The allow-by-default lint `missing_copy_implementations` does not honor `!Copy` impls

[fmease]

I would expect the following code to compile successfully since we explicitly opt out of implementing Copy:

#![feature(negative_impls)]
#![deny(missing_copy_implementations)]
#![crate_type = "lib"]

pub struct Struct {
    pub field: i32,
}

impl !Copy for Struct {}

However, the code fails to compile with the following error message:

error: type could implement `Copy`; consider adding `impl Copy`
 --> src/lib.rs:5:1
  |
5 | / pub struct Struct {
6 | |     pub field: i32,
7 | | }
  | |_^
  |
note: the lint level is defined here
 --> src/lib.rs:2:9
  |
2 | #![deny(missing_copy_implementations)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

@rustbot label C-bug A-lint F-negative_impls

[fmease]

The compiler only checks if the type implements Copy already to return early and not emit the lint diagnostic but it doesn't check if the type implements !Copy: https://github.com/rust-lang/rust/blob/4af79ccd5e23c7cbaabcd7aefdda3b715abde606/compiler/rustc_lint/src/builtin.rs#L753

[vincenzopalazzo]

I think this makes the trick, you are right! the last think is to explore what is the code of the method

https://github.com/rust-lang/rust/blob/9b0a099dfc9a97ecd10adb319396c731c4b2d169/compiler/rustc_middle/src/ty/util.rs#L824-L830

The code of the method looks not so trivial :)

https://github.com/rust-lang/rust/blob/32717603f61a566ff0b8293ef3177cb7c4f50fa9/compiler/rustc_middle/src/ty/sty.rs#L2201-L2245

If you did not find already I can play with your on it by mentoring you, I had some experience in "git archeology" :)

@rustbot label +E-mentor

[Kohei316]

@rustbot claim