Fix min_ident_chars: ignore on trait impl.

[t-webber]

fixes #13396

changelog: [min_ident_chars]: ignore lint when implementing a trait, to respect [renamed_function_params]

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[github-actions]

Lintcheck changes for baded8c

Lint	Added	Removed	Changed
clippy::min_ident_chars	0	2453	39

This comment will be updated if you push new changes

[samueltardieu]

For a local trait, it should warn about the short parameter name in the trait method definition. I don't see a test for this, and I suspect it won't warn. To suppress warnings about trait implementations, we should ensure it warns at the trait definition so that users are warned at least once about the usage of short identifiers.

[t-webber]

Yes, I didn't look at that case, and it seems it never fired the lint even before my PR:

#![allow(dead_code)]
#![warn(clippy::min_ident_chars)]

trait Trait {
    fn func(x: u32) -> u32;
}

struct Struct;

impl Trait for Struct {
    fn func(x: u32) -> u32 {
        x
    }
}

fn main() {}

This code doesn't produce any warnings.

For functions with one char, it works though, and it warns on trait definition only, so I guess I should do the same thing for one-char-long params