Feature-gate non-Rust ABI methods

[Aatch]

Currently we allow methods to be marked with the extern qualifier indicating that they have a non-Rust ABI (default is "C"). They are rather bug-ridden, only really working by accident, see #26997 and #26049.

While I personally think they shouldn't be allowed to start with, putting them behind a feature-gate until we make a decision either way seems like a good idea.

Hopefully this doesn't require an RFC, but if it does, I'll write one up and this can be a tracking issue.

/cc @rust-lang/lang

[nrc]

+1 for feature gating, but I think it probably does need an RFC

[nikomatsakis]

Didn't we write an RFC about this already? I have some vague memory of this.

[pnkfelix]

(When I first read this, it seemed absurdly broad, but then I saw another interpretation, so here's my Q...)

@Aatch just to be clear, you're just talking about methods (as in, "stuff in impl blocks") and not free-fn's, right?

I.e. we would not be feature-gating

extern fn printf(fmt: *const u8);

but we would be feature-gating:

impl Print { extern fn printf(fmt: *const u8); }

Right?

[steveklabnik]

@pnkfelix i had the same reaction, and then read closely and realized that yeah, I think @Aatch is talking about only methods.