bindings_with_variant_name is unhelpful and incorrect for function arguments

[RDambrosio016]

Given the following code:

#![allow(non_camel_case_types, unused_variables, dead_code)]

enum Foo {
    c
}

fn foo(c: Foo) {}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=e6829290d29151853fdd98844ba6391c

The current output is:

warning[E0170]: pattern binding `c` is named the same as one of the variants of the type `Foo`
 --> src/lib.rs:7:8
  |
7 | fn foo(c: Foo) {}
  |        ^ help: to match on the variant, qualify the path: `Foo::c`
  |
  = note: `#[warn(bindings_with_variant_name)]` on by default

Ideally the lint should not trigger on such a pattern, it does not really make sense and the suggestion is wrong altogether,

[RDambrosio016]

@rustbot label: +D-confusing +D-incorrect

[nbdd0121]

Note that the suggestion isn't wrong in this particular case, because Foo::c can actually match any Foo. It's wrong only when Foo has 2+ variants.

[RDambrosio016]

Right, the real world case of this was with the ascii crate with the AsciiChar enum, this is just a tiny test case