Internal compiler error (traits & associated consts)

[idubrov]

Internal compiler error happens compiling weird code with traits and associated consts.

I tried this code:

use std::fmt::Debug;

pub trait Foo<T> {
  const FOO: *const u8;
}

impl <T: Debug> Foo<T> for Debug {
  const FOO: *const u8 = <T as Debug>::fmt as *const u8;
}

pub trait Bar {
  const BAR: *const u8;
}

pub trait Baz {
  type Data: Debug;
}

pub struct BarStruct<S: Baz>(S);

impl<S: Baz> Bar for BarStruct<S> {
  const BAR: *const u8 = <Debug as Foo<<S as Baz>::Data>>::FOO;
}

struct AnotherStruct;
#[derive(Debug)]
struct SomeStruct;

impl Baz for AnotherStruct {
  type Data = SomeStruct;
}

fn main() {
  let _x = <BarStruct<AnotherStruct> as Bar>::BAR;
}

I expected to see this happen: compiler not to crash

Instead, this happened: compiler crashed with:

error: internal compiler error: src/librustc_traits/normalize_erasing_regions.rs:53: could not fully normalize `for<'r, 's, 't0> fn(&'r <S as Baz>::Data, &'s mut std::fmt::Formatter<'t0>) -> std::result::Result<(), std::fmt::Error>`

thread 'main' panicked at 'Box<Any>', src/librustc_errors/lib.rs:600:9
query stack during panic:
#0 [normalize_ty_after_erasing_regions] normalizing `ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: All }, value: for<'r, 's, 't0> fn(&'r <S as Baz>::Data, &'s mut std::fmt::Formatter<'t0>) -> std::result::Result<(), std::fmt::Error> }`
#1 [const_eval_raw] const-evaluating `<(dyn std::fmt::Debug + 'static) as Foo<T>>::FOO`
  --> src/main.rs:22:26
   |
22 |   const BAR: *const u8 = <Debug as Foo<<S as Baz>::Data>>::FOO;
   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#2 [optimized_mir] processing `<BarStruct<S> as Bar>::BAR`
#3 [const_eval_raw] const-evaluating `<BarStruct<S> as Bar>::BAR`
  --> src/main.rs:34:12
   |
34 |   let _x = <BarStruct<AnotherStruct> as Bar>::BAR;
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#4 [optimized_mir] processing `main`
#5 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error

Meta

Playground nightly: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=59adf803ef87a6d4b1251408811542dd
Playground stable: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=59adf803ef87a6d4b1251408811542dd

[Centril]

Regression seems to have happened in 1.26; Testing this with godbolt (1.25) works but it doesn't with 1.26.

[Goddard]

error: internal compiler error: src/librustc/ty/subst.rs:491: Type parameter T/#1 (T/1) out of range when substituting (root type=Some(T)) substs=[T]

thread 'main' panicked at 'Box', src/librustc_errors/lib.rs:538:9
note: Run with RUST_BACKTRACE=1 for a backtrace.

[andjo403]

the example do not ICE any more from version 1.34.0 see https://godbolt.org/z/rQN231

[adrian-budau]

I'd like to work on this (specifically to add the tests). I've tested and can confirm as @andjo403 that the example does not trigger an ICE anymore.

Tested on:

• rustc 1.36.0 (a53f9df32 2019-07-03)
• rustc 1.38.0-nightly (9703ef666 2019-08-10)