"impl Trait" causes error: overflow evaluating the requirement impl Trait: std::marker::Freeze

[theemathas]

Code:

trait Trait {}

struct Wrap<T>(T);
impl<T: Trait> Trait for Wrap<T> {}

fn f(x: impl Trait) -> impl Trait {
    f(Wrap(x))
}

playground link

This code probably shouldn't compile, since it would probably need to instantiate infinitely many versions of the f function.

It currently doesn't compile as it should. However, it does produce the following error, which is very confusing:

   Compiling playground v0.0.1 (file:///playground)
error[E0275]: overflow evaluating the requirement `impl Trait: std::marker::Freeze`
  |
  = help: consider adding a `#![recursion_limit="128"]` attribute to your crate
  = note: required because it appears within the type `impl Trait`
  = note: required because it appears within the type `impl Trait`
  = note: required because it appears within the type `impl Trait`
  = note: required because it appears within the type `impl Trait`
(many more lines repeating this here...)
  = note: required because it appears within the type `impl Trait`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0275`.
error: Could not compile `playground`.

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

I have no idea what std::marker::Freeze is, and it appearing in the error message will probably be confusing to other users as well. I'm not sure whether this will happen in actual code though.

Most likely a reincarnation of #44349.

[ishitatsuyuki]

Basically the diagnostic is useless if you did any endless recursion. There are a few issues open for this already.

[theemathas]

Which is why I said "I'm not sure whether this will happen in actual code though."

[WaffleLapkin]

On latest stable (1.45.0) this code correctly (?) errors with

error[E0720]: opaque type expands to a recursive type
 --> src/main.rs:6:24
  |
6 | fn f(x: impl Trait) -> impl Trait {
  |                        ^^^^^^^^^^ expands to a recursive type
  |
  = note: type resolves to itself

I think the issue is resolved