#![feature(arbitrary_self_types)] - "type of self must not be a method generic parameter type"

[johamster]

Up to version 1.83 the following code compiled successfully:

Code

#![feature(arbitrary_self_types)]                                                                                                                                                                                                                                 

struct Foo;
impl Foo {
    fn foo(self: &impl std::ops::Deref<Target = Self>) {}
}

With version 1.84 we get the following error:

error[E0801]: invalid generic `self` parameter type: `&impl std::ops::Deref<Target = Self>`
...
note: "type of `self` must not be a method generic parameter type"

Version it worked on

It most recently worked on: 1.83

Version with regression

rustc --version --verbose:

rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: aarch64-apple-darwin
release: 1.84.0
LLVM version: 19.1.5

[Noratrieb]

I assume this is expected and not a bug, but it would be good to confirm that, I recommend using cargo-bisect-rustc to figure out which PR made this info an error.

[schreter]

I assume this is expected and not a bug, but it would be good to confirm that, I recommend using cargo-bisect-rustc to figure out which PR made this info an error.

What do you mean with "info"? Previously, it didn't generate any message, no warnings or so. It was correctly compiled and worked as expected. So I'd personally see this as a regression/error.

[Noratrieb]

I meant "into" that was a typo. This is an unstable feature, so it is expected to change. I know there were some recent changes to it which have probably causes this. That's why I asked for a bisection, so we can find the PR that regressed this, which will probably tell us whether it was an accidental regression (and therefore a bug) or an expected change (not a bug).

[ranger-ross]

bisects to #130098 in nightly-2024-10-31

[ranger-ross]

@rustbot label +S-has-bisection -E-needs-bisection

[Noratrieb]

Thank you for the bisection. Yeah that look very intentional. You will need to change your code to remove the use of generic self types.

[johamster]

Ack, thanks for the clarification. Will close the bug.

[schreter]

Thanks for the explanation why the error happens. I looked at the reasoning in the relevant threads. Basically, the feature does work with generic self types w/o problems, the worries are with people using explicit generics via turbofish operator, which potentially causes strange compile errors.

Would it be at least possible to allow something like fn method(self: &impl SomethingImplementingDeref<Target = Self>), i.e., implicit generic, where there is no possibility explicitly specify the type?

Background: We have a set of handle types all implementing a trait Handle: Deref + Clone and want to declare a method working on all handle types. So far, it worked with fn method(self: &impl Handle<Target = Self>).

[Noratrieb]

I'd recommend opening a new issue about that topic