From 60b227acccf40cff72df2104fbdedc8185e8a57f Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Esteban=20K=C3=BCber?= Date: Tue, 4 Nov 2025 18:41:04 +0000 Subject: [PATCH] Recognize `type Alias = dyn Trait` in `fn` return types ``` error[E0277]: the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time --> $DIR/dyn-trait-type-alias-return-type.rs:4:11 | LL | fn f() -> T { loop {} } | ^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `(dyn Debug + 'static)` note: this type alias is unsized --> $DIR/dyn-trait-type-alias-return-type.rs:1:1 | LL | type T = dyn core::fmt::Debug; | ^^^^^^ = note: the return type of a function must have a statically known size ``` --- .../src/error_reporting/traits/suggestions.rs | 13 +++++++ .../clippy/tests/ui/future_not_send.stderr | 37 +++++++++---------- .../dyn-trait-type-alias-return-type.rs | 10 +++-- .../dyn-trait-type-alias-return-type.stderr | 19 +++++----- tests/ui/unsized/issue-91801.rs | 2 +- tests/ui/unsized/issue-91801.stderr | 17 ++++----- 6 files changed, 54 insertions(+), 44 deletions(-) diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs index 16bb4c186447..822025b986b8 100644 --- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs +++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs @@ -1886,6 +1886,19 @@ pub(super) fn suggest_impl_trait( let ty::Dynamic(_, _) = trait_pred.self_ty().skip_binder().kind() else { return false; }; + if let Node::Item(hir::Item { kind: hir::ItemKind::Fn { sig: fn_sig, .. }, .. }) = + self.tcx.hir_node_by_def_id(obligation.cause.body_id) + && let hir::FnRetTy::Return(ty) = fn_sig.decl.output + && let hir::TyKind::Path(qpath) = ty.kind + && let hir::QPath::Resolved(None, path) = qpath + && let Res::Def(DefKind::TyAlias, def_id) = path.res + { + // Do not suggest + // type T = dyn Trait; + // fn foo() -> impl T { .. } + err.span_note(self.tcx.def_span(def_id), "this type alias is unsized"); + return false; + } err.code(E0746); err.primary_message("return type cannot be a trait object without pointer indirection"); diff --git a/src/tools/clippy/tests/ui/future_not_send.stderr b/src/tools/clippy/tests/ui/future_not_send.stderr index e366dc2d2195..8b8af1ebaed3 100644 --- a/src/tools/clippy/tests/ui/future_not_send.stderr +++ b/src/tools/clippy/tests/ui/future_not_send.stderr @@ -1,8 +1,8 @@ error: future cannot be sent between threads safely - --> tests/ui/future_not_send.rs:8:1 + --> tests/ui/future_not_send.rs:8:62 | LL | async fn private_future(rc: Rc<[u8]>, cell: &Cell) -> bool { - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `private_future` is not `Send` + | ^^^^ future returned by `private_future` is not `Send` | note: future is not `Send` as this value is used across an await --> tests/ui/future_not_send.rs:11:20 @@ -23,10 +23,10 @@ LL | async fn private_future(rc: Rc<[u8]>, cell: &Cell) -> bool { = help: to override `-D warnings` add `#[allow(clippy::future_not_send)]` error: future cannot be sent between threads safely - --> tests/ui/future_not_send.rs:14:1 + --> tests/ui/future_not_send.rs:14:41 | LL | pub async fn public_future(rc: Rc<[u8]>) { - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `public_future` is not `Send` + | ^ future returned by `public_future` is not `Send` | note: future is not `Send` as this value is used across an await --> tests/ui/future_not_send.rs:17:20 @@ -39,10 +39,10 @@ LL | async { true }.await; = note: `std::rc::Rc<[u8]>` doesn't implement `std::marker::Send` error: future cannot be sent between threads safely - --> tests/ui/future_not_send.rs:24:1 + --> tests/ui/future_not_send.rs:24:63 | LL | async fn private_future2(rc: Rc<[u8]>, cell: &Cell) -> bool { - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `private_future2` is not `Send` + | ^^^^ future returned by `private_future2` is not `Send` | note: captured value is not `Send` --> tests/ui/future_not_send.rs:24:26 @@ -58,10 +58,10 @@ LL | async fn private_future2(rc: Rc<[u8]>, cell: &Cell) -> bool { = note: `std::cell::Cell` doesn't implement `std::marker::Sync` error: future cannot be sent between threads safely - --> tests/ui/future_not_send.rs:30:1 + --> tests/ui/future_not_send.rs:30:42 | LL | pub async fn public_future2(rc: Rc<[u8]>) {} - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `public_future2` is not `Send` + | ^ future returned by `public_future2` is not `Send` | note: captured value is not `Send` --> tests/ui/future_not_send.rs:30:29 @@ -71,10 +71,10 @@ LL | pub async fn public_future2(rc: Rc<[u8]>) {} = note: `std::rc::Rc<[u8]>` doesn't implement `std::marker::Send` error: future cannot be sent between threads safely - --> tests/ui/future_not_send.rs:42:5 + --> tests/ui/future_not_send.rs:42:39 | LL | async fn private_future(&self) -> usize { - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `private_future` is not `Send` + | ^^^^^ future returned by `private_future` is not `Send` | note: future is not `Send` as this value is used across an await --> tests/ui/future_not_send.rs:45:24 @@ -87,10 +87,10 @@ LL | async { true }.await; = note: `std::rc::Rc<[u8]>` doesn't implement `std::marker::Sync` error: future cannot be sent between threads safely - --> tests/ui/future_not_send.rs:49:5 + --> tests/ui/future_not_send.rs:49:38 | LL | pub async fn public_future(&self) { - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `public_future` is not `Send` + | ^ future returned by `public_future` is not `Send` | note: captured value is not `Send` because `&` references cannot be sent unless their referent is `Sync` --> tests/ui/future_not_send.rs:49:32 @@ -100,13 +100,10 @@ LL | pub async fn public_future(&self) { = note: `std::rc::Rc<[u8]>` doesn't implement `std::marker::Sync` error: future cannot be sent between threads safely - --> tests/ui/future_not_send.rs:61:1 + --> tests/ui/future_not_send.rs:61:37 | -LL | / async fn generic_future(t: T) -> T -LL | | -LL | | where -LL | | T: Send, - | |____________^ future returned by `generic_future` is not `Send` +LL | async fn generic_future(t: T) -> T + | ^ future returned by `generic_future` is not `Send` | note: future is not `Send` as this value is used across an await --> tests/ui/future_not_send.rs:67:20 @@ -118,10 +115,10 @@ LL | async { true }.await; = note: `T` doesn't implement `std::marker::Sync` error: future cannot be sent between threads safely - --> tests/ui/future_not_send.rs:83:1 + --> tests/ui/future_not_send.rs:83:51 | LL | async fn generic_future_always_unsend(_: Rc) { - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `generic_future_always_unsend` is not `Send` + | ^ future returned by `generic_future_always_unsend` is not `Send` | note: future is not `Send` as this value is used across an await --> tests/ui/future_not_send.rs:86:20 diff --git a/tests/ui/type-alias/dyn-trait-type-alias-return-type.rs b/tests/ui/type-alias/dyn-trait-type-alias-return-type.rs index f96cd989a4ce..fe1322a48d65 100644 --- a/tests/ui/type-alias/dyn-trait-type-alias-return-type.rs +++ b/tests/ui/type-alias/dyn-trait-type-alias-return-type.rs @@ -1,8 +1,10 @@ type T = dyn core::fmt::Debug; - +//~^ NOTE this type alias is unsized + fn f() -> T { loop {} } -//~^ ERROR return type cannot be a trait object without pointer indirection -//~| HELP -//~| HELP +//~^ ERROR the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time +//~| HELP