`mismatched_lifetime_syntaxes` should treat &T and ContainsLifetime as equivalent

[stevenroose]

Problem

warning: hiding a lifetime that's elided elsewhere is confusing
   --> lib/src/connectors.rs:175:20
    |
175 |     pub fn connectors(&self) -> ConnectorIter {
    |                       ^^^^^     ------------- the same lifetime is hidden here
    |                       |
    |                       the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
help: use `'_` for type paths
    |
175 |     pub fn connectors(&self) -> ConnectorIter<'_> {
    |                                              ++++

I just tried to upgrade to Rust v1.89.0 and suddenly I see various of these compiler warnings.

The argument it seems to make is that "hiding" a lifetime is not the same as "eliding" it. However, in both cases, the lifetime is hidden.

Both &self and ConnectorIter hide the lifetime, implicitly eliding all possible lifetimes it could have.

The warning suggests to explicitly elide the lifetime on the return value, ConnectorIter<'_>, but it doesn't recommend also explicitly eliding it on self: f.e. &'_ self. It seems that this lint is quite broken then.

I can agree that having &'a self and a hidden lifetime on the return type is confusing and could be linted, but in the example I gave, both ref and return type hide (and hence elide) the lifetime, which should be no problem.

Steps

pub struct HasRef<'a>(pub &'a str);

pub fn return_has_ref(_v: &str) -> HasRef { todo!() }

Possible Solution(s)

Consider &self as a hidden lifetime just as hiding the lifetime of a struct. Or disable mismatched_lifetime_syntaxes by default.

Notes

No response

Version

1.89.0

[shepmaster]

pub fn connectors(&self) -> ConnectorIter is exactly the case that the lint was created to warn against, so I'm pretty sure that this issue should be closed as "code working as intended".

In more detail, the language team declared that there are three groups, one group named "elided" and one group named "hidden" (and yes, "hidden" lifetimes still participate in lifetime elision).

[stevenroose]

I strongly disagree with the decision to introduce this lint. There is absolutely nothing that can possible go wrong with a signature like this pub fn connectors(&self) -> ConnectorIter. If all lifetimes in a signature are either elided or hidden, there is nothing that can be confusing about it because a hidden lifetime is automatically also elided.

More generally, though, I would also like to use this opportunity to voice my strong opposition to introducing more opinionated linting in rustc. I disagree with many of clippy or rust-analyzer's default lints (let alone rustfmt format), but at least these are optional tools, so it is ok if they are opinionated. Compiler warnings like unused variables make sense, as they almost always indicate an error. But opinionated linting does not belong in a compiler.