Misleading/Incomplete errors involving Pattern

[glandium]

Given the following code:

pub fn strip_lf(s: &str) -> &str {
    s.strip_suffix(b'\n').unwrap_or(s)
}

The current output is:

error[E0277]: expected a `FnMut<(char,)>` closure, found `u8`
 --> src/lib.rs:2:20
  |
2 |     s.strip_suffix(b'\n').unwrap_or(s)
  |                    ^^^^^ expected an `FnMut<(char,)>` closure, found `u8`
  |
  = help: the trait `FnMut<(char,)>` is not implemented for `u8`
  = note: required because of the requirements on the impl of `Pattern<'_>` for `u8`

Pattern<'a> has implementations for:

• char
• &str
• &String
• &[char]
• &&str
• F where F: FnMut(char) -> bool

While it is true that u8 doesn't match any of the non-generic impls, and that it doesn't have an impl for FnMut<(char,)> -> bool, it's sad that the error message doesn't mention that those other possibilities exist.
(Also, the error message doesn't mention the closure return type)

[JakobDegen]

@rustbot claim

[estebank]

Linking to the Zulip discussion for the record.

[JakobDegen]

I thought about this a bunch, and don't really know the best solution here. Happy to let someone else try

[taladar]

I encountered a related issue today with the pattern parameter on replace

fn main() {
    let s1: String = "Foo".to_string();
    let s2: String = "Bar".to_string();
    let mut s: String = "FooBarBaz".to_string();
    s.replace(s1, &s2);
    format!("Replacement result: {}", s);
}

(no & for s1 in the replace call) results in the error:

   Compiling compiler-bug-test v0.1.0 (/home/taladar/temp/20211223/compiler-bug-test)
error[E0277]: expected a `FnMut<(char,)>` closure, found `String`
 --> src/main.rs:5:7
  |
5 |     s.replace(s1, &s2);
  |       ^^^^^^^ expected an `FnMut<(char,)>` closure, found `String`
  |
  = help: the trait `FnMut<(char,)>` is not implemented for `String`
  = note: required because of the requirements on the impl of `Pattern<'_>` for `String`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `compiler-bug-test` due to previous error

which is misleading because it marks "replace" instead of the first parameter in addition to the issue mentioned above.

[edmorley]

See also #87437 (which may be a dupe of this or vice versa, unless I'm misunderstanding).