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

estebank · estebank · commit 50d116814517 · 2025-07-18T23:31:56.000Z
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
  --&gt; $DIR/assoc-type.rs:37:16
   |
LL |     type Bar = NonConstAdd;
   |                ^^^^^^^^^^^
   |
note: required by a bound in `Foo::Bar`
  --&gt; $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
    --&gt; tests/ui/traits/const-traits/call-generic-method-fail.rs:5:5
     |
5    |     *t == *t
     |     ^^^^^^^^
     |
note: trait `PartialEq` is implemented but not `const`
    --&gt; /home/gh-estebank/rust/library/core/src/ptr/const_ptr.rs:1590:1
     |
1590 | impl&lt;T: PointeeSized&gt; PartialEq for *const T {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: trait `PartialEq` is implemented but not `const`
    --&gt; /home/gh-estebank/rust/library/core/src/ptr/mut_ptr.rs:2011:1
     |
2011 | impl&lt;T: PointeeSized&gt; PartialEq for *mut T {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
@@ -806,6 +806,34 @@ 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.path.span.shrink_to_lo(),
+                        format!("make the `impl` of trait `{trait_name}` `const`"),
+                        "const ".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                } else {
+                    diag.span_note(
+                        impl_span,
+                        format!("trait `{trait_name}` is implemented but not `const`"),
+                    );
+                }
+            }
+        }
         diag
     }
 
diff --git a/tests/ui/traits/const-traits/assoc-type.current.stderr b/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
 
diff --git a/tests/ui/traits/const-traits/assoc-type.next.stderr b/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
 
diff --git a/tests/ui/traits/const-traits/call-const-closure.stderr b/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
 
diff --git a/tests/ui/traits/const-traits/call-const-trait-method-fail.stderr b/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
 
diff --git a/tests/ui/traits/const-traits/call-generic-method-fail.stderr b/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
 
diff --git a/tests/ui/traits/const-traits/call-generic-method-nonconst.stderr b/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
 
diff --git a/tests/ui/traits/const-traits/const-closure-trait-method-fail.stderr b/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
 
diff --git a/tests/ui/traits/const-traits/const-default-method-bodies.stderr b/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
 
diff --git a/tests/ui/traits/const-traits/const-drop-fail-2.precise.stderr b/tests/ui/traits/const-traits/const-drop-fail-2.precise.stderr
@@ -16,6 +16,10 @@ note: required by a bound in `check`
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
+help: make the `impl` of trait `A` `const`
+   |
+LL | impl const A for NonTrivialDrop {}
+   |      +++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/const-drop-fail-2.stock.stderr b/tests/ui/traits/const-traits/const-drop-fail-2.stock.stderr
@@ -16,6 +16,10 @@ note: required by a bound in `check`
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
+help: make the `impl` of trait `A` `const`
+   |
+LL | impl const A for NonTrivialDrop {}
+   |      +++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/const-impl-trait.stderr b/tests/ui/traits/const-traits/const-impl-trait.stderr
@@ -6,6 +6,8 @@ LL |     assert!(cmp(&()));
    |             |
    |             required by a bound introduced by this call
    |
+note: trait `PartialEq` is implemented but not `const`
+  --> $SRC_DIR/core/src/cmp.rs:LL:COL
 note: required by a bound in `cmp`
   --> $DIR/const-impl-trait.rs:11:23
    |
diff --git a/tests/ui/traits/const-traits/const-opaque.no.stderr b/tests/ui/traits/const-traits/const-opaque.no.stderr
@@ -11,12 +11,21 @@ note: required by a bound in `bar`
    |
 LL | const fn bar<T: [const] Foo>(t: T) -> impl [const] Foo {
    |                 ^^^^^^^^^^^ required by this bound in `bar`
+help: make the `impl` of trait `Foo` `const`
+   |
+LL | impl const Foo for () {
+   |      +++++
 
 error[E0277]: the trait bound `(): const Foo` is not satisfied
   --> $DIR/const-opaque.rs:33:12
    |
 LL |     opaque.method();
    |            ^^^^^^
+   |
+help: make the `impl` of trait `Foo` `const`
+   |
+LL | impl const Foo for () {
+   |      +++++
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/traits/const-traits/const_derives/derive-const-use.stderr b/tests/ui/traits/const-traits/const_derives/derive-const-use.stderr
@@ -5,6 +5,9 @@ LL | #[derive_const(Default, PartialEq)]
    |                         --------- in this derive macro expansion
 LL | pub struct S((), A);
    |              ^^
+   |
+note: trait `PartialEq` is implemented but not `const`
+  --> $SRC_DIR/core/src/cmp.rs:LL:COL
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/cross-crate.gatednc.stderr b/tests/ui/traits/const-traits/cross-crate.gatednc.stderr
@@ -3,6 +3,12 @@ error[E0277]: the trait bound `cross_crate::NonConst: [const] cross_crate::MyTra
    |
 LL |     NonConst.func();
    |              ^^^^
+   |
+note: trait `MyTrait` is implemented but not `const`
+  --> $DIR/auxiliary/cross-crate.rs:12:1
+   |
+LL | impl MyTrait for NonConst {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/default-method-body-is-const-body-checking.stderr b/tests/ui/traits/const-traits/default-method-body-is-const-body-checking.stderr
@@ -9,6 +9,10 @@ note: required by a bound in `foo`
    |
 LL | const fn foo<T>() where T: [const] Tr {}
    |                            ^^^^^^^^^^ required by this bound in `foo`
+help: make the `impl` of trait `Tr` `const`
+   |
+LL | impl const Tr for () {}
+   |      +++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/default-method-body-is-const-same-trait-ck.stderr b/tests/ui/traits/const-traits/default-method-body-is-const-same-trait-ck.stderr
@@ -3,6 +3,11 @@ error[E0277]: the trait bound `(): [const] Tr` is not satisfied
    |
 LL |         ().a()
    |            ^
+   |
+help: make the `impl` of trait `Tr` `const`
+   |
+LL | impl const Tr for () {}
+   |      +++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/item-bound-entailment-fails.stderr b/tests/ui/traits/const-traits/item-bound-entailment-fails.stderr
@@ -9,6 +9,10 @@ note: required by a bound in `Foo::Assoc`
    |
 LL |     type Assoc<T>: [const] Bar
    |                    ^^^^^^^^^^^ required by this bound in `Foo::Assoc`
+help: make the `impl` of trait `Bar` `const`
+   |
+LL | impl<T> const Bar for N<T> where T: Bar {}
+   |         +++++
 
