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Recognize type Alias = dyn Trait in fn return types

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```
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
```
This commit is contained in:
Esteban Küber
2025-11-04 18:41:04 +00:00
parent fabf5746a2
commit 60b227accc
6 changed files with 54 additions and 44 deletions
Download Patch File Download Diff File Expand all files Collapse all files
compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+13
View File
@@ -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");
src/tools/clippy/tests/ui/future_not_send.stderr
+17 -20
View File
@@ -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<usize>) -> 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<usize>) -> 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<usize>) -> 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<usize>) -> bool {
= note: `std::cell::Cell<usize>` 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) -> T
LL | |
LL | | where
LL | | T: Send,
| |____________^ future returned by `generic_future` is not `Send`
LL | async fn generic_future<T>(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<T>(_: Rc<T>) {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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
tests/ui/type-alias/dyn-trait-type-alias-return-type.rs
+6 -4
View File
@@ -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() {}
tests/ui/type-alias/dyn-trait-type-alias-return-type.stderr
+9 -10
View File
@@ -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<dyn T> { 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`.
tests/ui/unsized/issue-91801.rs
+1 -1
View File
@@ -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))
});
tests/ui/unsized/issue-91801.stderr
+8 -9
View File
@@ -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<dyn Validator<'a>> {
| +++++++ +
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`.