Nonsensical error in rustc output.

[zakarumych]

Given a code with few issues I got completely unrelated error bound to line with one of those issues.

I tried this code:

#![recursion_limit="8"] // default value makes compilation longer.

extern crate num_traits; // 0.2.12

struct Foo(u8);

#[repr(transparent)]
struct Bar(Foo);

impl Bar {
    fn cast(foo: &mut Foo) -> &mut Self {
        unsafe { &*(foo as *mut _ as *mut _) } // It should be error about coercing `&_` to `&mut _`
    }
}

impl Baz {} // If you delete this, or replace `Baz` with existing type - weired error vanishes.

I expected to see this happen:

error[E0308]: mismatched types
  --> src/lib.rs:12:18
   |
12 |         unsafe { &*(foo as *mut _ as *mut _) } // It should be error about coercing `&_` to `&mut _`
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
   |
   = note: expected mutable reference `&mut Bar`
                      found reference `&_`

Instead, this happened:

error[E0275]: overflow evaluating the requirement `&num_rational::Ratio<_>: num_traits::Pow<u128>`
  --> src/lib.rs:12:18
   |
12 |         unsafe { &*(foo as *mut _ as *mut _) } // It should be error about coercing `&_` to `&mut _`
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider adding a `#![recursion_limit="16"]` attribute to your crate (`playground`)
   = note: required because of the requirements on the impl of `num_traits::Pow<u128>` for `&num_rational::Ratio<num_rational::Ratio<_>>`
   = note: required because of the requirements on the impl of `num_traits::Pow<u128>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>`
   = note: required because of the requirements on the impl of `num_traits::Pow<u128>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>`
   = note: required because of the requirements on the impl of `num_traits::Pow<u128>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>`
   = note: required because of the requirements on the impl of `num_traits::Pow<u128>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>`
   = note: required because of the requirements on the impl of `num_traits::Pow<u128>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>`
   = note: required because of the requirements on the impl of `num_traits::Pow<u128>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>`
   = note: required because of the requirements on the impl of `num_traits::Pow<&i128>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>`

Meta

Simplified reproduction.
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=4ded409a017da7b3d6d0a4dcbfc1bebb

[rukai]

I also hit this issue on stable rust 1.46
I wasnt able to reproduce it with a smaller sample. (Although I didnt try super hard)
But for me it occurred when I passed the result of String::as_ref to a function expecting a String.

   Compiling canon_collision v0.0.1 (/home/rukai2/Projects/Crates/canon_collision/canon_collision/canon_collision)
error[E0275]: overflow evaluating the requirement `&num_rational::Ratio<_>: entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>`
   --> canon_collision/src/game.rs:104:56
    |
104 |                     ty: EntityType::Player(Player::new(fighter.as_ref(), team, i, &stage, &package, &setup.rules)),
    |                                                        ^^^^^^^^^^^^^^^^
    |
    = help: consider adding a `#![recursion_limit="256"]` attribute to your crate (`canon_collision`)
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<_>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>>>>>>>>`
    = note: required because of the requirements on the impl of `entity::item::_IMPL_NUM_FromPrimitive_FOR_ItemAction::_num_traits::Pow<u32>` for `&num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<num_rational::Ratio<_>>>>>>>>>>>>>>>>>>>>>>>`

[estebank]

Now that #89576 has landed, I think this issue can be resolved. Can you try to reproduce with a recent nightly? If it's still present, please reopen.