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unused-lifetimes lint false-positives with async functions #61115

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Seeker14491 opened this issue May 24, 2019 · 4 comments
Closed

unused-lifetimes lint false-positives with async functions #61115

Seeker14491 opened this issue May 24, 2019 · 4 comments

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@Seeker14491
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@Seeker14491 Seeker14491 commented May 24, 2019

Async functions that take references seem to always trigger the unused-lifetimes lint (which is allow-by-default). Example:

#![feature(async_await)]

#![warn(unused_lifetimes)]

pub async fn print(s: &str) {
    println!("{}", s);
}
warning: lifetime parameter `'_` never used
 --> src/lib.rs:5:23
  |
5 | pub async fn print(s: &str) {
  |                       ^
  |
@Seeker14491
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@Seeker14491 Seeker14491 commented May 24, 2019

I found this clippy issue which describes basically the same problem.

@nikomatsakis
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@nikomatsakis nikomatsakis commented Jun 4, 2019

Seeing as this lint is not enabled by default, marking as deferred, though this would be good to fix and probably not too hard.

bors added a commit to rust-lang/rust-clippy that referenced this issue Jul 23, 2019
Ignore generated fresh lifetimes in elision check

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fixes #3988

changelog: Ignore generated fresh lifetimes in elision check.

**HELP**: It seems `tests/ui` are compiled under edition 2015, and I don't know how to add tests for this properly.

Here is the test input it had already passed:
```rust
#![feature(async_await)]
#![allow(dead_code)]

async fn sink1<'a>(_: &'a str) {} // lint
async fn sink1_elided(_: &str) {} // ok

async fn one_to_one<'a>(s: &'a str) -> &'a str { s } // lint
async fn one_to_one_elided(s: &str) -> &str { s } // ok
async fn all_to_one<'a>(a: &'a str, _b: &'a str) -> &'a str { a } // ok
// async fn unrelated(_: &str, _: &str) {} // Not allowed in async fn

// #3988
struct Foo;
impl Foo {
    pub async fn foo(&mut self) {} // ok
}

// rust-lang/rust#61115
async fn print(s: &str) { // ok
    println!("{}", s);
}

fn main() {}

```
@gilescope
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@gilescope gilescope commented Sep 17, 2019

@rustbot claim

@gilescope
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@gilescope gilescope commented Sep 17, 2019

Looks the same issue as #64493.

A little inelegant but if

    !"5 | pub async fn print(s: &str) {".contains("'_")

we can suppress the lint I think...

Centril added a commit to Centril/rust that referenced this issue Oct 15, 2019
…r=matthewjasper

Reducing spurious unused lifetime warnings.

Fixes rust-lang#61115, fixes rust-lang#64493.
@bors bors closed this in fcef4b1 Oct 16, 2019
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