Reimplement ~const trait specialization

compiler/rustc_hir_analysis/messages.ftl

@@ -113,8 +113,6 @@ hir_analysis_const_param_ty_impl_on_unsized =
     the trait `ConstParamTy` may not be implemented for this type
     .label = type is not `Sized`
 
-hir_analysis_const_specialize = cannot specialize on const impl with non-const impl
-
 hir_analysis_copy_impl_on_non_adt =
     the trait `Copy` cannot be implemented for this type
     .label = type is not a structure or enumeration

compiler/rustc_hir_analysis/src/errors.rs

@@ -1079,13 +1079,6 @@ pub(crate) struct EmptySpecialization {
     pub base_impl_span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(hir_analysis_const_specialize)]
-pub(crate) struct ConstSpecialize {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(hir_analysis_static_specialize)]
 pub(crate) struct StaticSpecialize {

compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs

@@ -66,7 +66,6 @@
 //! on traits with methods can.
 
 use rustc_data_structures::fx::FxHashSet;
-use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
@@ -134,7 +133,6 @@ fn check_always_applicable(
         unconstrained_parent_impl_args(tcx, impl2_def_id, impl2_args)
     };
 
-    res = res.and(check_constness(tcx, impl1_def_id, impl2_node, span));
     res = res.and(check_static_lifetimes(tcx, &parent_args, span));
     res = res.and(check_duplicate_params(tcx, impl1_args, parent_args, span));
     res = res.and(check_predicates(tcx, impl1_def_id, impl1_args, impl2_node, impl2_args, span));
@@ -157,30 +155,6 @@ fn check_has_items(
     Ok(())
 }
 
-/// Check that the specializing impl `impl1` is at least as const as the base
-/// impl `impl2`
-fn check_constness(
-    tcx: TyCtxt<'_>,
-    impl1_def_id: LocalDefId,
-    impl2_node: Node,
-    span: Span,
-) -> Result<(), ErrorGuaranteed> {
-    if impl2_node.is_from_trait() {
-        // This isn't a specialization
-        return Ok(());
-    }
-
-    let impl1_constness = tcx.constness(impl1_def_id.to_def_id());
-    let impl2_constness = tcx.constness(impl2_node.def_id());
-
-    if let hir::Constness::Const = impl2_constness {
-        if let hir::Constness::NotConst = impl1_constness {
-            return Err(tcx.dcx().emit_err(errors::ConstSpecialize { span }));
-        }
-    }
-    Ok(())
-}
-
 /// Given a specializing impl `impl1`, and the base impl `impl2`, returns two
 /// generic parameters `(S1, S2)` that equate their trait references.
 /// The returned types are expressed in terms of the generics of `impl1`.

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -15,6 +15,7 @@ use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::codes::*;
 use rustc_errors::{Diag, EmissionGuarantee};
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_infer::traits::Obligation;
 use rustc_middle::bug;
 use rustc_middle::query::LocalCrate;
 use rustc_middle::ty::print::PrintTraitRefExt as _;
@@ -224,29 +225,38 @@ pub(super) fn specialization_enabled_in(tcx: TyCtxt<'_>, _: LocalCrate) -> bool
     tcx.features().specialization() || tcx.features().min_specialization()
 }
 
-/// Is `impl1` a specialization of `impl2`?
+/// Is `specializing_impl_def_id` a specialization of `parent_impl_def_id`?
 ///
-/// Specialization is determined by the sets of types to which the impls apply;
-/// `impl1` specializes `impl2` if it applies to a subset of the types `impl2` applies
-/// to.
+/// For every type that could apply to `specializing_impl_def_id`, we prove that
+/// the `parent_impl_def_id` also applies (i.e. it has a valid impl header and
+/// its where-clauses hold).
+///
+/// For the purposes of const traits, we also check that the specializing
+/// impl is not more restrictive than the parent impl. That is, if the
+/// `parent_impl_def_id` is a const impl (conditionally based off of some `~const`
+/// bounds), then `specializing_impl_def_id` must also be const for the same
+/// set of types.
 #[instrument(skip(tcx), level = "debug")]
-pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId, DefId)) -> bool {
+pub(super) fn specializes(
+    tcx: TyCtxt<'_>,
+    (specializing_impl_def_id, parent_impl_def_id): (DefId, DefId),
+) -> bool {
     // We check that the specializing impl comes from a crate that has specialization enabled,
     // or if the specializing impl is marked with `allow_internal_unstable`.
     //
     // We don't really care if the specialized impl (the parent) is in a crate that has
     // specialization enabled, since it's not being specialized, and it's already been checked
     // for coherence.
-    if !tcx.specialization_enabled_in(impl1_def_id.krate) {
-        let span = tcx.def_span(impl1_def_id);
+    if !tcx.specialization_enabled_in(specializing_impl_def_id.krate) {
+        let span = tcx.def_span(specializing_impl_def_id);
         if !span.allows_unstable(sym::specialization)
             && !span.allows_unstable(sym::min_specialization)
         {
             return false;
         }
     }
 
-    let impl1_trait_header = tcx.impl_trait_header(impl1_def_id).unwrap();
+    let specializing_impl_trait_header = tcx.impl_trait_header(specializing_impl_def_id).unwrap();
 
     // We determine whether there's a subset relationship by:
     //
@@ -261,27 +271,123 @@ pub(super) fn specializes(tcx: TyCtxt<'_>, (impl1_def_id, impl2_def_id): (DefId,
     // See RFC 1210 for more details and justification.
 
