-return-value.fixed b/tests/ui/type/type-check/coerce-result-return-value.fixed
new file mode 100644
index 0000000000000..8a05407070dad
--- /dev/null
+++ b/tests/ui/type/type-check/coerce-result-return-value.fixed
@@ -0,0 +1,24 @@
+// run-rustfix
+struct A;
+struct B;
+impl From<A> for B {
+    fn from(_: A) -> Self { B }
+}
+fn foo1(x: Result<(), A>) -> Result<(), B> {
+    Ok(x?) //~ ERROR mismatched types
+}
+fn foo2(x: Result<(), A>) -> Result<(), B> {
+    return Ok(x?); //~ ERROR mismatched types
+}
+fn foo3(x: Result<(), A>) -> Result<(), B> {
+    if true {
+        Ok(x?) //~ ERROR mismatched types
+    } else {
+        Ok(x?) //~ ERROR mismatched types
+    }
+}
+fn main() {
+    let _ = foo1(Ok(()));
+    let _ = foo2(Ok(()));
+    let _ = foo3(Ok(()));
+}
diff --git a/tests/ui/type/type-check/coerce-result-return-value.rs b/tests/ui/type/type-check/coerce-result-return-value.rs
new file mode 100644
index 0000000000000..442203addb787
--- /dev/null
+++ b/tests/ui/type/type-check/coerce-result-return-value.rs
@@ -0,0 +1,24 @@
+// run-rustfix
+struct A;
+struct B;
+impl From<A> for B {
+    fn from(_: A) -> Self { B }
+}
+fn foo1(x: Result<(), A>) -> Result<(), B> {
+    x //~ ERROR mismatched types
+}
+fn foo2(x: Result<(), A>) -> Result<(), B> {
+    return x; //~ ERROR mismatched types
+}
+fn foo3(x: Result<(), A>) -> Result<(), B> {
+    if true {
+        x //~ ERROR mismatched types
+    } else {
+        x //~ ERROR mismatched types
+    }
+}
+fn main() {
+    let _ = foo1(Ok(()));
+    let _ = foo2(Ok(()));
+    let _ = foo3(Ok(()));
+}
diff --git a/tests/ui/type/type-check/coerce-result-return-value.stderr b/tests/ui/type/type-check/coerce-result-return-value.stderr
new file mode 100644
index 0000000000000..550153520782c
--- /dev/null
+++ b/tests/ui/type/type-check/coerce-result-return-value.stderr
@@ -0,0 +1,65 @@
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value.rs:8:5
+   |
+LL | fn foo1(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     x
+   |     ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |     Ok(x?)
+   |     +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value.rs:11:12
+   |
+LL | fn foo2(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     return x;
+   |            ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |     return Ok(x?);
+   |            +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value.rs:15:9
+   |
+LL | fn foo3(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+LL |     if true {
+LL |         x
+   |         ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |         Ok(x?)
+   |         +++ ++
+
+error[E0308]: mismatched types
+  --> $DIR/coerce-result-return-value.rs:17:9
+   |
+LL | fn foo3(x: Result<(), A>) -> Result<(), B> {
+   |                              ------------- expected `Result<(), B>` because of return type
+...
+LL |         x
+   |         ^ expected struct `B`, found struct `A`
+   |
+   = note: expected enum `Result<_, B>`
+              found enum `Result<_, A>`
+help: use `?` to coerce and return an appropriate `Err`, and wrap the resulting value in `Ok` so the expression remains of type `Result`
+   |
+LL |         Ok(x?)
+   |         +++ ++
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.