Disentangle ForwardGenericParamBan and ConstParamTy ribs

compiler/rustc_resolve/messages.ftl

@@ -153,9 +153,13 @@ resolve_extern_crate_self_requires_renaming =
     `extern crate self;` requires renaming
     .suggestion = rename the `self` crate to be able to import it
 
+resolve_forward_declared_generic_in_const_param_ty =
+    const parameter types cannot reference parameters before they are declared
+    .label = const parameter type cannot reference `{$param}` before it is declared
+
 resolve_forward_declared_generic_param =
-    generic parameters with a default cannot use forward declared identifiers
-    .label = defaulted generic parameters cannot be forward declared
+    generic parameter defaults cannot reference parameters before they are declared
+    .label = cannot reference `{$param}` before it is declared
 
 resolve_found_an_item_configured_out =
     found an item that was configured out
@@ -377,9 +381,11 @@ resolve_self_imports_only_allowed_within_multipart_suggestion =
 resolve_self_imports_only_allowed_within_suggestion =
     consider importing the module directly
 
+resolve_self_in_const_generic_ty =
+    cannot use `Self` in const parameter type
+
 resolve_self_in_generic_param_default =
     generic parameters cannot use `Self` in their defaults
-    .label = `Self` in generic parameter default
 
 resolve_similarly_named_defined_here =
     similarly named {$candidate_descr} `{$candidate}` defined here

compiler/rustc_resolve/src/diagnostics.rs

@@ -42,10 +42,10 @@ use crate::imports::{Import, ImportKind};
 use crate::late::{PatternSource, Rib};
 use crate::{
     AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, BindingError, BindingKey, Finalize,
-    HasGenericParams, LexicalScopeBinding, MacroRulesScope, Module, ModuleKind,
-    ModuleOrUniformRoot, NameBinding, NameBindingKind, ParentScope, PathResult, PrivacyError,
-    ResolutionError, Resolver, Scope, ScopeSet, Segment, UseError, Used, VisResolutionError,
-    errors as errs, path_names_to_string,
+    ForwardGenericParamBanReason, HasGenericParams, LexicalScopeBinding, MacroRulesScope, Module,
+    ModuleKind, ModuleOrUniformRoot, NameBinding, NameBindingKind, ParentScope, PathResult,
+    PrivacyError, ResolutionError, Resolver, Scope, ScopeSet, Segment, UseError, Used,
+    VisResolutionError, errors as errs, path_names_to_string,
 };
 
 type Res = def::Res<ast::NodeId>;
@@ -887,9 +887,14 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 participle,
                 name,
             }),
-            ResolutionError::ForwardDeclaredGenericParam => {
-                self.dcx().create_err(errs::ForwardDeclaredGenericParam { span })
-            }
+            ResolutionError::ForwardDeclaredGenericParam(param, reason) => match reason {
+                ForwardGenericParamBanReason::Default => {
+                    self.dcx().create_err(errs::ForwardDeclaredGenericParam { param, span })
+                }
+                ForwardGenericParamBanReason::ConstParamTy => self
+                    .dcx()
+                    .create_err(errs::ForwardDeclaredGenericInConstParamTy { param, span }),
+            },
             ResolutionError::ParamInTyOfConstParam { name } => {
                 self.dcx().create_err(errs::ParamInTyOfConstParam { span, name })
             }
@@ -908,9 +913,14 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             ResolutionError::ParamInEnumDiscriminant { name, param_kind: is_type } => self
                 .dcx()
                 .create_err(errs::ParamInEnumDiscriminant { span, name, param_kind: is_type }),
-            ResolutionError::SelfInGenericParamDefault => {
-                self.dcx().create_err(errs::SelfInGenericParamDefault { span })
-            }
+            ResolutionError::ForwardDeclaredSelf(reason) => match reason {
+                ForwardGenericParamBanReason::Default => {
+                    self.dcx().create_err(errs::SelfInGenericParamDefault { span })
+                }
+                ForwardGenericParamBanReason::ConstParamTy => {
+                    self.dcx().create_err(errs::SelfInConstGenericTy { span })
+                }
+            },
             ResolutionError::UnreachableLabel { name, definition_span, suggestion } => {
                 let ((sub_suggestion_label, sub_suggestion), sub_unreachable_label) =
                     match suggestion {

compiler/rustc_resolve/src/errors.rs

@@ -338,6 +338,16 @@ pub(crate) struct ForwardDeclaredGenericParam {
     #[primary_span]
     #[label]
     pub(crate) span: Span,
+    pub(crate) param: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(resolve_forward_declared_generic_in_const_param_ty)]
+pub(crate) struct ForwardDeclaredGenericInConstParamTy {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    pub(crate) param: Symbol,
 }
 