     // Currently we do not allow e.g., a negative impl to specialize a positive one
-    if impl1_trait_header.polarity != tcx.impl_polarity(impl2_def_id) {
+    if specializing_impl_trait_header.polarity != tcx.impl_polarity(parent_impl_def_id) {
         return false;
     }
 
-    // create a parameter environment corresponding to an identity instantiation of impl1,
-    // i.e. the most generic instantiation of impl1.
-    let param_env = tcx.param_env(impl1_def_id);
+    // create a parameter environment corresponding to an identity instantiation of the specializing impl,
+    // i.e. the most generic instantiation of the specializing impl.
+    let param_env = tcx.param_env(specializing_impl_def_id);
 
-    // Create an infcx, taking the predicates of impl1 as assumptions:
+    // Create an infcx, taking the predicates of the specializing impl as assumptions:
     let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
 
-    // Attempt to prove that impl2 applies, given all of the above.
-    fulfill_implication(
-        &infcx,
+    let specializing_impl_trait_ref =
+        specializing_impl_trait_header.trait_ref.instantiate_identity();
+    let cause = &ObligationCause::dummy();
+    debug!(
+        "fulfill_implication({:?}, trait_ref={:?} |- {:?} applies)",
+        param_env, specializing_impl_trait_ref, parent_impl_def_id
+    );
+
+    // Attempt to prove that the parent impl applies, given all of the above.
+
+    let ocx = ObligationCtxt::new(&infcx);
+    let specializing_impl_trait_ref = ocx.normalize(cause, param_env, specializing_impl_trait_ref);
+
+    if !ocx.select_all_or_error().is_empty() {
+        infcx.dcx().span_delayed_bug(
+            infcx.tcx.def_span(specializing_impl_def_id),
+            format!("failed to fully normalize {specializing_impl_trait_ref}"),
+        );
+        return false;
+    }
+
+    let parent_args = infcx.fresh_args_for_item(DUMMY_SP, parent_impl_def_id);
+    let parent_impl_trait_ref = ocx.normalize(
+        cause,
         param_env,
-        impl1_trait_header.trait_ref.instantiate_identity(),
-        impl1_def_id,
-        impl2_def_id,
-        &ObligationCause::dummy(),
-    )
-    .is_ok()
+        infcx
+            .tcx
+            .impl_trait_ref(parent_impl_def_id)
+            .expect("expected source impl to be a trait impl")
+            .instantiate(infcx.tcx, parent_args),
+    );
+
+    // do the impls unify? If not, no specialization.
+    let Ok(()) = ocx.eq(cause, param_env, specializing_impl_trait_ref, parent_impl_trait_ref)
+    else {
+        return false;
+    };
+
+    // Now check that the source trait ref satisfies all the where clauses of the target impl.
+    // This is not just for correctness; we also need this to constrain any params that may
+    // only be referenced via projection predicates.
+    let predicates = ocx.normalize(
+        cause,
+        param_env,
+        infcx.tcx.predicates_of(parent_impl_def_id).instantiate(infcx.tcx, parent_args),
+    );
+    let obligations = predicates_for_generics(|_, _| cause.clone(), param_env, predicates);
+    ocx.register_obligations(obligations);
+
+    let errors = ocx.select_all_or_error();
+    if !errors.is_empty() {
+        // no dice!
+        debug!(
+            "fulfill_implication: for impls on {:?} and {:?}, \
+                 could not fulfill: {:?} given {:?}",
+            specializing_impl_trait_ref,
+            parent_impl_trait_ref,
+            errors,
+            param_env.caller_bounds()
+        );
+        return false;
+    }
+
+    // If the parent impl is const, then the specializing impl must be const,
+    // and it must not be *more restrictive* than the parent impl (that is,
+    // it cannot be const in fewer cases than the parent impl).
+    if tcx.is_conditionally_const(parent_impl_def_id) {
+        if !tcx.is_conditionally_const(specializing_impl_def_id) {
+            return false;
+        }
+
+        let const_conditions = ocx.normalize(
+            cause,
+            param_env,
+            infcx.tcx.const_conditions(parent_impl_def_id).instantiate(infcx.tcx, parent_args),
+        );
+        ocx.register_obligations(const_conditions.into_iter().map(|(trait_ref, _)| {
+            Obligation::new(
+                infcx.tcx,
+                cause.clone(),
+                param_env,
+                trait_ref.to_host_effect_clause(infcx.tcx, ty::BoundConstness::Maybe),
+            )
+        }));
+
+        let errors = ocx.select_all_or_error();
+        if !errors.is_empty() {
+            // no dice!
+            debug!(
+                "fulfill_implication: for impls on {:?} and {:?}, \
+                 could not fulfill: {:?} given {:?}",
+                specializing_impl_trait_ref,
+                parent_impl_trait_ref,
+                errors,
+                param_env.caller_bounds()
+            );
+            return false;
+        }
+    }
+
+    debug!(
+        "fulfill_implication: an impl for {:?} specializes {:?}",
+        specializing_impl_trait_ref, parent_impl_trait_ref
+    );
+
+    true
 }
 
