Removing return not sugggested by needless_return when calling some chained string methods

[cyrgani]

Summary

When having a return statement with a call to str::trim, the needless_return list does not warn. Replacing trim withlen in the following example correctly warns.

It seems to happen with other borrowing methods as well, such as in the following example:

fn f() -> String {
	return "".split("").next().unwrap().to_string();
}

Lint Name

needless_return

Reproducer

I tried this code:

fn main(){
	f();
}
fn f() -> String {
	return "".trim().to_string();
}

I expected to see this happen:

warning: unneeded `return` statement
 --> src/main.rs:5:2
  |
5 |     return "".trim().to_string();
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_return
  = note: `#[warn(clippy::needless_return)]` on by default
help: remove `return`
  |
5 -     return "".trim().to_string();
5 +     "".trim().to_string()
  |

Instead, this happened:

No output

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

[apoisternex]

@rustbot claim

[y21]

We do need to be careful here because allowing these cases can sometimes also end up as false positives when temporaries are involved, e.g.:

fn f() -> i32 {
  let r = RefCell::new(42);
  return *r.borrow();
}

Trying to replace that return <expr>; with simply <expr> results in a compile error because the temporary that references the local RefCell gets dropped after the local itself

(Though it should be possible to relax the heuristic in the lint that prevents that while stile allowing the repro in the OP)