Suggest `PartialEq` bound for binary operation `==` error

[tesuji]

Try this snippet: playground

pub fn strip_prefix<'a, T>(s: &'a [T], prefix: &[T]) -> Option<&'a [T]>
// where 
//     T: PartialEq
{
    let n = prefix.len();
    if n <= s.len() {
        let (head, tail) = s.split_at(n);
        if head == prefix {
            return Some(tail);
        }
    }
    None
}

The compiler errors out:

error[E0369]: binary operation `==` cannot be applied to type `&[T]`
 --> src/lib.rs:8:17
  |
8 |         if head == prefix {
  |            ---- ^^ ------ &[T]
  |            |
  |            &[T]

error: aborting due to previous error

For more information about this error, try `rustc --explain E0369`.

It would be nice if the compiler could suggest adding PartialEq bound for T.

[estebank]

related: #73261

[tesuji]

Note: The stderr for Rust 1.42 and below:

error[E0369]: binary operation `==` cannot be applied to type `&[T]`
 --> <source>:8:17
  |
8 |         if head == prefix {
  |            ---- ^^ ------ &[T]
  |            |
  |            &[T]
  |
  = note: an implementation of `std::cmp::PartialEq` might be missing for `&[T]`

[estebank]

Yep. The ideal output would be something along the lines of

error[E0369]: binary operation `==` cannot be applied to type `&[T]`
 --> src/lib.rs:8:17
  |
8 |         if head == prefix {
  |            ---- ^^ ------ &[T]
  |            |
  |            &[T]
help: consider restricting `T` to require it implements `std::cmp::PartialEq`
  |
1 | pub fn strip_prefix<'a, T: std::cmp::PartialEq>(s: &'a [T], prefix: &[T]) -> Option<&'a [T]>
  |                          ^^^^^^^^^^^^^^^^^^^^^

[estebank]

How does this look like?