Point at non-const trait impl when encountering unmet [const] bound

When encountering an unmet `Ty: [const] Trait` bound, if `Trait` is `#[const_trait]` and there's an `impl Trait for Ty` point at it. If local, suggest `impl const Trait for Ty`, otherwise just point at it. ``` error[E0277]: the trait bound `NonConstAdd: [const] Add` is not satisfied --> $DIR/assoc-type.rs:37:16 | LL | type Bar = NonConstAdd; | ^^^^^^^^^^^ | note: required by a bound in `Foo::Bar` --> $DIR/assoc-type.rs:33:15 | LL | type Bar: [const] Add; | ^^^^^^^^^^^ required by this bound in `Foo::Bar` help: make the `impl` of trait `Add` `const` | LL | impl const Add for NonConstAdd { | +++++ ``` ``` error[E0277]: the trait bound `T: [const] PartialEq` is not satisfied --> tests/ui/traits/const-traits/call-generic-method-fail.rs:5:5 | 5 | *t == *t | ^^^^^^^^ | note: trait `PartialEq` is implemented but not `const` --> /home/gh-estebank/rust/library/core/src/ptr/const_ptr.rs:1590:1 | 1590 | impl<T: PointeeSized> PartialEq for *const T { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ note: trait `PartialEq` is implemented but not `const` --> /home/gh-estebank/rust/library/core/src/ptr/mut_ptr.rs:2011:1 | 2011 | impl<T: PointeeSized> PartialEq for *mut T { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ```

Author: estebank

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -839,6 +839,34 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
  )) {
  diag.downgrade_to_delayed_bug();
  }
+ for candidate in self.find_similar_impl_candidates(trait_ref) {
+ let CandidateSimilarity::Exact { .. } = candidate.similarity else { continue };
+ let impl_did = candidate.impl_def_id;
+ let trait_did = candidate.trait_ref.def_id;
+ let impl_span = self.tcx.def_span(impl_did);
+ let trait_name = self.tcx.item_name(trait_did);
+
+ if self.tcx.is_const_trait(trait_did) && !self.tcx.is_const_trait_impl(impl_did) {
+ if let Some(impl_did) = impl_did.as_local()
+ && let item = self.tcx.hir_expect_item(impl_did)
+ && let hir::ItemKind::Impl(item) = item.kind
+ && let Some(of_trait) = item.of_trait
+ {
+ // trait is const, impl is local and not const
+ diag.span_suggestion_verbose(
+ of_trait.trait_ref.path.span.shrink_to_lo(),
+ format!("make the `impl` of trait `{trait_name}` `const`"),
+ "const ".to_string(),
+ Applicability::MaybeIncorrect,
+ );
+ } else {
+ diag.span_note(
+ impl_span,
+ format!("trait `{trait_name}` is implemented but not `const`"),
+ );
+ }
+ }
+ }
  diag
  }
 

tests/ui/consts/const_cmp_type_id.stderr

@@ -3,6 +3,9 @@ error[E0277]: the trait bound `TypeId: const PartialOrd` is not satisfied
  |
 LL | let _a = TypeId::of::<u8>() < TypeId::of::<u16>();
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+ |
+note: trait `PartialOrd` is implemented but not `const`
+ --> $SRC_DIR/core/src/any.rs:LL:COL
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/assoc-type.current.stderr

@@ -9,6 +9,10 @@ note: required by a bound in `Foo::Bar`
  |
 LL | type Bar: [const] Add;
  | ^^^^^^^^^^^ required by this bound in `Foo::Bar`
+help: make the `impl` of trait `Add` `const`
+ |
+LL | impl const Add for NonConstAdd {
+ | +++++
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/assoc-type.next.stderr

@@ -9,6 +9,10 @@ note: required by a bound in `Foo::Bar`
  |
 LL | type Bar: [const] Add;
  | ^^^^^^^^^^^ required by this bound in `Foo::Bar`
+help: make the `impl` of trait `Add` `const`
+ |
+LL | impl const Add for NonConstAdd {
+ | +++++
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/call-const-closure.stderr

@@ -3,6 +3,11 @@ error[E0277]: the trait bound `(): [const] Bar` is not satisfied
  |
 LL | (const || ().foo())();
  | ^^^
+ |
+help: make the `impl` of trait `Bar` `const`
+ |
+LL | impl const Bar for () {
+ | +++++
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/call-const-trait-method-fail.stderr

@@ -3,6 +3,11 @@ error[E0277]: the trait bound `u32: [const] Plus` is not satisfied
  |
 LL | a.plus(b)
  | ^
+ |
+help: make the `impl` of trait `Plus` `const`
+ |
+LL | impl const Plus for u32 {
+ | +++++
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/call-generic-method-fail.stderr

@@ -3,6 +3,11 @@ error[E0277]: the trait bound `T: [const] PartialEq` is not satisfied
  |
 LL | *t == *t
  | ^^^^^^^^
+ |
+note: trait `PartialEq` is implemented but not `const`
+ --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+note: trait `PartialEq` is implemented but not `const`
+ --> $SRC_DIR/core/src/ptr/mut_ptr.rs:LL:COL
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/call-generic-method-nonconst.stderr

@@ -11,6 +11,10 @@ note: required by a bound in `equals_self`
  |
 LL | const fn equals_self<T: [const] Foo>(t: &T) -> bool {
  | ^^^^^^^^^^^ required by this bound in `equals_self`
+help: make the `impl` of trait `Foo` `const`
+ |
+LL | impl const Foo for S {
+ | +++++
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/const-closure-trait-method-fail.stderr

@@ -3,6 +3,11 @@ error[E0277]: the trait bound `(): const Tr` is not satisfied
  |
 LL | const _: () = assert!(need_const_closure(Tr::a) == 42);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
+ |
+help: make the `impl` of trait `Tr` `const`
+ |
+LL | impl const Tr for () {
+ | +++++
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/const-default-method-bodies.stderr

@@ -3,6 +3,11 @@ error[E0277]: the trait bound `NonConstImpl: [const] ConstDefaultFn` is not sati
  |
 LL | NonConstImpl.a();
  | ^
+ |
+help: make the `impl` of trait `ConstDefaultFn` `const`
+ |
+LL | impl const ConstDefaultFn for NonConstImpl {
+ | +++++
 
 error: aborting due to 1 previous error