const_item_mutation incorrectly fires when using raw pointers

[jonas-schievink]

I tried this code:

fn main() {
    const PTR: *mut () = 0x8000_0000 as *mut _;
    
    unsafe { &mut *PTR };
}

I expected to see this happen: This should compile without warnings, as I am just turning the 0x8000_0000 raw pointer into a mutable reference.

Instead, this happened:

warning: taking a mutable reference to a `const` item
 --> src/main.rs:4:14
  |
4 |     unsafe { &mut *PTR };
  |              ^^^^^^^^^
  |
  = note: `#[warn(const_item_mutation)]` on by default
  = note: each usage of a `const` item creates a new temporary
  = note: the mutable reference will refer to this temporary, not the original `const` item
note: `const` item defined here
 --> src/main.rs:2:5
  |
2 |     const PTR: *mut () = 0x8000_0000 as *mut _;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The lint claims that I'm "taking a mutable reference to a const item", but I'm not – the const item is the reference.

This warning started popping up a lot for the cortex-m crate, which uses this pattern heavily.

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This is on current nightly (2020-11-05).