Compiler panics when creating a trait object for a trait which uses generic_associated_types

[TopologicallySpeaking]

I think the trait object isn't aware of the parameters in the associated type, so it ends up with a different number of parameters than it should.

Code

main.rs:

#![feature(generic_associated_types)]

pub trait SubTrait {}

pub trait SuperTrait {
    type SubType<'a>: SubTrait;

    fn get_sub<'a>(&'a mut self) -> Self::SubType<'a>;
}

pub struct SubStruct<'a> {
    sup: &'a mut SuperStruct,
}

impl<'a> SubTrait for SubStruct<'a> {}

pub struct SuperStruct {
    value: u8,
}

impl SuperStruct {
    pub fn new(value: u8) -> SuperStruct {
        SuperStruct { value }
    }
}

impl SuperTrait for SuperStruct {
    type SubType<'a> = SubStruct<'a>;

    fn get_sub<'a>(&'a mut self) -> Self::SubType<'a> {
        SubStruct { sup: self }
    }
}

fn main() {
    let sub: Box<dyn SuperTrait<SubType = SubStruct>> = Box::new(SuperStruct::new(0));
}

Meta

rustc --version --verbose:

rustc 1.47.0-nightly (8e21bd063 2020-08-14)
binary: rustc
commit-hash: 8e21bd0633b8d970646ee6eb706c9e8acfad19af
commit-date: 2020-08-14
host: x86_64-unknown-linux-gnu
release: 1.47.0-nightly
LLVM version: 10.0

Error output

warning: the feature `generic_associated_types` is incomplete and may not be safe to use and/or cause compiler crashes
 --> src/main.rs:1:12
  |
1 | #![feature(generic_associated_types)]
  |            ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #44265 <https://github.com/rust-lang/rust/issues/44265> for more information

warning: unused variable: `sub`
  --> src/main.rs:36:9
   |
36 |     let sub: Box<dyn SuperTrait<SubType = SubStruct>> = Box::new(SuperStruct::new(0));
   |         ^^^ help: if this is intentional, prefix it with an underscore: `_sub`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: field is never read: `sup`
  --> src/main.rs:12:5
   |
12 |     sup: &'a mut SuperStruct,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: field is never read: `value`
  --> src/main.rs:18:5
   |
18 |     value: u8,
   |     ^^^^^^^^^

warning: Error finalizing incremental compilation session directory `/home/alex/Documents/Development/ice_minimal/target/debug/incremental/ice_minimal-12sq6hsbdt9f2/s-fr1eb3vgke-1vvozop-working`: No such file or directory (os error 2)

warning: 5 warnings emitted

error: internal compiler error: impl item and trait item have different parameter counts
  |
  = note: delayed at /rustc/8e21bd0633b8d970646ee6eb706c9e8acfad19af/src/librustc_session/session.rs:441:27

error: internal compiler error: TyKind::Error constructed but no error reported
  |
  = note: delayed at /rustc/8e21bd0633b8d970646ee6eb706c9e8acfad19af/src/librustc_session/session.rs:441:27

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:366:17
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.47.0-nightly (8e21bd063 2020-08-14) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

error: could not compile `ice_minimal`.

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

Backtrace

```warning: the feature generic_associated_types is incomplete and may not be safe to use and/or cause compiler crashes
--> src/main.rs:1:12
|
1 | #![feature(generic_associated_types)]
| ^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(incomplete_features)]` on by default
= note: see issue #44265 #44265 for more information

warning: unused variable: sub
--> src/main.rs:36:9
|
36 | let sub: Box<dyn SuperTrait<SubType = SubStruct>> = Box::new(SuperStruct::new(0));
| ^^^ help: if this is intentional, prefix it with an underscore: _sub
|
= note: #[warn(unused_variables)] on by default

warning: field is never read: sup
--> src/main.rs:12:5
|
12 | sup: &'a mut SuperStruct,
| ^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: #[warn(dead_code)] on by default

warning: field is never read: value
--> src/main.rs:18:5
|
18 | value: u8,
| ^^^^^^^^^

warning: Error finalizing incremental compilation session directory /home/alex/Documents/Development/ice_minimal/target/debug/incremental/ice_minimal-12sq6hsbdt9f2/s-fr1ebr4tpj-1habbbz-working: No such file or directory (os error 2)

warning: 5 warnings emitted

error: internal compiler error: impl item and trait item have different parameter counts
|
= note: delayed at /rustc/8e21bd0633b8d970646ee6eb706c9e8acfad19af/src/librustc_session/session.rs:441:27

error: internal compiler error: TyKind::Error constructed but no error reported
|
= note: delayed at /rustc/8e21bd0633b8d970646ee6eb706c9e8acfad19af/src/librustc_session/session.rs:441:27

thread 'rustc' panicked at 'no errors encountered even though delay_span_bug issued', src/librustc_errors/lib.rs:366:17
stack backtrace:
0: std::panicking::begin_panic
1: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
2: core::ptr::drop_in_place
3: <alloc::rc::Rc as core::ops::drop::Drop>::drop
4: core::ptr::drop_in_place
5: rustc_span::with_source_map
6: rustc_interface::interface::create_compiler_and_run
7: rustc_span::with_session_globals
note: Some details are omitted, run with RUST_BACKTRACE=full for a verbose 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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.47.0-nightly (8e21bd0 2020-08-14) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
end of query stack
error: could not compile ice_minimal.

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

</p>
</details>

[matthiaskrgr]

Might be caused by 08e2d46 (When I check the code without the feature flag, rustc panics on the code since that commit)

[nikomatsakis]

Might be fixed by #79554