Incorrect suggestion for ptr_eq warning leads to vtable_address_comparisons error

Suppose we start with the following code:

use std::rc::Rc;
pub trait Trait {}
pub fn eq(a: Rc<dyn Trait>, b: Rc<dyn Trait>) -> bool {
    Rc::ptr_eq(&a, &b)
}

error: comparing trait object pointers compares a non-unique vtable address
 --> src/lib.rs:4:5
  |
4 |     Rc::ptr_eq(&a, &b)
  |     ^^^^^^^^^^^^^^^^^^
  |
  = note: `#[deny(clippy::vtable_address_comparisons)]` on by default
  = help: consider extracting and comparing data pointers only
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#vtable_address_comparisons

Okay, let’s do that:

pub fn eq(a: Rc<dyn Trait>, b: Rc<dyn Trait>) -> bool {
    &*a as *const dyn Trait as *const u8 == &*b as *const dyn Trait as *const u8
}

warning: use `std::ptr::eq` when comparing raw pointers
 --> src/lib.rs:4:5
  |
4 |     &*a as *const dyn Trait as *const u8 == &*b as *const dyn Trait as *const u8
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: try: `std::ptr::eq(&*a as *const dyn Trait, &*b as *const dyn Trait)`
  |
  = note: `#[warn(clippy::ptr_eq)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_eq

warning: 1 warning emitted

This suggestion is wrong. The cast to *const u8 is important, and removing it has led back to the first error:

pub fn eq(a: Rc<dyn Trait>, b: Rc<dyn Trait>) -> bool {
    std::ptr::eq(&*a as *const dyn Trait, &*b as *const dyn Trait)
}

error: comparing trait object pointers compares a non-unique vtable address
 --> src/lib.rs:4:5
  |
4 |     std::ptr::eq(&*a as *const dyn Trait, &*b as *const dyn Trait)
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[deny(clippy::vtable_address_comparisons)]` on by default
  = help: consider extracting and comparing data pointers only
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#vtable_address_comparisons

Meta

• cargo clippy -V: clippy 0.0.212 (0b644e4 2020-12-26)
• rustc -Vv:

  rustc 1.51.0-nightly (0b644e419 2020-12-26)
  binary: rustc
  commit-hash: 0b644e419681835bd0f5871c3bfbd648aa04f157
  commit-date: 2020-12-26
  host: x86_64-unknown-linux-gnu
  release: 1.51.0-nightly
  

Reproduce the reported Rc comparison with the vtable_address_comparisons and ptr_eq lints, then trace how the ptr_eq suggestion is generated. Done means the diagnostic no longer suggests comparing trait-object pointers without the required data-pointer cast, with a regression test covering the example.