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Provide additional context to errors involving const traits #144194

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23cecd7
Point at non-const trait impl when encountering unmet `[const]` bound
estebank Jul 18, 2025
0fc3d5f
Point at non-const trait when using them in const context
estebank Jul 19, 2025
8b89cb3
Only suggest `#[const_trait]` only on local traits
estebank Jul 19, 2025
90ff38b
Point at the enclosing const context
estebank Jul 19, 2025
fd07c3f
Account for `#![feature(const_trait_impl)]` state
estebank Jul 19, 2025
c2d7915
review comments
estebank Jul 23, 2025
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28 changes: 28 additions & 0 deletions compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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
}

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4 changes: 4 additions & 0 deletions tests/ui/traits/const-traits/assoc-type.current.stderr
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Expand Up @@ -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

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4 changes: 4 additions & 0 deletions tests/ui/traits/const-traits/assoc-type.next.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/call-const-closure.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/call-const-trait-method-fail.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/call-generic-method-fail.stderr
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Expand Up @@ -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
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Why is it pointing at the raw ptr impls?

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I think that they are incorrectly detected as CandidateSimilarity::Exact, which is incorrect. I can skip them explicitly on this PR by adding some other check, or look into it at another time.

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It's because of this:

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

Lines 1689 to 1701 in 4ff3fa0

let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
loop {
match t.kind() {
ty::Ref(_, inner, _) | ty::RawPtr(inner, _) => t = *inner,
_ => break t,
}
}
};
if !ignoring_lifetimes {
a = strip_references(a);
b = strip_references(b);
}


error: aborting due to 1 previous error

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4 changes: 4 additions & 0 deletions tests/ui/traits/const-traits/call-generic-method-nonconst.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/const-closure-trait-method-fail.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/const-default-method-bodies.stderr
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Expand Up @@ -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

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4 changes: 4 additions & 0 deletions tests/ui/traits/const-traits/const-drop-fail-2.precise.stderr
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Expand Up @@ -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

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4 changes: 4 additions & 0 deletions tests/ui/traits/const-traits/const-drop-fail-2.stock.stderr
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Expand Up @@ -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

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2 changes: 2 additions & 0 deletions tests/ui/traits/const-traits/const-impl-trait.stderr
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Expand Up @@ -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
|
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9 changes: 9 additions & 0 deletions tests/ui/traits/const-traits/const-opaque.no.stderr
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Expand Up @@ -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

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3 changes: 3 additions & 0 deletions tests/ui/traits/const-traits/const_derives/derive-const-use.stderr
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Expand Up @@ -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

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6 changes: 6 additions & 0 deletions tests/ui/traits/const-traits/cross-crate.gatednc.stderr
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Expand Up @@ -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

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4 changes: 4 additions & 0 deletions tests/ui/traits/const-traits/default-method-body-is-const-body-checking.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/default-method-body-is-const-same-trait-ck.stderr
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Expand Up @@ -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

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4 changes: 4 additions & 0 deletions tests/ui/traits/const-traits/item-bound-entailment-fails.stderr
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Expand Up @@ -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
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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/minicore-deref-fail.stderr
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Expand Up @@ -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

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4 changes: 4 additions & 0 deletions tests/ui/traits/const-traits/minicore-fn-fail.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/no-explicit-const-params.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/specializing-constness-2.stderr
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Expand Up @@ -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

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5 changes: 5 additions & 0 deletions tests/ui/traits/const-traits/super-traits-fail.stderr
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Expand Up @@ -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

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