pin_ergonomics leads to bizarre "expected Pin<X> found Pin<X>" type errors

[yotamofek]

I know pin_ergonomics is in very early stages, but I was eager to try it out and ran into this somewhat fundamental issue.
cc #130494

I tried this code:

#![feature(pin_ergonomics)]

use std::pin::Pin;

pub fn foo() {
    let _: Pin<Box<()>> = Box::pin(());
}

I expected to see this happen: should compile

Instead, this happened: got a compilation error

error[E0308]: mismatched types
 --> src/lib.rs:6:27
  |
6 |     let _: Pin<Box<()>> = Box::pin(());
  |                           ^^^^^^^^^^^^ types differ
  |
  = note: expected struct `Pin<_>`
             found struct `Pin<_>`

Meta

rustc --version --verbose:

rustc 1.84.0-nightly (f2a35426b 2024-11-16)
binary: rustc
commit-hash: f2a35426b6586178c44b27cedae182502092e898
commit-date: 2024-11-16
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.3

[fmease]

Conceptually simpler reproducer:

#![feature(pin_ergonomics)]
fn mk() -> std::pin::Pin<()> { loop {} }

rustc file.rs --crate-type=lib -Zverbose-internals -Ztrim-diagnostic-paths=no -Aincomplete-features:

error[E0308]: mismatched types
 --> file.rs:2:32
  |
2 | fn mk() -> std::pin::Pin<()> { loop {} }
  |                                ^^^^^^^ types differ
  |
  = note: expected struct `std::pin::Pin<()>`
             found struct `std::pin::Pin<()>`

[fmease]

And less artificial:

#![feature(pin_ergonomics)]

struct X;
impl std::ops::Deref for X { type Target = u32; fn deref(&self) -> &Self::Target { &0 } }

fn main() {
    let _: std::pin::Pin<X> = std::pin::Pin::new(X);
}

rustc file.rs -Zverbose-internals -Ztrim-diagnostic-paths=no -Aincomplete-features:

error[E0308]: mismatched types
 --> file.rs:8:31
  |
8 |     let _: std::pin::Pin<X> = std::pin::Pin::new(X);
  |                               ^^^^^^^^^^^^^^^^^^^^^ types differ
  |
  = note: expected struct `std::pin::Pin<X>`
             found struct `std::pin::Pin<X>`

[fmease]

Possibly due to the following:

rust/compiler/rustc_hir_typeck/src/coercion.rs

Line 821 in ee612c4

Err(TypeError::Mismatch)

---

Where Coercible(Pin<X>, Pin<Y>) always results in a TypeMismatch if X and Y are not of the form &[mut] Xp and &[mut] Yp respectively even if Coercible(X, Y). For context, this happens in extract_pin_mut inside coerce_pin inside coerce. Rephrased, the coercion fails if extract_pin_mut fails to find a "coercion candidate" (Pin<&[mut] _>) which is the case in all three reproducers given above.

[fmease]

cc @eholk