ICE w/ const_generics when trying to use the size of an associated type size as an array length

[petertodd]

#![allow(incomplete_features)]
#![feature(const_generics)]

use std::mem;

pub trait Trait {
    type Associated : Sized;

    fn associated_size(&self) -> usize {
        [0u8; mem::size_of::<Self::Associated>()];
        0   
    }   
}

fn main() {
}

results in:

error: internal compiler error: src/librustc_traits/normalize_erasing_regions.rs:34: could not fully normalize `fn() -> usize {std::mem::size_of::<<Self as Trait>::Associated>}`

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:889:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

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

error: aborting due to previous error

Removing the const_generics feature flag gets the expected error message:

error[E0220]: associated type `Associated` not found for `Self`
  --> normalize-err.rs:10:36
   |
10 |         [0u8; mem::size_of::<Self::Associated>()];
   |                                    ^^^^^^^^^^ associated type `Associated` not found

error: aborting due to previous error

For more information about this error, try `rustc --explain E0220`.

(IIUC that itself is a bug related to #43408)

Using mem::size_of::<Self::Associated>() directly outside of a const-context works, which I guess is something const_generics enables that isn't otherwise allowed.

Finally, the error message is the same whether or not the code is in a provided method or a impl block; I actually ran into the bug in the later.

[Aaron1011]

I believe this should be fixed by #67800

[JohnTitor]

Now, it shows different ICE(s) (playground):

error: internal compiler error: cat_expr Errd
error: internal compiler error: cat_expr Errd
error: internal compiler error: PromoteTemps: MIR had errors
error: internal compiler error: broken MIR in DefId(0:6 ~ playground[a80e]::Trait[0]::associated_size[0]) ("return type"): bad type [type error]
error: internal compiler error: broken MIR in DefId(0:6 ~ playground[a80e]::Trait[0]::associated_size[0]) (LocalDecl { mutability: Mut, local_info: Other, internal: false, is_block_tail: None, ty: [type error], user_ty: UserTypeProjections { contents: [] }, source_info: SourceInfo { span: src/main.rs:9:5: 12:6, scope: scope[0] } }): bad type [type error]

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:355:17

[JohnTitor]

Now ICE no longer occurs. I think it's fixed by #69859.