Closed
Description
Sorry, this is not necessarily the smallest possible example. Please let me know if this is unhelpful and I'll come up with something more concise.
Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8a1f669598035e38e7e141eb799d6738
use std::io;
fn real_dispatch<T, F>(f: F) -> Result<(), io::Error>
where
F: FnOnce(&mut UIView<T>) -> Result<(), io::Error> + Send + 'static,
{
todo!()
}
#[derive(Debug)]
struct UIView<'a, T: 'a> {
_phantom: std::marker::PhantomData<&'a mut T>,
}
trait Handle<'a, T: 'a, V, R> {
fn dispatch<F>(&self, f: F) -> Result<(), io::Error>
where
F: FnOnce(&mut V) -> R + Send + 'static;
}
#[derive(Debug, Clone)]
struct TUIHandle<T> {
_phantom: std::marker::PhantomData<T>,
}
impl<'a, T: 'a> Handle<'a, T, UIView<'a, T>, Result<(), io::Error>> for TUIHandle<T> {
fn dispatch<F>(&self, f: F) -> Result<(), io::Error>
where
F: FnOnce(&mut UIView<'a, T>) -> Result<(), io::Error> + Send + 'static,
{
real_dispatch(f)
}
}The output in current stable (1.63.0) is:
(Please note the "expected closure with ... found closure with ..." hint.)
error[E0277]: expected a `FnOnce<(&mut UIView<'_, T>,)>` closure, found `F`
--> src/lib.rs:31:23
|
31 | real_dispatch(f)
| ------------- ^ expected an `FnOnce<(&mut UIView<'_, T>,)>` closure, found `F`
| |
| required by a bound introduced by this call
|
= note: expected a closure with arguments `(&mut UIView<'a, T>,)`
found a closure with arguments `(&mut UIView<'_, T>,)`
note: required by a bound in `real_dispatch`
--> src/lib.rs:5:8
|
3 | fn real_dispatch<T, F>(f: F) -> Result<(), io::Error>
| ------------- required by a bound in this
4 | where
5 | F: FnOnce(&mut UIView<T>) -> Result<(), io::Error> + Send + 'static,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `real_dispatch`
For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` due to previous error
From 1.64 onwards, the expected/found hint is omitted:
error[E0277]: expected a `FnOnce<(&mut UIView<'_, T>,)>` closure, found `F`
--> src/lib.rs:31:23
|
31 | real_dispatch(f)
| ------------- ^ expected an `FnOnce<(&mut UIView<'_, T>,)>` closure, found `F`
| |
| required by a bound introduced by this call
|
note: required by a bound in `real_dispatch`
--> src/lib.rs:5:8
|
3 | fn real_dispatch<T, F>(f: F) -> Result<(), io::Error>
| ------------- required by a bound in this
4 | where
5 | F: FnOnce(&mut UIView<T>) -> Result<(), io::Error> + Send + 'static,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `real_dispatch`
For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` due to previous error
To me the 1.63 behaviour is preferable to that from 1.64+ as it can often be non-obvious where the differences lie.