Syntactically-incorrect suggestion given by Rustc for #[derive()] that does not satisfy trait bounds

[eddieantonio]

rustc produces a syntactically-invalid fix; namely, it suggests to add a trait bound within the #[derive(...)] attribute, which does not seem right!

• Suppose I have written my own impl<T> PartialEq for MyStruct<T> but only for T: Copy
• Then I want to #[derive(Eq)] on MyStruct<T>

Given the following code: Rust Playground

#[derive(Eq)]
struct MyStruct<T>{
    x: T
}

impl<T> PartialEq for MyStruct<T>
where T: Copy + Eq
{
    fn eq(&self, other: &Self) -> bool {
        self.x == other.x
    }
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `T: Copy` is not satisfied
   --> src/lib.rs:1:10
    |
1   | #[derive(Eq)]
    |          ^^ the trait `Copy` is not implemented for `T`
    |
note: required because of the requirements on the impl of `PartialEq` for `MyStruct<T>`
   --> src/lib.rs:6:9
    |
6   | impl<T> PartialEq for MyStruct<T>
    |         ^^^^^^^^^     ^^^^^^^^^^^
note: required by a bound in `Eq`
    = note: this error originates in the derive macro `Eq` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider further restricting this bound
    |
1   | #[derive(Eq + std::marker::Copy)]
    |             +++++++++++++++++++

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

The issue is with the help: portion:

help: consider further restricting this bound
    |
1   | #[derive(Eq + std::marker::Copy)]
    |             +++++++++++++++++++

However, if I were to non-judgementally apply the suggestion... 💣💥 Rust Playground

   Compiling playground v0.0.1 (/playground)
error: expected one of `(`, `,`, `::`, or `=`, found `+`
 --> src/lib.rs:1:13
  |
1 | #[derive(Eq + std::marker::Copy)]
  |             ^ expected one of `(`, `,`, `::`, or `=`

error[E0412]: cannot find type `MyStruct` in this scope
 --> src/lib.rs:6:23
  |
6 | impl<T> PartialEq for MyStruct<T>
  |                       ^^^^^^^^ not found in this scope

For more information about this error, try `rustc --explain E0412`.
error: could not compile `playground` due to 2 previous errors

Possilbe resolutions to this issue:

• omit the help: suggestion entirely because of how misleading it is
• for this case in particular, a more appropriate help: suggestion is to remove the #[derive(...)] and provide an implementation of Eq manually Rust playground

[estebank]

I believe this will be fixed by #90519.