ptr_arg triggers on &mut String despite clone_into

[ronnodas]

Summary

If a &mut String argument is only used as the input to clone_into, then ptr_arg triggers erroneously.

Lint Name

ptr_arg

Reproducer

I tried this code:

fn foo(source: &str, destination: &mut String)  {
    source.clone_into(destination)
}

fn main() {
    foo("bar", &mut String::new());
}

I saw this happen:

warning: writing `&mut String` instead of `&mut str` involves a new object where a slice will do
 --> src/main.rs:1:35
  |
1 | fn foo(source: &str, destination: &mut String)  {
  |                                   ^^^^^^^^^^^ help: change this to: `&mut str`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg
  = note: `#[warn(clippy::ptr_arg)]` on by default

I expected to see this happen:

Nothing

Version

rustc 1.80.1 (3f5fd8dd4 2024-08-06)
binary: rustc
commit-hash: 3f5fd8dd41153bc5fdca9427e9e05be2c767ba23
commit-date: 2024-08-06
host: x86_64-unknown-linux-gnu
release: 1.80.1
LLVM version: 18.1.7

Additional Labels

@rustbot label +I-suggestion-causes-error

[rustbot]

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