 error[E0277]: the trait bound `T: [const] Bar` is not satisfied
   --> $DIR/item-bound-entailment-fails.rs:24:21
diff --git a/tests/ui/traits/const-traits/minicore-deref-fail.stderr b/tests/ui/traits/const-traits/minicore-deref-fail.stderr
@@ -3,6 +3,11 @@ error[E0277]: the trait bound `Ty: [const] minicore::Deref` is not satisfied
    |
 LL |     *Ty;
    |     ^^^
+   |
+help: make the `impl` of trait `Deref` `const`
+   |
+LL | impl const Deref for Ty {
+   |      +++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/minicore-fn-fail.stderr b/tests/ui/traits/const-traits/minicore-fn-fail.stderr
@@ -11,6 +11,10 @@ note: required by a bound in `call_indirect`
    |
 LL | const fn call_indirect<T: [const] Fn()>(t: &T) { t() }
    |                           ^^^^^^^^^^^^ required by this bound in `call_indirect`
+help: make the `impl` of trait `Foo` `const`
+   |
+LL | impl const Foo for () {}
+   |      +++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/no-explicit-const-params.stderr b/tests/ui/traits/const-traits/no-explicit-const-params.stderr
@@ -59,6 +59,11 @@ error[E0277]: the trait bound `(): const Bar` is not satisfied
    |
 LL |     <() as Bar<false>>::bar();
    |      ^^
+   |
+help: make the `impl` of trait `Bar` `const`
+   |
+LL | impl const Bar for () {
+   |      +++++
 
 error: aborting due to 5 previous errors
 
diff --git a/tests/ui/traits/const-traits/specializing-constness-2.stderr b/tests/ui/traits/const-traits/specializing-constness-2.stderr
@@ -3,6 +3,11 @@ error[E0277]: the trait bound `T: [const] A` is not satisfied
    |
 LL |     <T as A>::a();
    |      ^
+   |
+help: make the `impl` of trait `A` `const`
+   |
+LL | impl<T: Default> const A for T {
+   |                  +++++
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/traits/const-traits/super-traits-fail.stderr b/tests/ui/traits/const-traits/super-traits-fail.stderr
@@ -3,6 +3,11 @@ error[E0277]: the trait bound `S: [const] Foo` is not satisfied
    |
 LL | impl const Bar for S {}
    |                    ^
+   |
+help: make the `impl` of trait `Foo` `const`
+   |
+LL | impl const Foo for S {
+   |      +++++
 
 error: aborting due to 1 previous error
 

Original file line number	Diff line number	Diff line change
@@ -9,6 +9,10 @@ note: required by a bound in `Foo::Bar`
`9`	`9`	`\|`
`10`	`10`	`LL \| type Bar: [const] Add;`
`11`	`11`	\| ^^^^^^^^^^^ required by this bound in `Foo::Bar`
	`12`	+help: make the `impl` of trait `Add` `const`
	`13`	`+ \|`
	`14`	`+LL \| impl const Add for NonConstAdd {`
	`15`	`+ \| +++++`
`12`	`16`
`13`	`17`	`error: aborting due to 1 previous error`
`14`	`18`
Original file line number	Diff line number	Diff line change
@@ -11,6 +11,10 @@ note: required by a bound in `equals_self`
`11`	`11`	`\|`
`12`	`12`	`LL \| const fn equals_self<T: [const] Foo>(t: &T) -> bool {`
`13`	`13`	\| ^^^^^^^^^^^ required by this bound in `equals_self`
	`14`	+help: make the `impl` of trait `Foo` `const`
	`15`	`+ \|`
	`16`	`+LL \| impl const Foo for S {`
	`17`	`+ \| +++++`
`14`	`18`
`15`	`19`	`error: aborting due to 1 previous error`
`16`	`20`
Original file line number	Diff line number	Diff line change
@@ -16,6 +16,10 @@ note: required by a bound in `check`
`16`	`16`	`\|`
`17`	`17`	`LL \| const fn check<T: [const] Destruct>(_: T) {}`
`18`	`18`	\| ^^^^^^^^^^^^^^^^ required by this bound in `check`
	`19`	+help: make the `impl` of trait `A` `const`
	`20`	`+ \|`
	`21`	`+LL \| impl const A for NonTrivialDrop {}`
	`22`	`+ \| +++++`
`19`	`23`
`20`	`24`	`error: aborting due to 1 previous error`
`21`	`25`