Code
pub trait Foo {
fn bar<F>(&mut self, func: F) -> impl std::future::Future<Output = ()> + Send
where
F: FnMut();
}
struct Baz {}
impl Foo for Baz {
async fn bar<F>(&mut self, _func: F) -> ()
where
F: FnMut() + Send,
{
()
}
}Current output
$ cargo check
Checking playground v0.0.1 (/playground)
error[E0277]: `F` cannot be sent between threads safely
--> src/lib.rs:10:5
|
10 | / async fn bar<F>(&mut self, _func: F) -> ()
11 | | where
12 | | F: FnMut() + Send,
| |__________________________^ `F` cannot be sent between threads safely
|
note: required by a bound in `<Baz as Foo>::bar`
--> src/lib.rs:12:22
|
10 | async fn bar<F>(&mut self, _func: F) -> ()
| --- required by a bound in this associated function
11 | where
12 | F: FnMut() + Send,
| ^^^^ required by this bound in `<Baz as Foo>::bar`
help: consider further restricting this bound
|
12 | F: FnMut() + Send + std::marker::Send,
| +++++++++++++++++++
error[E0276]: impl has stricter requirements than trait
--> src/lib.rs:12:22
|
2 | / fn bar<F>(&mut self, func: F) -> impl std::future::Future<Output = ()> + Send
3 | | where
4 | | F: FnMut();
| |___________________- definition of `bar` from trait
...
12 | F: FnMut() + Send,
| ^^^^ impl has extra requirement `F: Send`
Some errors have detailed explanations: E0276, E0277.
For more information about an error, try `rustc --explain E0276`.
error: could not compile `playground` (lib) due to 2 previous errors
Desired output
$ cargo check
Checking playground v0.0.1 (/playground)
error[E0277]: `F` cannot be sent between threads safely
--> src/lib.rs:10:5
|
10 | / async fn bar<F>(&mut self, _func: F) -> ()
11 | | where
12 | | F: FnMut() + Send,
| |__________________________^ `F` cannot be sent between threads safely
|
note: required by a bound in `<Baz as Foo>::bar`
--> src/lib.rs:12:22
|
10 | async fn bar<F>(&mut self, _func: F) -> ()
| --- required by a bound in this associated function
11 | where
12 | F: FnMut() + Send,
| ^^^^ required by this bound in `<Baz as Foo>::bar`
help: consider further restricting this bound
|
4 | F: FnMut() + std::marker::Send,
| +++++++++++++++++++
error[E0276]: impl has stricter requirements than trait
--> src/lib.rs:12:22
|
2 | / fn bar<F>(&mut self, func: F) -> impl std::future::Future<Output = ()> + Send
3 | | where
4 | | F: FnMut();
| |___________________- definition of `bar` from trait
...
12 | F: FnMut() + Send,
| ^^^^ impl has extra requirement `F: Send`
Some errors have detailed explanations: E0276, E0277.
For more information about an error, try `rustc --explain E0276`.
error: could not compile `playground` (lib) due to 2 previous errors
Rationale and extra context
rustc's suggestion to add the std::marker::Send trait to the impl Foo for Baz impl block is incorrect -- the function in that block already has the marker trait. Instead, it should suggest to add the trait in the where F: FnMut() in the trait definition instead, or raise the error on the trait definition instead, because the real error is made there.
If the suggestion is applied, it will result in a second Send bound added to line 12, which is definitely not correct.
Other cases
No response
Rust Version
$ rustc --version --verbose
rustc 1.79.0 (129f3b996 2024-06-10)
binary: rustc
commit-hash: 129f3b9964af4d4a709d1383930ade12dfe7c081
commit-date: 2024-06-10
host: x86_64-pc-windows-msvc
release: 1.79.0
LLVM version: 18.1.7
Anything else?
This is my first bug on the Rust issue tracker, so apologies if I'm confusing or did something wrong. I've been using Rust for around a year now on and off and it's really such a good language!
Code
Current output
Desired output
Rationale and extra context
rustc's suggestion to add thestd::marker::Sendtrait to theimpl Foo for Bazimpl block is incorrect -- the function in that block already has the marker trait. Instead, it should suggest to add the trait in thewhere F: FnMut()in the trait definition instead, or raise the error on the trait definition instead, because the real error is made there.If the suggestion is applied, it will result in a second
Sendbound added to line 12, which is definitely not correct.Other cases
No response
Rust Version
Anything else?
This is my first bug on the Rust issue tracker, so apologies if I'm confusing or did something wrong. I've been using Rust for around a year now on and off and it's really such a good language!