redundant_slicing false positive on bytes::buf::UninitSlice move

[roblabla]

Lint name: redundant_slicing

While updating rust to 1.51, we hit a false positive on one of the new lints. Here's a minimal reproducer:

use bytes::*; // 1.0.1

fn main() {
    let mut storage = &mut [0u8; 512][..];
    let bytes = storage.chunk_mut();

    {
        let mut _bytes_msg = &mut bytes[..];
    }

    println!("{:?}", bytes);    
}

I expected to see this happen: No lint on the slicing at &mut bytes[..], as it is required: instead of moving the bytes variable to _bytes_msg, thus shortening its lifetime, it will instead create a new borrow with its own lifetime inside of _bytes_msg.

Instead, this happened: Clippy complained about redundant slicing:

warning: redundant slicing of the whole range
 --> src/main.rs:8:30
  |
8 |         let mut _bytes_msg = &mut bytes[..];
  |                              ^^^^^^^^^^^^^^ help: use the original slice instead: `bytes`
  |
  = note: `#[warn(clippy::redundant_slicing)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_slicing

warning: 1 warning emitted

Note that this does not reproduce when using a normal &mut [u8] slice. But bytes.chunk_mut() returns its own slice-like object UninitSlice, on which this rule triggers.

Meta

• cargo clippy -V: 0.1.52 (2021-03-24 07e0e2e)
• rustc -Vv:

  rustc 1.51.0 (2fd73fabe 2021-03-23)
  binary: rustc
  commit-hash: 2fd73fabe469357a12c2c974c140f67e7cdd76d0
  commit-date: 2021-03-23
  host: x86_64-unknown-linux-gnu
  release: 1.51.0
  LLVM version: 11.0.1
  

[koivunej]

Reproduction with a static slice: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6f42468aa0f8251ede82bb902904053c

With a workaround: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f4268028fa191c00285bc8caeec8793e

[Jarcho]

I'll deal with this later today.

@rustbot claim

[Jarcho]

This isn't a false positive, but a bad suggestion. Correct code is:

use bytes::*; // 1.0.1

fn main() {
    let mut storage = &mut [0u8; 512][..];
    let bytes = storage.chunk_mut();

    {
        let mut _bytes_msg = &mut *bytes;
    }

    println!("{:?}", bytes);    
}

Basically the indexing is being used to reborrow the reference.