const generics: error[E0391]: cycle detected when const-evaluating + checking `Foo::{{constant}}#0`

[is8ac]

Source

#![feature(const_generics)]

trait Baz {
    type Quaks;
}
impl Baz for u8 {
    type Quaks = [u16; 3];
}

trait Bar {}
impl Bar for [u16; 3] {}
impl Bar for [[u16; 3]; 2] {}

trait Foo
where
    [<u8 as Baz>::Quaks; 2]: Bar,
    <u8 as Baz>::Quaks: Bar,
{
}

fn main() {}

Error

[isaac@d6-arch bitnn]$ rustc src/bin/cycle_demo.rs 
warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> src/bin/cycle_demo.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default

error[E0391]: cycle detected when const-evaluating + checking `Foo::{{constant}}#0`
  --> src/bin/cycle_demo.rs:17:26
   |
17 |     [<u8 as Baz>::Quaks; 2]: Bar,
   |                          ^
   |
note: ...which requires const-evaluating + checking `Foo::{{constant}}#0`...
  --> src/bin/cycle_demo.rs:17:26
   |
17 |     [<u8 as Baz>::Quaks; 2]: Bar,
   |                          ^
note: ...which requires const-evaluating `Foo::{{constant}}#0`...
  --> src/bin/cycle_demo.rs:17:26
   |
17 |     [<u8 as Baz>::Quaks; 2]: Bar,
   |                          ^
note: ...which requires processing `Foo::{{constant}}#0`...
  --> src/bin/cycle_demo.rs:17:26
   |
17 |     [<u8 as Baz>::Quaks; 2]: Bar,
   |                          ^
note: ...which requires processing `Foo::{{constant}}#0`...
  --> src/bin/cycle_demo.rs:17:26
   |
17 |     [<u8 as Baz>::Quaks; 2]: Bar,
   |                          ^
   = note: ...which again requires const-evaluating + checking `Foo::{{constant}}#0`, completing the cycle
note: cycle used when processing `Foo`
  --> src/bin/cycle_demo.rs:14:1
   |
14 | / trait Foo
15 | | where
16 | |     Self: Sized,
17 | |     [<u8 as Baz>::Quaks; 2]: Bar,
18 | |     <u8 as Baz>::Quaks: Bar,
19 | | {
20 | | }
   | |_^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0391`.

Remove the line [<u8 as Baz>::Quaks; 2]: Bar, and it compiles.
Removing #![feature(const_generics)] also makes it compile.

Meta

[isaac@d6-arch bitnn]$ rustc --version --verbose
rustc 1.41.0-nightly (59947fcae 2019-12-08)
binary: rustc
commit-hash: 59947fcae6a40df12e33af8c8c7291014b7603e0
commit-date: 2019-12-08
host: x86_64-unknown-linux-gnu
release: 1.41.0-nightly
LLVM version: 9.0

[is8ac]

The example compiles fine with
rustc 1.41.0-nightly (412f43ac5 2019-11-24), but fails with
rustc 1.41.0-nightly (6d77e45f0 2019-12-04).

[jonas-schievink]

Caused by #66883, but sounds like expected fallout. cc @eddyb

[eddyb]

We probably need a way to aggregate these issues and avoid having dozens of duplicates, heh.

Making this work is blocked on lazy normalization.
You can disable const generics, or make your array impls const-generic, there's not really anything else we can properly support atm.

@varkor @yodaldevoid @oli-obk Maybe we should disable generics in scope for plain literals? It's inconsistent but it'd be behind the const_generics feature-gate anyway.

[varkor]

Maybe we should disable generics in scope for plain literals? It's inconsistent but it'd be behind the const_generics feature-gate anyway.

I had been meaning to do something like this a while ago: I agree there's not much point allowing users to write this if it just doesn't work at all.

[lcnr]

It seems like we already added tests for this issue in #71973 😅