needless_lifetimes removes lifetimes that are actually necessary

[AlfredAn]

Summary

The lifetimes seem to be unavoidable in the function I defined, but clippy still suggests removing them. If I run cargo clippy --fix, the resulting code fails to compile.

Lint Name

needless_lifetimes

Reproducer

I tried this code:

#![allow(dead_code)]

use std::collections::hash_map::{Entry, HashMap};

use petgraph::matrix_graph::{NodeIndex, UnMatrix};
use winnow::{
    ascii::line_ending,
    ascii::{alpha1, dec_uint},
    combinator::{opt, separated, seq, terminated},
    error::ParserError,
    PResult, Parser,
};

pub fn lines<'a, Output, Error>(
    parser: impl Parser<&'a str, Output, Error>,
) -> impl Parser<&'a str, Vec<Output>, Error>
where
    Error: ParserError<&'a str>,
{
    terminated(separated(.., parser, line_ending), opt(line_ending))
}

type G<'s> = UnMatrix<&'s str, u32>;

fn edge<'s>(input: &mut &'s str) -> PResult<(&'s str, &'s str, u32)> {
    seq!(
        alpha1,
        _: " to ",
        alpha1,
        _: " = ",
        dec_uint
    )
    .parse_next(input)
}

fn parse<'s>(input: &'s str) -> G<'s> {
    lines(edge)
        .map(|edges| {
            let mut g = G::default();

            let mut nodes = HashMap::<&'s str, NodeIndex<u16>>::default();
            let mut node = |g: &mut G<'s>, tag: &'s str| match nodes.entry(tag) {
                Entry::Occupied(e) => *e.get(),
                Entry::Vacant(e) => *e.insert(g.add_node(tag)),
            };

            for (a, b, weight) in edges {
                let a = node(&mut g, a);
                let b = node(&mut g, b);
                g.add_edge(a, b, weight);
            }

            g
        })
        .parse(input)
        .unwrap()
}

I saw this happen:

rygno@Alfred-Desktop:~/aoc/test_clippy$ cargo clippy --fix --allow-dirty
    Checking test_clippy v0.1.0 (/home/rygno/aoc/test_clippy)
warning: failed to automatically apply fixes suggested by rustc to crate `test_clippy`

after fixes were automatically applied the compiler reported errors within these files:

  * test_clippy/src/lib.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust-clippy/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error[E0261]: use of undeclared lifetime name `'s`
  --> test_clippy/src/lib.rs:41:40
   |
36 | fn parse(input: &str) -> G<'_> {
   |         - help: consider introducing lifetime `'s` here: `<'s>`
...
41 |             let mut nodes = HashMap::<&'s str, NodeIndex<u16>>::default();
   |                                        ^^ undeclared lifetime

error[E0261]: use of undeclared lifetime name `'s`
  --> test_clippy/src/lib.rs:42:39
   |
36 | fn parse(input: &str) -> G<'_> {
   |         - help: consider introducing lifetime `'s` here: `<'s>`
...
42 |             let mut node = |g: &mut G<'s>, tag: &'s str| match nodes.entry(tag) {
   |                                       ^^ undeclared lifetime

error[E0261]: use of undeclared lifetime name `'s`
  --> test_clippy/src/lib.rs:42:50
   |
36 | fn parse(input: &str) -> G<'_> {
   |         - help: consider introducing lifetime `'s` here: `<'s>`
...
42 |             let mut node = |g: &mut G<'s>, tag: &'s str| match nodes.entry(tag) {
   |                                                  ^^ undeclared lifetime

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0261`.
Original diagnostics will follow.

warning: the following explicit lifetimes could be elided: 's
  --> test_clippy/src/lib.rs:36:10
   |
36 | fn parse<'s>(input: &'s str) -> G<'s> {
   |          ^^          ^^           ^^
   |
   = 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
   |
36 - fn parse<'s>(input: &'s str) -> G<'s> {
36 + fn parse(input: &str) -> G<'_> {
   |

warning: `test_clippy` (lib) generated 1 warning (run `cargo clippy --fix --lib -p test_clippy` to apply 1 suggestion)
warning: failed to automatically apply fixes suggested by rustc to crate `test_clippy`

after fixes were automatically applied the compiler reported errors within these files:

  * test_clippy/src/lib.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust-clippy/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error[E0261]: use of undeclared lifetime name `'s`
  --> test_clippy/src/lib.rs:41:40
   |
36 | fn parse(input: &str) -> G<'_> {
   |         - help: consider introducing lifetime `'s` here: `<'s>`
...
41 |             let mut nodes = HashMap::<&'s str, NodeIndex<u16>>::default();
   |                                        ^^ undeclared lifetime

error[E0261]: use of undeclared lifetime name `'s`
  --> test_clippy/src/lib.rs:42:39
   |
36 | fn parse(input: &str) -> G<'_> {
   |         - help: consider introducing lifetime `'s` here: `<'s>`
...
42 |             let mut node = |g: &mut G<'s>, tag: &'s str| match nodes.entry(tag) {
   |                                       ^^ undeclared lifetime

error[E0261]: use of undeclared lifetime name `'s`
  --> test_clippy/src/lib.rs:42:50
   |
36 | fn parse(input: &str) -> G<'_> {
   |         - help: consider introducing lifetime `'s` here: `<'s>`
...
42 |             let mut node = |g: &mut G<'s>, tag: &'s str| match nodes.entry(tag) {
   |                                                  ^^ undeclared lifetime

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0261`.
Original diagnostics will follow.

warning: `test_clippy` (lib test) generated 1 warning (1 duplicate)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s

I expected to see this happen:
No suggestions, as the lifetime annotation is necessary here as far as I know.

Version

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-unknown-linux-gnu
release: 1.78.0
LLVM version: 18.1.2

Additional Labels

@rustbot label +l-suggestion-causes-error

[y21]

Looks like the lint's logic for checking if a lifetime is referred to in the function body stops at closures (probably a visitor not configured to enter nested bodies).

Minimized repro:

fn f<'a>(s: &'a str) {
  || { let _: &'a str; };
}

[m-rph]

Hey @y21 I have a bunch of other issues on needless_lifetimes in my backlog that I am currently working on, do you mind if I take this?

[y21]

Feel free to take it, I wasn't planning to work on it (am currently fairly busy with other things). Just decided to minimize this since the original reproducer was not minimal and relied on external types/deps