Suggests replacing &mut Vec with &mut [] (ptr_arg)

[ijackson]

Summary

ptr_arg ought only to trigger on &Vec, not &mut Vec. In general, if someone passes &mut SomeObject, they are likely to be meaning to give access to its methods.

Current stable seems to work fine.

Reproducer

I tried this code:

pub trait Body: Sized {
    fn encode_onto(self, w: &mut Vec<u8>);
}

struct Wombat;

impl Body for Wombat {
    fn encode_onto(self, w: &mut Vec<u8>) {
        w.push(0);
    }
}

fn main() {
    let mut w = vec![42];
    Wombat.encode_onto(&mut w);
    println!("{:?}", &w);
}

I expected to see this happen:

No complaints.

Instead, this happened:

nailing-cargo: invoking: cargo clippy --manifest-path=/home/ian/Rustup/Arti/experiments/Cargo.toml --locked --target-dir=target --offline
    Checking foo v0.1.0 (/home/ian/Rustup/Arti/experiments)
warning: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
 --> src/main.rs:3:29
  |
3 |     fn encode_onto(self, w: &mut Vec<u8>);
  |                             ^^^^^^^^^^^^ help: change this to: `&mut [u8]`
  |
  = note: `#[warn(clippy::ptr_arg)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

warning: `foo` (bin "foo") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
nailing-cargo: unnailed.  status 0.

Compilation finished at Wed Feb  2 18:01:56

Needless to say, the suggestion doesn't compile.

Version

rustcargo@zealot:/home/ian/Rustup/Arti/experiments$ rustc -Vv
rustc 1.60.0-nightly (a00e130da 2022-01-29)
binary: rustc
commit-hash: a00e130dae74a213338e2b095ec855156d8f3d8a
commit-date: 2022-01-29
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0
rustcargo@zealot:/home/ian/Rustup/Arti/experiments$

Additional Labels

@rustbot label +I-suggestion-causes-error

[ijackson]

@rustbot modify labels +regressoin-from-stable-to-nightly

[ijackson]

@rustbod modify labels +regession-from-stable-to-nightly

[ijackson]

@rustbot modify labels +regession-from-stable-to-nightly

this time for sure

[ijackson]

@rustbot modify labels +regression-from-stable-to-nightly

[xFrednet]

What label isn't listed as an allowed label in our configuration file, I didn't even knew we had it until now ^^

[Jarcho]

Trait functions will no longer suggest &mut changes as of #8369.

[ijackson]

Ah, cool, thanks. Closing this as a dupe of #8366