FP: needless_lifetimes

[beyarkay]

Summary

I'm getting an error about unnecessary lifetimes, but when I action the suggestion, rustc throws an error.

Lint Name

needless_lifetimes

Reproducer

I tried this code:

// #[allow(clippy::needless_lifetimes)]
fn iterate_hashtags<'a>(txt: &'a str) -> impl Iterator<Item = (&'a str, &'a str)> {
    txt.split('#')
        .filter(|item| !item.is_empty())
        .filter_map(move |item| {
            item.find(' ')
                .map(|byte_of_first_space| item.split_at(byte_of_first_space))
        })
}

fn main() {
    let str_with_hashtags = "#thing this is a description of the thing\n#otherthing another thingy\nthat spans\nmultiple lines\n#hashtag hash taggery inception";
        for (hashtag, line) in iterate_hashtags(str_with_hashtags) {
            println!("{}: `{}`", hashtag, line);
        }
}

I saw this happen:

╰→ cargo clippy
warning: the following explicit lifetimes could be elided: 'a
  --> src/main.rs:14:21
   |
14 | fn iterate_hashtags<'a>(txt: &'a str) -> impl Iterator<Item = (&'a str, &'a str)> {
   |                     ^^        ^^                                ^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `#[warn(clippy::needless_lifetimes)]` on by default
help: elide the lifetimes
   |
14 - fn iterate_hashtags<'a>(txt: &'a str) -> impl Iterator<Item = (&'a str, &'a str)> {
14 + fn iterate_hashtags(txt: &str) -> impl Iterator<Item = (&'_ str, &'a str)> {
   |

Which implies the following code should work:

// #[allow(clippy::needless_lifetimes)]
fn iterate_hashtags(txt: &str) -> impl Iterator<Item = (&'_ str, &'a str)> {
    txt.split('#')
        .filter(|item| !item.is_empty())
        .filter_map(move |item| {
            item.find(' ')
                .map(|byte_of_first_space| item.split_at(byte_of_first_space))
            //.map(|split| (split.0.trim(), split.1.trim()))
            //.filter(|split| !split.1.is_empty())
        })
}

But the above function throws this error:

╰→ cargo c
    Checking boyd-bot v0.1.0 (/Users/brk/projects/boyd-bot)
error[E0261]: use of undeclared lifetime name `'a`
  --> src/main.rs:14:67
   |
14 | fn iterate_hashtags(txt: &str) -> impl Iterator<Item = (&'_ str, &'a str)> {
   |                                                                   ^^ undeclared lifetime
   |
   = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'a` lifetime
   |
14 | fn iterate_hashtags(txt: &str) -> impl for<'a> Iterator<Item = (&'_ str, &'a str)> {
   |                                        +++++++
help: consider introducing lifetime `'a` here
   |
14 | fn iterate_hashtags<'a>(txt: &str) -> impl Iterator<Item = (&'_ str, &'a str)> {
   |                    ++++

error: lifetime may not live long enough
  --> src/main.rs:15:5
   |
14 |   fn iterate_hashtags(txt: &str) -> impl Iterator<Item = (&'_ str, &'a str)> {
   |                            - let's call the lifetime of this reference `'1`
15 | /     txt.split('#')
16 | |         .filter(|item| !item.is_empty())
17 | |         .filter_map(move |item| {
18 | |             item.find(' ')
...  |
21 | |             //.filter(|split| !split.1.is_empty())
22 | |         })
   | |__________^ returning this value requires that `'1` must outlive `'static`
   |
help: to declare that `impl Iterator<Item = (&str, &str)>` captures data from argument `txt`, you can add an explicit `'_` lifetime bound
   |
14 | fn iterate_hashtags(txt: &str) -> impl Iterator<Item = (&'_ str, &'a str)> + '_ {
   |                                                                            ++++

Version

rustc 1.75.0 (82e1608df 2023-12-21)
binary: rustc
commit-hash: 82e1608dfa6e0b5569232559e3d385fea5a93112
commit-date: 2023-12-21
host: aarch64-apple-darwin
release: 1.75.0
LLVM version: 17.0.6

I updated clippy in case that was the issue, but I'm on clippy 0.1.75 (82e1608d 2023-12-21) and it's still a problem.

Additional Labels

@rustbot label +l-suggestion-causes-error

[m-rph]

@rustbot claim

[m-rph]

Given that the same lifetime is given, to all refs, these could be elided completely, which suggests that the code doesn't consider all lifetimes, perhaps this is related to nesting?

Edit: I had asked Samuel whether they are familiar with the codebase and wanted to tackle this, but deleted the comment.

[samueltardieu]

Note that this is not a false positive: the lint should be triggered indeed, but the suggestion is wrong because it forgets to replace the latest &'a. Also, I'm surprised it suggests &'_ str in the output instead of the simpler &str.

@rustbot label +I-suggestion-causes-error -I-false-positive

[samueltardieu]

Please go ahead, I don't specially know this part of the codebase.

[m-rph]

Closing as duplicate. This appears to be the same issue as #12291.