 #[derive(Diagnostic)]
@@ -353,7 +363,13 @@ pub(crate) struct ParamInTyOfConstParam {
 #[diag(resolve_self_in_generic_param_default, code = E0735)]
 pub(crate) struct SelfInGenericParamDefault {
     #[primary_span]
-    #[label]
+    pub(crate) span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(resolve_self_in_const_generic_ty)]
+pub(crate) struct SelfInConstGenericTy {
+    #[primary_span]
     pub(crate) span: Span,
 }
 

compiler/rustc_resolve/src/ident.rs

@@ -1117,31 +1117,21 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         debug!("validate_res_from_ribs({:?})", res);
         let ribs = &all_ribs[rib_index + 1..];
 
-        // An invalid forward use of a generic parameter from a previous default.
-        if let RibKind::ForwardGenericParamBan = all_ribs[rib_index].kind {
+        // An invalid forward use of a generic parameter from a previous default
+        // or in a const param ty.
+        if let RibKind::ForwardGenericParamBan(reason) = all_ribs[rib_index].kind {
             if let Some(span) = finalize {
                 let res_error = if rib_ident.name == kw::SelfUpper {
-                    ResolutionError::SelfInGenericParamDefault
+                    ResolutionError::ForwardDeclaredSelf(reason)
                 } else {
-                    ResolutionError::ForwardDeclaredGenericParam
+                    ResolutionError::ForwardDeclaredGenericParam(rib_ident.name, reason)
                 };
                 self.report_error(span, res_error);
             }
             assert_eq!(res, Res::Err);
             return Res::Err;
         }
 
-        if let RibKind::ConstParamTy = all_ribs[rib_index].kind {
-            if let Some(span) = finalize {
-                self.report_error(
-                    span,
-                    ResolutionError::ParamInTyOfConstParam { name: rib_ident.name },
-                );
-            }
-            assert_eq!(res, Res::Err);
-            return Res::Err;
-        }
-
         match res {
             Res::Local(_) => {
                 use ResolutionError::*;
@@ -1153,7 +1143,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         | RibKind::FnOrCoroutine
                         | RibKind::Module(..)
                         | RibKind::MacroDefinition(..)
-                        | RibKind::ForwardGenericParamBan => {
+                        | RibKind::ForwardGenericParamBan(_) => {
                             // Nothing to do. Continue.
                         }
                         RibKind::Item(..) | RibKind::AssocItem => {
@@ -1247,12 +1237,27 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         | RibKind::MacroDefinition(..)
                         | RibKind::InlineAsmSym
                         | RibKind::AssocItem
-                        | RibKind::ConstParamTy
-                        | RibKind::ForwardGenericParamBan => {
+                        | RibKind::ForwardGenericParamBan(_) => {
                             // Nothing to do. Continue.
                             continue;
                         }
 
+                        RibKind::ConstParamTy => {
+                            if !self.tcx.features().generic_const_parameter_types() {
+                                if let Some(span) = finalize {
+                                    self.report_error(
+                                        span,
+                                        ResolutionError::ParamInTyOfConstParam {
+                                            name: rib_ident.name,
+                                        },
+                                    );
+                                }
+                                return Res::Err;
+                            } else {
+                                continue;
+                            }
+                        }
+
                         RibKind::ConstantItem(trivial, _) => {
                             if let ConstantHasGenerics::No(cause) = trivial {
                                 // HACK(min_const_generics): If we encounter `Self` in an anonymous
@@ -1325,8 +1330,23 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         | RibKind::MacroDefinition(..)
                         | RibKind::InlineAsmSym
                         | RibKind::AssocItem
-                        | RibKind::ConstParamTy
-                        | RibKind::ForwardGenericParamBan => continue,
+                        | RibKind::ForwardGenericParamBan(_) => continue,
+
+                        RibKind::ConstParamTy => {
+                            if !self.tcx.features().generic_const_parameter_types() {
+                                if let Some(span) = finalize {
+                                    self.report_error(
+                                        span,
+                                        ResolutionError::ParamInTyOfConstParam {
+                                            name: rib_ident.name,
+                                        },
+                                    );
+                                }
+                                return Res::Err;
+                            } else {
+                                continue;
+                            }
+                        }
 
