transmute_ptr_to_ptr: False positive when using transmute in consts

[WaffleLapkin]

Using transmute in consts (recently stabilized) to cast one pointer to another raises transmute_ptr_to_ptr warning, despite the fact that dereferencing raw pointers in constants is unstable.

const _: &() = {
    struct ZST;
    let zst = &ZST;

    unsafe {
        core::mem::transmute::<&'static ZST, &'static ()>(zst)   
    }
};

warning: transmute from a reference to a reference
 --> src/lib.rs:6:9
  |
6 |         core::mem::transmute::<&'static ZST, &'static ()>(zst)   
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*(zst as *const _::ZST as *const ())`
  |
  = note: `#[warn(clippy::transmute_ptr_to_ptr)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#transmute_ptr_to_ptr

(playground)

const _: &() = {
    struct ZST;
    let zst = &ZST;

    unsafe {
        &*(zst as *const ZST as *const ())
    }
};

error[E0658]: dereferencing raw pointers in constants is unstable
 --> src/lib.rs:6:9
  |
6 |         &*(zst as *const ZST as *const ())
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #51911 <https://github.com/rust-lang/rust/issues/51911> for more information

(playground)

Meta

• cargo clippy -V: clippy 0.0.212 (cfbc6d4 2020-08-21)
• rustc -Vv:

  rustc 1.46.0-beta.5 (cfbc6d4cf 2020-08-21)
  binary: rustc
  commit-hash: cfbc6d4cf1acbac5c025acf8ed68929caa551df3
  commit-date: 2020-08-21
  host: x86_64-unknown-linux-gnu
  release: 1.46.0-beta.5
  LLVM version: 10.0