ICE when using generic array type depending on expression of const generics

[raku42]

I am writing my own little math library to learn rust. I was successful in implementing a mathematical vector type which uses a const generic parameter to specify its dimension/length and stores its values in an array of type [T; DIM] under the hood. When trying to implement the same pattern for a matrix that uses a flat array to store its values, I encounter the following error:

#![feature(const_generics)]

pub type MatrixArray<T, const N : usize, const M : usize> = [T; N * M];

pub struct Matrix<T, const N : usize, const M : usize> 
    where T : Copy + Sized
{
    store : MatrixArray<T, N, M>,
}

impl<T, const N : usize, const M : usize> Default for Matrix<T, N, M>
   where T : Copy + Sized + Default
{
    fn default() -> Self {
        let zero_comps = unsafe {
            let mut comps = std::mem::MaybeUninit::< MatrixArray<T, N, M> >::uninit().assume_init();
            for comp in comps.iter_mut() {
                *comp = T::default();
            }
            comps
        };
        Self {
            store : zero_comps
        }
    }
}

fn main() {
    println!("Does not compile!");
}

(Playground)

Errors:

  Compiling playground v0.0.1 (/playground)
warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> src/main.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default

warning: field is never read: `store`
 --> src/main.rs:8:5
  |
8 |     store : MatrixArray<T, N, M>,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

error: internal compiler error: constant in type had an ignored error: TooGeneric
 --> src/main.rs:8:5
  |
8 |     store : MatrixArray<T, N, M>,
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: constant in type had an ignored error: TooGeneric
  --> src/main.rs:16:29
   |
16 |             let mut comps = std::mem::MaybeUninit::< MatrixArray<T, N, M> >::uninit().assume_init();
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: constant in type had an ignored error: TooGeneric
  --> src/main.rs:16:29
   |
16 |             let mut comps = std::mem::MaybeUninit::< MatrixArray<T, N, M> >::uninit().assume_init();
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: broken MIR in DefId(0:17 ~ playground[a92e]::{{impl}}[0]::default[0]) (CanonicalUserTypeAnnotation { user_ty: Canonical { max_universe: U0, variables: [CanonicalVarInfo { kind: Ty(General(U0)) }], value: TypeOf(DefId(2:1154 ~ core[77d3]::mem[0]::maybe_uninit[0]::{{impl}}[2]::uninit[0]), UserSubsts { substs: [^0], user_self_ty: Some(UserSelfTy { impl_def_id: DefId(2:1151 ~ core[77d3]::mem[0]::maybe_uninit[0]::{{impl}}[2]), self_ty: std::mem::MaybeUninit<[T; _]> }) }) }, span: src/main.rs:16:29: 16:84, inferred_ty: fn() -> std::mem::MaybeUninit<[T; _]> {std::mem::MaybeUninit::<[T; _]>::uninit} }): bad user type AscribeUserType(fn() -> std::mem::MaybeUninit<[T; _]> {std::mem::MaybeUninit::<[T; _]>::uninit}, DefId(2:1154 ~ core[77d3]::mem[0]::maybe_uninit[0]::{{impl}}[2]::uninit[0]) UserSubsts { substs: [_], user_self_ty: Some(UserSelfTy { impl_def_id: DefId(2:1151 ~ core[77d3]::mem[0]::maybe_uninit[0]::{{impl}}[2]), self_ty: std::mem::MaybeUninit<[T; _]> }) }): NoSolution

error: internal compiler error: broken MIR in DefId(0:17 ~ playground[a92e]::{{impl}}[0]::default[0]) (NoSolution): could not prove WellFormed(std::mem::MaybeUninit<[T; _]>)
  --> src/main.rs:16:29
   |
16 |             let mut comps = std::mem::MaybeUninit::< MatrixArray<T, N, M> >::uninit().assume_init();
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: broken MIR in DefId(0:17 ~ playground[a92e]::{{impl}}[0]::default[0]) (NoSolution): could not prove WellFormed(std::mem::MaybeUninit<[T; _]>)
  --> src/main.rs:14:5
   |
14 | /     fn default() -> Self {
15 | |         let zero_comps = unsafe {
16 | |             let mut comps = std::mem::MaybeUninit::< MatrixArray<T, N, M> >::uninit().assume_init();
17 | |             for comp in comps.iter_mut() {
...  |
24 | |         }
25 | |     }
   | |_____^

error: internal compiler error: broken MIR in DefId(0:17 ~ playground[a92e]::{{impl}}[0]::default[0]) (NoSolution): could not prove WellFormed([T; _])
  --> src/main.rs:14:5
   |
14 | /     fn default() -> Self {
15 | |         let zero_comps = unsafe {
16 | |             let mut comps = std::mem::MaybeUninit::< MatrixArray<T, N, M> >::uninit().assume_init();
17 | |             for comp in comps.iter_mut() {
...  |
24 | |         }
25 | |     }
   | |_____^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:345:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.42.0-nightly (da3629b05 2019-12-29) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: could not compile `playground`.

To learn more, run the command again with --verbose.

Interestingly, I only get the MIR error message in my full project, but not the TooGeneric error, which I put down to having more instances where the type is used therefore triggering monomorphization.

When I do not define the generic type but only use [T; N * M] directly, I get normal error messages about non-matching types which I expect because of the treatment of this expression as a projection [T; _].

I suspect this to be another instance of #43408.

[Alexendoo]

Fixed by #68388