                         RibKind::ConstantItem(trivial, _) => {
                             if let ConstantHasGenerics::No(cause) = trivial {
@@ -1377,6 +1397,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             }
             _ => {}
         }
+
         res
     }
 

compiler/rustc_resolve/src/late.rs

@@ -207,7 +207,7 @@ pub(crate) enum RibKind<'ra> {
     /// All bindings in this rib are generic parameters that can't be used
     /// from the default of a generic parameter because they're not declared
     /// before said generic parameter. Also see the `visit_generics` override.
-    ForwardGenericParamBan,
+    ForwardGenericParamBan(ForwardGenericParamBanReason),
 
     /// We are inside of the type of a const parameter. Can't refer to any
     /// parameters.
@@ -218,6 +218,12 @@ pub(crate) enum RibKind<'ra> {
     InlineAsmSym,
 }
 
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub(crate) enum ForwardGenericParamBanReason {
+    Default,
+    ConstParamTy,
+}
+
 impl RibKind<'_> {
     /// Whether this rib kind contains generic parameters, as opposed to local
     /// variables.
@@ -232,7 +238,7 @@ impl RibKind<'_> {
             RibKind::ConstParamTy
             | RibKind::AssocItem
             | RibKind::Item(..)
-            | RibKind::ForwardGenericParamBan => true,
+            | RibKind::ForwardGenericParamBan(_) => true,
         }
     }
 
@@ -246,7 +252,7 @@ impl RibKind<'_> {
             | RibKind::Item(..)
             | RibKind::ConstantItem(..)
             | RibKind::Module(..)
-            | RibKind::ForwardGenericParamBan
+            | RibKind::ForwardGenericParamBan(_)
             | RibKind::ConstParamTy
             | RibKind::InlineAsmSym => true,
         }
@@ -1561,8 +1567,10 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         // provide previous type parameters as they're built. We
         // put all the parameters on the ban list and then remove
         // them one by one as they are processed and become available.
-        let mut forward_ty_ban_rib = Rib::new(RibKind::ForwardGenericParamBan);
-        let mut forward_const_ban_rib = Rib::new(RibKind::ForwardGenericParamBan);
+        let mut forward_ty_ban_rib =
+            Rib::new(RibKind::ForwardGenericParamBan(ForwardGenericParamBanReason::Default));
+        let mut forward_const_ban_rib =
+            Rib::new(RibKind::ForwardGenericParamBan(ForwardGenericParamBanReason::Default));
         for param in params.iter() {
             match param.kind {
                 GenericParamKind::Type { .. } => {
@@ -1593,16 +1601,24 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             forward_ty_ban_rib.bindings.insert(Ident::with_dummy_span(kw::SelfUpper), Res::Err);
         }
 
+        // NOTE: We use different ribs here not for a technical reason, but just
+        // for better diagnostics.
         let mut forward_ty_ban_rib_const_param_ty = Rib {
             bindings: forward_ty_ban_rib.bindings.clone(),
             patterns_with_skipped_bindings: FxHashMap::default(),
-            kind: RibKind::ConstParamTy,
+            kind: RibKind::ForwardGenericParamBan(ForwardGenericParamBanReason::ConstParamTy),
         };
         let mut forward_const_ban_rib_const_param_ty = Rib {
             bindings: forward_const_ban_rib.bindings.clone(),
             patterns_with_skipped_bindings: FxHashMap::default(),
-            kind: RibKind::ConstParamTy,
+            kind: RibKind::ForwardGenericParamBan(ForwardGenericParamBanReason::ConstParamTy),
         };
+        // We'll ban these with a `ConstParamTy` rib, so just clear these ribs for better
+        // diagnostics, so we don't mention anything about const param tys having generics at all.
+        if !self.r.tcx.features().generic_const_parameter_types() {
+            forward_ty_ban_rib_const_param_ty.bindings.clear();
+            forward_const_ban_rib_const_param_ty.bindings.clear();
+        }
 
