The help for deprecated panic_implementation attribute is bad

[nagisa]

warning: use of deprecated attribute `panic_implementation`: This attribute was renamed to `panic_handler`. See https://github.com/rust-lang/rust/issues/44489#issuecomment-415140224
  --> test.rs:13:1
   |
13 | #[panic_implementation]
   | ^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute
   |
   = note: #[warn(deprecated)] on by default

Namely,

   | ^^^^^^^^^^^^^^^^^^^^^^^ help: remove this attribute

will not help at all in resolving the issue. The help should either not exist at all or suggest replacement to #[panic_handler] (moving the suggestion from the warning itself).

[varkor]

If anyone'd like to take this on, it should be straightforward.

https://github.com/rust-lang/rust/blob/6bd2d5703d5944def902ec9dca6ea8aae07c8963/src/libsyntax/feature_gate.rs#L720-L724

I think the clearest way to address this is to modify the Deprecated type to have a second Option<&'static str> field, which is None if there is no suggested replacement and Some(replacement_feature) if the feature has been deprecated in favour of replacement_feature.

You'd then need to update the occurrences of Deprecated (in the case of panic_implementation, it needs Some("panic_handler") — others may also have sensible substitutes) and make this lint:
https://github.com/rust-lang/rust/blob/6bd2d5703d5944def902ec9dca6ea8aae07c8963/src/librustc_lint/builtin.rs#L793-L798
suggest replacing the feature attribute if a substitute exists.

Some tests might also need updating afterwards.

(Also, This attribute should be this attribute in the panic_implementation deprecation notice.)

[illfygli]

I would like to try this one!

[varkor]

@Snaedis: great! Feel free to ask here if you need any help with the fix!

[illfygli]

@varkor first stab is ready; feedback would be much appreciated!