Regression of #53738: spurious single_use_lifetimes warning

[stouset]

Issue #53738 seems to have regressed.

I tried this code:

#[deny(single_use_lifetimes)]

#[derive(Eq)]
struct Foo<'a, T> {
    /// a reference to the underlying secret data that will be derefed
    pub data: &'a mut T,
}

I expected to see this happen: No warning should be produced.

Instead, this happened:

error: lifetime parameter `'a` only used once
 --> src/lib.rs:4:12
  |
3 | #[derive(Eq)]
  |          -- ...is used only here
4 | struct Foo<'a, T> {
  |            ^^ this lifetime...
  |
note: lint level defined here
 --> src/lib.rs:1:8
  |
1 | #[deny(single_use_lifetimes)]
  |        ^^^^^^^^^^^^^^^^^^^^

Meta

rustc --version --verbose:

rustc 1.42.0 (b8cedc004 2020-03-09)
binary: rustc
commit-hash: b8cedc00407a4c56a3bda1ed605c6fc166655447
commit-date: 2020-03-09
host: x86_64-apple-darwin
release: 1.42.0
LLVM version: 9.0

[jonas-schievink]

From what I can tell, the Eq case might have never worked, since Debug still doesn't warn (so the linked issue specifically maybe didn't regress, only a closely related example incorrectly warns). This needs confirmation though.

[cuviper]

It also fails in 1.40 and 1.41.

It gets a different error in 1.39, with Eq complaining that PartialEq is not implemented, but if you add that then it does compile successfully.

[spastorino]

@rustbot ping cleanup