         self.with_lifetime_rib(LifetimeRibKind::AnonymousReportError, |this| {
             for param in params {
@@ -1628,9 +1644,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                         // Allow all following defaults to refer to this type parameter.
                         let i = &Ident::with_dummy_span(param.ident.name);
                         forward_ty_ban_rib.bindings.remove(i);
-                        if this.r.tcx.features().generic_const_parameter_types() {
-                            forward_ty_ban_rib_const_param_ty.bindings.remove(i);
-                        }
+                        forward_ty_ban_rib_const_param_ty.bindings.remove(i);
                     }
                     GenericParamKind::Const { ref ty, kw_span: _, ref default } => {
                         // Const parameters can't have param bounds.
@@ -1641,9 +1655,13 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                         if this.r.tcx.features().generic_const_parameter_types() {
                             this.visit_ty(ty)
                         } else {
+                            this.ribs[TypeNS].push(Rib::new(RibKind::ConstParamTy));
+                            this.ribs[ValueNS].push(Rib::new(RibKind::ConstParamTy));
                             this.with_lifetime_rib(LifetimeRibKind::ConstParamTy, |this| {
                                 this.visit_ty(ty)
                             });
+                            this.ribs[TypeNS].pop().unwrap();
+                            this.ribs[ValueNS].pop().unwrap();
                         }
                         forward_const_ban_rib_const_param_ty = this.ribs[ValueNS].pop().unwrap();
                         forward_ty_ban_rib_const_param_ty = this.ribs[TypeNS].pop().unwrap();
@@ -1662,9 +1680,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                         // Allow all following defaults to refer to this const parameter.
                         let i = &Ident::with_dummy_span(param.ident.name);
                         forward_const_ban_rib.bindings.remove(i);
-                        if this.r.tcx.features().generic_const_parameter_types() {
-                            forward_const_ban_rib_const_param_ty.bindings.remove(i);
-                        }
+                        forward_const_ban_rib_const_param_ty.bindings.remove(i);
                     }
                 }
             }

compiler/rustc_resolve/src/lib.rs

@@ -32,7 +32,10 @@ use diagnostics::{ImportSuggestion, LabelSuggestion, Suggestion};
 use effective_visibilities::EffectiveVisibilitiesVisitor;
 use errors::{ParamKindInEnumDiscriminant, ParamKindInNonTrivialAnonConst};
 use imports::{Import, ImportData, ImportKind, NameResolution};
-use late::{HasGenericParams, PathSource, PatternSource, UnnecessaryQualification};
+use late::{
+    ForwardGenericParamBanReason, HasGenericParams, PathSource, PatternSource,
+    UnnecessaryQualification,
+};
 use macros::{MacroRulesBinding, MacroRulesScope, MacroRulesScopeRef};
 use rustc_arena::{DroplessArena, TypedArena};
 use rustc_ast::expand::StrippedCfgItem;
@@ -273,7 +276,7 @@ enum ResolutionError<'ra> {
         shadowed_binding_span: Span,
     },
     /// Error E0128: generic parameters with a default cannot use forward-declared identifiers.
-    ForwardDeclaredGenericParam,
+    ForwardDeclaredGenericParam(Symbol, ForwardGenericParamBanReason),
     // FIXME(generic_const_parameter_types): This should give custom output specifying it's only
     // problematic to use *forward declared* parameters when the feature is enabled.
     /// ERROR E0770: the type of const parameters must not depend on other generic parameters.
@@ -287,7 +290,7 @@ enum ResolutionError<'ra> {
     /// This error is emitted even with `generic_const_exprs`.
     ParamInEnumDiscriminant { name: Symbol, param_kind: ParamKindInEnumDiscriminant },
     /// Error E0735: generic parameters with a default cannot use `Self`
-    SelfInGenericParamDefault,
+    ForwardDeclaredSelf(ForwardGenericParamBanReason),
     /// Error E0767: use of unreachable label
     UnreachableLabel { name: Symbol, definition_span: Span, suggestion: Option<LabelSuggestion> },
     /// Error E0323, E0324, E0325: mismatch between trait item and impl item.

tests/ui/const-generics/const-param-type-depends-on-const-param.rs

@@ -9,11 +9,11 @@
 // We may want to lift this restriction in the future.
 
 pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
-//~^ ERROR: the type of const parameters must not depend on other generic parameters
-//[min]~^^ ERROR `[u8; N]` is forbidden
+//~^ ERROR the type of const parameters must not depend on other generic parameters
+//[min]~^^ ERROR `[u8; N]` is forbidden as the type of a const generic parameter
 
 pub struct SelfDependent<const N: [u8; N]>;
-//~^ ERROR: the type of const parameters must not depend on other generic parameters
-//[min]~^^ ERROR `[u8; N]` is forbidden
+//~^ ERROR the type of const parameters must not depend on other generic parameters
+//[min]~^^ ERROR `[u8; N]` is forbidden as the type of a const generic parameter
 
 fn main() {}

tests/ui/const-generics/const-param-type-depends-on-parent-param.rs

@@ -0,0 +1,8 @@
+#![feature(adt_const_params)]
+
+trait Trait<const N: usize> {
+    fn foo<const M: [u8; N]>() {}
+    //~^ ERROR the type of const parameters must not depend on other generic parameters
+}
+
+fn main() {}