Closed
Description
While looking through some clippy lint warnings in rust-lang/rust-clippy#5939 I found this very confusing error message:
use std::io::{self};
use std::path::{Path, PathBuf};
pub fn main() {
let path = &PathBuf::from("a");
do_op(path, "remove file", std::fs::remove_file); // redundant closure
}
fn do_op<F>(path: &Path, _desc: &str, mut f: F)
where
F: FnMut(&Path) -> io::Result<()>,
{
match f(path) {
Ok(()) => {}
Err(ref _e) => {}
Err(_e) => {}
}
}error[E0308]: mismatched types
--> src/main.rs:6:5
|
6 | do_op(path, "remove file", std::fs::remove_file); // redundant closure
| ^^^^^ one type is more general than the other
|
= note: expected type `std::ops::FnOnce<(&std::path::Path,)>`
found type `std::ops::FnOnce<(&std::path::Path,)>`
note that expected type and found type are identical which is not helpful 😅
This happens on
rustc 1.47.0-nightly (e15510ca3 2020-08-20)
binary: rustc
commit-hash: e15510ca33ea15c893b78710101c962b11459963
commit-date: 2020-08-20
host: x86_64-unknown-linux-gnu
release: 1.47.0-nightly
LLVM version: 10.0
and also on 1.46.0-beta.4.
On stable 1.45.2, the message looks different though:
error[E0631]: type mismatch in function arguments
--> src/main.rs:6:32
|
6 | do_op(path, "remove file", std::fs::remove_file); // redundant closure
| ^^^^^^^^^^^^^^^^^^^^
| |
| expected signature of `for<'r> fn(&'r std::path::Path) -> _`
| found signature of `fn(_) -> _`
...
9 | fn do_op<F>(path: &Path, _desc: &str, mut f: F)
| ----- required by a bound in this
10 | where
11 | F: FnMut(&Path) -> io::Result<()>,
| ------------------------------ required by this bound in `do_op`
error[E0271]: type mismatch resolving `for<'r> <fn(_) -> std::result::Result<(), std::io::Error> {std::fs::remove_file::<_>} as std::ops::FnOnce<(&'r std::path::Path,)>>::Output == std::result::Result<(), std::io::Error>`
--> src/main.rs:6:5
|
6 | do_op(path, "remove file", std::fs::remove_file); // redundant closure
| ^^^^^ expected bound lifetime parameter, found concrete lifetime
...
9 | fn do_op<F>(path: &Path, _desc: &str, mut f: F)
| ----- required by a bound in this
10 | where
11 | F: FnMut(&Path) -> io::Result<()>,
| -------------- required by this bound in `do_op`