Lint request: Prevent lifetimes that are only used for function output

[KamilaBorowska]

The following:

fn lifetime<'a>() -> &'a str {
    "Why some people do that"
}

Can be more clearly written as this. Also, using 'static lifetime makes it clear that it was intended, especially in unsafe code where returning unbound lifetime may as well be a mistake.

fn lifetime() -> &'static str {
    "Why some people do that"
}

[clarfonthey]

In the standard library the opposite is true; people tend to prefer the former style to the latter.

[oli-obk]

Yea, the static lifetime might mess with some inference around multiple lifetimes. If this lint is deemed too controversial, it can always become a restriction lint or allow-by-default

[clarfonthey]

I should also add that this only works for 'static types; for non-'static types you have to use the former style to get the same effect.

[KamilaBorowska]

Oh, there is a difference. What about making this a case only for public non-unsafe functions where this is definitely a mistake.

[felix91gr]

Where are we with regards to this lint?

What about making this a case only for public non-unsafe functions where this is definitely a mistake.

If this is what we want, I reckon I can probably implement it :)

[oli-obk]

The only way we can implement it without false positives is to check that the return type "has a 'static" bound, which I'm not sure how to check reliably. I'm not sure if the complexity required for this lint is worth it considering the minor improvement in API that it brings.

[felix91gr]

Should there be a needs-discussion label in this issue then, @oli-obk? Maybe we can close this one if the complexity is indeed too much for the improvement it brings :)

[clarfonthey]

I personally think that a means to not have both -> &'b T and <'a: 'b> -> &'a T (the static case is just 'b: 'static, which would make the 'a: 'static bound redundant). The only cases I can think of where you can't explicitly name 'b is when you're creating a type inside a block, which should be relatively easy to check for and bail out of linting.

Dunno if it's a good idea to toss this yet, I just think that it's not something that is high on the priority list.

[camsteffen]

Similar to #305