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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 · Fixed by #64603
Closed

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

Seeker14491 opened this issue May 24, 2019 · 4 comments · Fixed by #64603
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@gilescope
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A-async-await Area: Async & Await A-lifetimes Area: Lifetimes / regions A-lint Area: Lints (warnings about flaws in source code) such as unused_mut. AsyncAwait-Polish Async-await issues that are part of the "polish" area AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@Seeker14491
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Seeker14491 commented May 24, 2019 •
edited by rustbot
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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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I found this clippy issue which describes basically the same problem.

@jonas-schievink jonas-schievink added A-async-await Area: Async & Await A-lint Area: Lints (warnings about flaws in source code) such as unused_mut. A-lifetimes Area: Lifetimes / regions C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 24, 2019
@nikomatsakis nikomatsakis added the AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. label Jun 4, 2019
@nikomatsakis
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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
@bors
Auto merge of #4266 - uHOOCCOOHu:fix/async_fn_lifetime, r=flip1995
d71e9c4 
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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@rustbot claim

@gilescope
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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...

@gilescope gilescope mentioned this issue Sep 19, 2019
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@varkor varkor mentioned this issue Sep 19, 2019
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@nikomatsakis nikomatsakis added AsyncAwait-Polish Async-await issues that are part of the "polish" area AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. and removed AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. labels Oct 1, 2019
Centril added a commit to Centril/rust that referenced this issue Oct 15, 2019
@Centril
Rollup merge of rust-lang#64603 - gilescope:unused-lifetime-warning, …
c1f51b6 
…r=matthewjasper

Reducing spurious unused lifetime warnings.

Fixes rust-lang#61115, fixes rust-lang#64493.
@bors bors closed this as completed in fcef4b1 Oct 16, 2019
@Hawk777 Hawk777 mentioned this issue Sep 26, 2020
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Labels
A-async-await Area: Async & Await A-lifetimes Area: Lifetimes / regions A-lint Area: Lints (warnings about flaws in source code) such as unused_mut. AsyncAwait-Polish Async-await issues that are part of the "polish" area AsyncAwait-Triaged Async-await issues that have been triaged during a working group meeting. C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Reducing spurious unused lifetime warnings. gilescope/rust
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@nikomatsakis @gilescope @Centril @jonas-schievink @Seeker14491