compiler error: lifetimes in associated types leading to a compiler error.

[mmaker]

Hello!

I am getting a compiler error when trying to define the following trait:

trait Foo {
  type PublicKey<'a> : From<&'a [u8]>;
	     
}

fn main() {
  println!("hello, world!")
}

I would have expected my code to compile successfully, or to exit with some error explaining why static compilation failed.

Instead, I am getting a:

error: internal compiler error: src/librustc/ty/subst.rs:426: Region parameter out of range when substituting in region 'a (root type=Some(&'a [u8])) (index=1)

More specifically, I'm having troubles trying to define some type PublicKey that must implement AsRef<[u8]> and From<&[u8]>. If I try with lifetimes I get this compiler error; if I replace the from trait with From<[u8; N]> for some const N: usize (enabling generic_associated_types and const_generics) I get:

error: const generics in any position are currently unsupported
 --> foo.rs:4:24
  |
4 |   type PublicKey<const N: usize> : From<[u8; N]>;
  |                        ^

Am I doing something terribly stupid here or const generics for associated types are not yet ready and lifetimes on associated types lead to a compiler error?

Here's the full traceback

 --> foo.rs:3:3
  |
3 |   type PublicKey<'a> : From<&'a[u8]>;
  |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(generic_associated_types)] to the crate attributes to enable

error: internal compiler error: src/librustc/ty/subst.rs:426: Region parameter out of range when substituting in region 'a (root type=Some(&'a [u8])) (index=1)

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:558:9
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0658`.

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 1.34.0-nightly (aadbc459b 2019-02-23) running on x86_64-unknown-linux-gnu

[jonas-schievink]

type PublicKey<'a> defines a generic associated type, which aren't yet fully implemented.

[hellow554]

type PublicKey<'a> defines a generic associated type, which aren't yet fully implemented.

That's correct, but even without enabled feature it crashes, which should not happen, or am I wrong? :/

[pietroalbini]

cc @rust-lang/compiler @rust-lang/release

It's technically a stable-to-beta regression, but it goes from hard error to ICE so I don't think it's worth blocking the release on this.

(Also woops, mistakenly closed the issue)

[Centril]

As long as we're not breaking working code it's not really a breakage and so I agree we should not block a release on it.

[arielb1]

Also +1 for not blocking the release on this.

This error does not only occur in code that is illegal, it (apparently) only occurs in code that has already triggered a visible, clear, feature-gate error. Therefore, it's not even a particularly-high risk for confusing users (unlike e.g. an ICE that occurs instead of a valid error message, or an ICE that occurs after an obscure error message).

[pnkfelix]

triage: P-medium. Assigning to self for initial investigation.

(P-medium based on the reasons given by @arielb1 : the risks for end users here do seem quite low.)