the trait `Sized` is not implemented for `(dyn Debug + 'static)` +//~| NOTE doesn't have a size known at compile-time +//~| NOTE the return type of a function must have a statically known size fn main() {} diff --git a/tests/ui/type-alias/dyn-trait-type-alias-return-type.stderr b/tests/ui/type-alias/dyn-trait-type-alias-return-type.stderr index d67ba7759551..fc7f9f49d165 100644 --- a/tests/ui/type-alias/dyn-trait-type-alias-return-type.stderr +++ b/tests/ui/type-alias/dyn-trait-type-alias-return-type.stderr @@ -1,18 +1,17 @@ -error[E0746]: return type cannot be a trait object without pointer indirection - --> $DIR/dyn-trait-type-alias-return-type.rs:3:11 +error[E0277]: the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time + --> $DIR/dyn-trait-type-alias-return-type.rs:4:11 | LL | fn f() -> T { loop {} } | ^ doesn't have a size known at compile-time | -help: consider returning an `impl Trait` instead of a `dyn Trait` + = help: the trait `Sized` is not implemented for `(dyn Debug + 'static)` +note: this type alias is unsized + --> $DIR/dyn-trait-type-alias-return-type.rs:1:1 | -LL | fn f() -> impl T { loop {} } - | ++++ -help: alternatively, box the return type, and wrap all of the returned values in `Box::new` - | -LL | fn f() -> Box { Box::new(loop {}) } - | +++++++ + +++++++++ + +LL | type T = dyn core::fmt::Debug; + | ^^^^^^ + = note: the return type of a function must have a statically known size error: aborting due to 1 previous error -For more information about this error, try `rustc --explain E0746`. +For more information about this error, try `rustc --explain E0277`. diff --git a/tests/ui/unsized/issue-91801.rs b/tests/ui/unsized/issue-91801.rs index d906a08a55a2..8b4a3d214d5e 100644 --- a/tests/ui/unsized/issue-91801.rs +++ b/tests/ui/unsized/issue-91801.rs @@ -6,7 +6,7 @@ &[("validate that credits and debits balance", &validate_something)]; fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> Validator<'a> { - //~^ ERROR return type cannot be a trait object without pointer indirection + //~^ ERROR E0277 return Box::new(move |something: &'_ Something| -> Result<(), ()> { first(something).or_else(|_| second(something)) }); diff --git a/tests/ui/unsized/issue-91801.stderr b/tests/ui/unsized/issue-91801.stderr index 28e10f9caa41..73f9de92458e 100644 --- a/tests/ui/unsized/issue-91801.stderr +++ b/tests/ui/unsized/issue-91801.stderr @@ -1,18 +1,17 @@ -error[E0746]: return type cannot be a trait object without pointer indirection +error[E0277]: the size for values of type `(dyn Fn(&'a Something) -> Result<(), ()> + Send + Sync + 'a)` cannot be known at compilation time --> $DIR/issue-91801.rs:8:77 | LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> Validator<'a> { | ^^^^^^^^^^^^^ doesn't have a size known at compile-time | -help: consider returning an `impl Trait` instead of a `dyn Trait` + = help: the trait `Sized` is not implemented for `(dyn Fn(&'a Something) -> Result<(), ()> + Send + Sync + 'a)` +note: this type alias is unsized + --> $DIR/issue-91801.rs:3:1 | -LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> impl Validator<'a> { - | ++++ -help: alternatively, box the return type, and wrap all of the returned values in `Box::new` - | -LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> Box> { - | +++++++ + +LL | type Validator<'a> = dyn 'a + Send + Sync + Fn(&'a Something) -> Result<(), ()>; + | ^^^^^^^^^^^^^^^^^^ + = note: the return type of a function must have a statically known size error: aborting due to 1 previous error -For more information about this error, try `rustc --explain E0746`. +For more information about this error, try `rustc --explain E0277`.