Incorrect "conflicting implementations" error between two non-overlapping blanket impls

I tried this code:

pub trait Pod: Copy + 'static {}

pub trait Marshal {}

impl<T> Marshal for T where T: Pod {}
impl<T> Marshal for &mut T where T: Pod {}

I expected to see this happen: Compiles.

Instead, this happened:

error[E0119]: conflicting implementations of trait `Marshal` for type `&mut _`
 --> src/lib.rs:6:1
  |
5 | impl<T> Marshal for T where T: Pod {}
  | ---------------------------------- first implementation here
6 | impl<T> Marshal for &mut T where T: Pod {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&mut _`
  |
  = note: downstream crates may implement trait `std::clone::Clone` for type `&mut _`
  = note: downstream crates may implement trait `std::marker::Copy` for type `&mut _`
  = note: downstream crates may implement trait `Pod` for type `&mut _`

Notably, the notes are simply wrong: while it is possible to implement some upstream traits for &mut T as long as T is local (since &mut T is fundamental), a Clone or Copy impl cannot be written for any mutable reference type, since libcore has a negative impl !Clone for &mut T.
Attempting to do so anyways will yield "error[E0751]: found both positive and negative implementation of trait Clone for type &mut S".

Start by running the reproducer in src/lib.rs and examining the E0119 diagnostic for the two blanket implementations. Trace how the compiler reasons about downstream Clone, Copy, and Pod implementations for &mut T. Done means the non-overlapping implementations compile or the diagnostic no longer makes the incorrect claims described here.