False positive wrong_self_convention inside trait impls

[dtolnay]

Beginning with the most recent nightly (clippy 0.1.53 5e65467 2021-03-26, rustc 1.53.0-nightly 5e65467 2021-03-26) the following code incorrectly triggers wrong_self_convention. The trait method signature is controlled by the trait; it doesn't matter that the impl is for a type implementing Copy, that doesn't mean that the trait can or should take self by value. In particular there are likely other impls which are not Copy.

pub struct Thing;

pub trait Trait {
    fn to_thing(&self) -> Thing;
}

impl Trait for u8 {
    fn to_thing(&self) -> Thing {
        Thing
    }
}

warning: methods with the following characteristics: (`to_*` and `self` type is `Copy`) usually take `self` by value
 --> src/main.rs:8:17
  |
8 |     fn to_thing(&self) -> Thing {
  |                 ^^^^^
  |
  = note: `#[warn(clippy::wrong_self_convention)]` on by default
  = help: consider choosing a less ambiguous name
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention

Mentioning @mgacek8 @llogiq who touched this lint recently in #6924.

[dtolnay]

Mentioning @matthiaskrgr too since I am surprised this was not caught by b9a7a2a -- all three of the above affected crates (quote, syn, cxx) are in lintcheck_crates.

rust-clippy/lintcheck/lintcheck_crates.toml

Line 17 in 6f2a6fe

quote = {name = "quote", versions = ['1.0.7']}

rust-clippy/lintcheck/lintcheck_crates.toml

Line 21 in 6f2a6fe

syn = {name = "syn", versions = ['1.0.54']}

rust-clippy/lintcheck/lintcheck_crates.toml

Line 26 in 6f2a6fe

cxx = {name = "cxx", versions = ['1.0.32']}

[llogiq]

Oops. I was under the impression that methods taking &self should be named as_* instead of to_.

[extrawurst]

this is even more problematic because the trait ToString in std expects me to pass self as &self in to_string and now clippy starts complaining about that :)