Bad suggestion for &mut *(&mut &mut T) from clippy::needless_borrow

[SarthakSingh31]

Summary

clippy::needless_borrow suggested a change that only works if the compiler was going to reborrow the reference or the reference could be moved/copied. Since neither conditions were true the suggested code produced an error.

Reproducer

I originally encounter the issue here
https://github.com/bevyengine/bevy/blob/601cc0cbe3dc73c0db1993184f4ec47783e83884/crates/bevy_ecs/src/bundle.rs#L492

A reduced version:

struct Inner {
    num: u8,
}

enum Outer<'a> {
    InnerBorrowed(&'a mut Inner),
    _InnerOwned(Inner),
}

impl<'a> Outer<'a> {
    fn get_inner_mut(&mut self) -> &mut Inner {
        let inner = match self {
            Outer::InnerBorrowed(inner) => &mut *inner,
            Outer::_InnerOwned(inner) => inner,
        };
        inner.num += 1;
        inner
    }
}

fn main() {
    let mut inner = Inner { num: 0 };
    let mut outer = Outer::InnerBorrowed(&mut inner);
    let _inner_inside_outer = outer.get_inner_mut();
}

I expected to see this happen:

cargo clippy --fix should change Outer::InnerBorrowed(inner) => &mut *inner, into Outer::InnerBorrowed(inner) => inner,

Instead, this happened:

cargo clippy --fix tries to change Outer::InnerBorrowed(inner) => &mut *inner, into Outer::InnerBorrowed(inner) => *inner, and then produces the following console output

Console Output

❯ cargo clippy --fix                                 
    Checking test-234234 v0.1.0 (/extra/Projects/test)
warning: failed to automatically apply fixes suggested by rustc to crate `test_234234`

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

* src/main.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/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[E0507]: cannot move out of `*inner` which is behind a mutable reference
--> src/main.rs:13:44
|
13 |             Outer::InnerBorrowed(inner) => *inner,
|                                            ^^^^^^
|                                            |
|                                            move occurs because `*inner` has type `&mut Inner`, which does not implement the `Copy` trait
|                                            help: consider borrowing here: `&*inner`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0507`.
Original diagnostics will follow.

warning: this expression borrows a reference (`&mut Inner`) that is immediately dereferenced by the compiler
--> src/main.rs:13:44
|
13 |             Outer::InnerBorrowed(inner) => &mut *inner,
|                                            ^^^^^^^^^^^ help: change this to: `*inner`
|
= note: `#[warn(clippy::needless_borrow)]` on by default
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

warning: `test-234234` (bin "test-234234") generated 1 warning (1 duplicate)
warning: `test-234234` (bin "test-234234" test) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.44s

Version

rustc 1.59.0-nightly (f8abed9ed 2021-12-26)
binary: rustc
commit-hash: f8abed9ed48bace6be0087bcd44ed534e239b8d8
commit-date: 2021-12-26
host: x86_64-unknown-linux-gnu
release: 1.59.0-nightly
LLVM version: 13.0.0

Additional Labels

@rustbot label +I-suggestion-causes-error