const fn with `_` as an arg name vomits

[Gankra]

Doesn't work:

#![feature(const_fn)]
fn main(){}
const fn size_ofs(_: usize) { }

<anon>:3:19: 3:20 error: arguments of constant functions can only be immutable by-value bindings [E0022]
<anon>:3 const fn size_ofs(_: usize) { }
                           ^
<anon>:3:19: 3:20 help: see the detailed explanation for E0022
error: aborting due to previous error
playpen: application terminated with error code 101

---

Works:

#![feature(const_fn)]
fn main(){}
const fn size_ofs(_foo: usize) { }

<anon>:3:1: 3:35 warning: function is never used: `size_ofs`, #[warn(dead_code)] on by default
<anon>:3 const fn size_ofs(_foo: usize) { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[Manishearth]

Willing to mentor this.

Fix is simple, just need to check for PatWild where E0022 is emitted. We should also add a run-pass test to ensure that the above code continues to compile.

Feel free to ask me if you have further questions!

[GlenDC]

I've been willing for a long time to start getting involved in some contributions to Rust, so if it's ok for you I would like to give this one a go and make it my first contribution.

[Manishearth]

Go ahead!