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rust-bors merged 1 commit into from
Apr 22, 2026
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Improve E0308 error message for impl Trait return mismatches #155546

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6 changes: 6 additions & 0 deletions compiler/rustc_hir_typeck/src/errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -173,6 +173,12 @@ pub(crate) enum ExpectedReturnTypeLabel<'tcx> {
span: Span,
expected: Ty<'tcx>,
},
#[label("expected a single type implementing `{$trait_name}` because of return type")]
ImplTrait {
#[primary_span]
span: Span,
trait_name: String,
},
}

#[derive(Diagnostic)]
Expand Down
36 changes: 36 additions & 0 deletions compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
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Expand Up @@ -931,6 +931,42 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
}
hir::FnRetTy::Return(hir_ty) => {
if let hir::TyKind::OpaqueDef(op_ty, ..) = hir_ty.kind
&& let [hir::GenericBound::Trait(trait_ref)] = op_ty.bounds
&& !trait_ref
.trait_ref
.path
.segments
.last()
.and_then(|seg| seg.args)
.map_or(false, |args| !args.constraints.is_empty())
{
// Use the path to get the trait name string
let trait_name = trait_ref
.trait_ref
.path
.segments
.iter()
.map(|seg| seg.ident.as_str())
.collect::<Vec<_>>()
.join("::");

err.subdiagnostic(errors::ExpectedReturnTypeLabel::ImplTrait {
span: hir_ty.span,
trait_name,
});

if let Some(ret_coercion_span) = self.ret_coercion_span.get() {
let expected_name = expected.to_string();
err.span_label(
ret_coercion_span,
format!("return type resolved to be `{expected_name}`"),
);
}

self.try_suggest_return_impl_trait(err, expected, found, fn_id);
self.try_note_caller_chooses_ty_for_ty_param(err, expected, found);
return true;
} else if let hir::TyKind::OpaqueDef(op_ty, ..) = hir_ty.kind
// FIXME: account for RPITIT.
&& let [hir::GenericBound::Trait(trait_ref)] = op_ty.bounds
&& let Some(hir::PathSegment { args: Some(generic_args), .. }) =
Expand Down
14 changes: 10 additions & 4 deletions tests/ui/impl-trait/dyn-incompatible-trait-in-return-position-impl-trait.stderr
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Expand Up @@ -2,17 +2,23 @@ error[E0308]: mismatched types
--> $DIR/dyn-incompatible-trait-in-return-position-impl-trait.rs:36:5
|
LL | fn can() -> impl DynIncompatible {
| -------------------- expected `A` because of return type
...
| -------------------- expected a single type implementing `DynIncompatible` because of return type
LL | if true {
LL | return A;
| - return type resolved to be `A`
LL | }
LL | B
| ^ expected `A`, found `B`

error[E0308]: mismatched types
--> $DIR/dyn-incompatible-trait-in-return-position-impl-trait.rs:43:5
|
LL | fn cat() -> impl DynCompatible {
| ------------------ expected `A` because of return type
...
| ------------------ expected a single type implementing `DynCompatible` because of return type
LL | if true {
LL | return A;
| - return type resolved to be `A`
LL | }
LL | B
| ^ expected `A`, found `B`

Expand Down
7 changes: 5 additions & 2 deletions tests/ui/impl-trait/equality.stderr
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Expand Up @@ -2,8 +2,11 @@ error[E0308]: mismatched types
--> $DIR/equality.rs:17:5
|
LL | fn two(x: bool) -> impl Foo {
| -------- expected `i32` because of return type
...
| -------- expected a single type implementing `Foo` because of return type
LL | if x {
LL | return 1_i32;
| ----- return type resolved to be `i32`
LL | }
LL | 0_u32
| ^^^^^ expected `i32`, found `u32`
|
Expand Down
35 changes: 25 additions & 10 deletions tests/ui/impl-trait/point-to-type-err-cause-on-impl-trait-return.stderr
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Expand Up @@ -64,8 +64,11 @@ error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:5:5
|
LL | fn foo() -> impl std::fmt::Display {
| ---------------------- expected `i32` because of return type
...
| ---------------------- expected a single type implementing `std::fmt::Display` because of return type
LL | if false {
LL | return 0i32;
| ---- return type resolved to be `i32`
LL | }
LL | 1u32
| ^^^^ expected `i32`, found `u32`
|
Expand All @@ -79,8 +82,11 @@ error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:12:16
|
LL | fn bar() -> impl std::fmt::Display {
| ---------------------- expected `i32` because of return type
...
| ---------------------- expected a single type implementing `std::fmt::Display` because of return type
LL | if false {
LL | return 0i32;
| ---- return type resolved to be `i32`
LL | } else {
LL | return 1u32;
| ^^^^ expected `i32`, found `u32`
|
Expand All @@ -94,8 +100,11 @@ error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:20:9
|
LL | fn baz() -> impl std::fmt::Display {
| ---------------------- expected `i32` because of return type
...
| ---------------------- expected a single type implementing `std::fmt::Display` because of return type
LL | if false {
LL | return 0i32;
| ---- return type resolved to be `i32`
LL | } else {
LL | 1u32
| ^^^^ expected `i32`, found `u32`
|
Expand Down Expand Up @@ -137,8 +146,10 @@ error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:35:14
|
LL | fn bat() -> impl std::fmt::Display {
| ---------------------- expected `i32` because of return type
...
| ---------------------- expected a single type implementing `std::fmt::Display` because of return type
LL | match 13 {
LL | 0 => return 0i32,
| ---- return type resolved to be `i32`
LL | _ => 1u32,
| ^^^^ expected `i32`, found `u32`
|
Expand All @@ -151,9 +162,10 @@ error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:40:5
|
LL | fn can() -> impl std::fmt::Display {
| ---------------------- expected `i32` because of return type
| ---------------------- expected a single type implementing `std::fmt::Display` because of return type
LL | / match 13 {
LL | | 0 => return 0i32,
| | ---- return type resolved to be `i32`
LL | | 1 => 1u32,
LL | | _ => 2u32,
LL | | }
Expand All @@ -168,7 +180,10 @@ error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:53:13
|
LL | fn cat() -> impl std::fmt::Display {
| ---------------------- expected `i32` because of return type
| ---------------------- expected a single type implementing `std::fmt::Display` because of return type
...
LL | return 0i32;
| ---- return type resolved to be `i32`
...
LL | 1u32
| ^^^^ expected `i32`, found `u32`
Expand Down
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