Drop guards in slice sorting derive src pointers from &mut T, which is invalidated by interior mutation in comparison

[saethlin]

I tried to run https://github.com/rust-lang/miri-test-libstd on alloc with -Zmiri-track-raw-pointers, and got a failure on the test slice::panic_safe. The test failure has nothing to do with panic safety, it's from how the test tests for panic safety.

I minimized the test failure into this very silly program:

use std::cell::Cell;
use std::cmp::Ordering;

#[derive(Clone)]
struct Evil(Cell<usize>);

fn main() {
    let mut input = vec![Evil(Cell::new(0)); 3];

    // Hits the bug pattern via CopyOnDrop in core
    input.sort_unstable_by(|a, _b| {
        a.0.set(0);
        Ordering::Less
    });

    // Hits the bug pattern via InsertionHole in alloc
    input.sort_by(|_a, b| {
        b.0.set(0);
        Ordering::Less
    });
}

To fix this, I'm just removing the mutability/uniqueness where it wasn't required.

[rust-highfive]

r? @yaahc

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[danielhenrymantilla]

Unnecessary muts indeed!

[m-ou-se]

@bors r=danielhenrymantilla

[bors]

📌 Commit a5a91c8 has been approved by danielhenrymantilla