Friendlier compiler message for incorrect trait method signatures

[andersforsgren]

When implementing a trait, it would be useful to see the expected method signature in compiler error messages. Example

impl std::fmt::Display for MyType {
   // Not sure what to put here so I stub it and hope to get help from error
   fn fmt() -> () { } 
}   

The current error message for E0186 only lets me partially resolve the issue:

error: method `fmt` has a `&self` declaration in the trait, but not in the impl [E0186]

It would be more friendly to the user if the message showed both the actual and expected method signatures.

Incorrect method signature for trait `Display` method `fmt`
Actual signature    fn fmt() -> ()
Expected signature  fn fmt(&self, &mut Formatter) -> Result<(), Error>

[nagisa]

I wanted similar behaviour occasionally as well. Looking signatures up in the docs is painful.

[birkenfeld]

related: #24626, which requests the same for missing methods in impls.

[estebank]

Related: #37370.

[Mark-Simulacrum]

Worth pointing out that the signature is relatively easy to get today if the method is removed. Still a problem though.