 /// Query provider for `specialization_graph_of`.

tests/ui/traits/const-traits/overlap-const-with-nonconst.min_spec.stderr

@@ -0,0 +1,14 @@
+error[E0119]: conflicting implementations of trait `Foo` for type `(_,)`
+  --> $DIR/overlap-const-with-nonconst.rs:23:1
+   |
+LL | / impl<T> const Foo for T
+LL | | where
+LL | |     T: ~const Bar,
+   | |__________________- first implementation here
+...
+LL |   impl<T> Foo for (T,) {
+   |   ^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(_,)`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/traits/const-traits/overlap-const-with-nonconst.rs

@@ -0,0 +1,38 @@
+//@ revisions: spec min_spec
+
+#![feature(const_trait_impl)]
+#![cfg_attr(spec, feature(specialization))]
+//[spec]~^ WARN the feature `specialization` is incomplete
+#![cfg_attr(min_spec, feature(min_specialization))]
+
+#[const_trait]
+trait Bar {}
+impl<T> const Bar for T {}
+
+#[const_trait]
+trait Foo {
+    fn method(&self);
+}
+impl<T> const Foo for T
+where
+    T: ~const Bar,
+{
+    default fn method(&self) {}
+}
+// specializing impl:
+impl<T> Foo for (T,) {
+//~^ ERROR conflicting implementations
+    fn method(&self) {
+        println!("hi");
+    }
+}
+
+const fn dispatch<T: ~const Bar + Copy>(t: T) {
+    t.method();
+}
+
+fn main() {
+    const {
+        dispatch(((),));
+    }
+}

tests/ui/traits/const-traits/overlap-const-with-nonconst.spec.stderr

@@ -0,0 +1,24 @@
+warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/overlap-const-with-nonconst.rs:4:27
+   |
+LL | #![cfg_attr(spec, feature(specialization))]
+   |                           ^^^^^^^^^^^^^^
+   |
+   = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
+   = help: consider using `min_specialization` instead, which is more stable and complete
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0119]: conflicting implementations of trait `Foo` for type `(_,)`
+  --> $DIR/overlap-const-with-nonconst.rs:23:1
+   |
+LL | / impl<T> const Foo for T
+LL | | where
+LL | |     T: ~const Bar,
+   | |__________________- first implementation here
+...
+LL |   impl<T> Foo for (T,) {
+   |   ^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(_,)`
+
+error: aborting due to 1 previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/traits/const-traits/specialization/const-default-bound-non-const-specialized-bound.stderr

@@ -1,11 +1,31 @@
-error: cannot specialize on const impl with non-const impl
+error[E0119]: conflicting implementations of trait `Bar`
   --> $DIR/const-default-bound-non-const-specialized-bound.rs:28:1
    |
+LL | / impl<T> const Bar for T
+LL | | where
+LL | |     T: ~const Foo,
+   | |__________________- first implementation here
+...
 LL | / impl<T> Bar for T
 LL | | where
 LL | |     T: Foo, //FIXME ~ ERROR missing `~const` qualifier
 LL | |     T: Specialize,
-   | |__________________^
+   | |__________________^ conflicting implementation
+
+error[E0119]: conflicting implementations of trait `Baz`
+  --> $DIR/const-default-bound-non-const-specialized-bound.rs:48:1
+   |
+LL | / impl<T> const Baz for T
+LL | | where
+LL | |     T: ~const Foo,
+   | |__________________- first implementation here
+...
+LL | / impl<T> const Baz for T //FIXME ~ ERROR conflicting implementations of trait `Baz`
+LL | | where
+LL | |     T: Foo,
+LL | |     T: Specialize,
+   | |__________________^ conflicting implementation
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0119`.

tests/ui/traits/const-traits/specialization/const-default-impl-non-const-specialized-impl.min_spec.stderr

@@ -0,0 +1,12 @@
+error[E0119]: conflicting implementations of trait `Value` for type `FortyTwo`
+  --> $DIR/const-default-impl-non-const-specialized-impl.rs:22:1
+   |
+LL | impl<T> const Value for T {
+   | ------------------------- first implementation here
+...
+LL | impl Value for FortyTwo {
+   | ^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `FortyTwo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.