Allow attributes in logical parameter position

[Robbepop]

Allow attributes in logical parameter position like here

fn foonction(#[my_attribute] bar: u32) -> bool;

Current Situation

The compiler currently simply does not allow for attributes in this position:

error: expected pattern, found `#`
  --> file.rs:15:13
   |
15 |     pub fn foo(#![bar] capacity: usize) -> Self {
   |                ^ expected pattern

Rational

This could be very useful for some procedural macros which was effectively my use case yesterday.
Also I can imagine use cases for something like an #[ignore] attribute to say that a parameter is not in use which is currently implied by prefixing its name with an underscore. However, at least to me such an #[ignore] attribute (or come up with a better name for it) would be much more explicit which sticks better to the explicit nature of the language itself.

Other Sources

On march this year (2018) @petrochenkov was already talking about the usefulness of attributes for formal function parameters in this thread.

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If there is anybody who is free of time and wants to give it a try I would be very grateful. Otherwise I would try implementing it in my spare time.

[Robbepop]

@Centril created RFC: rust-lang/rfcs#2565

[Centril]

Closed therefore.