Error when an arg to #[allow_internal_unstable] is a key-value

[scrabsha]

It looks like we don't check that the features passed to #[allow_internal_unstable] are "just features" (as in, with no arguments). This means the following compiles:

#![feature(allow_internal_unstable)]

#[allow_internal_unstable(cat = "meow")]
macro_rules! foo {
    () => {};
}

Playground link

This PR makes it an error to pass anything other than an identifier as argument to #[allow_internal_unstable]. I believe it is ok to make it an error (and technically breaking some code) because allow_internal_unstable is an internal feature, which (quoting rustc's output) is internal to the compiler or standard library. Happy to change to a warning if my assumption is wrong though :)

Discovered while writing the followup of #155696.

[mejrs]

You could do something like this:

rust/compiler/rustc_attr_parsing/src/attributes/diagnostic/mod.rs

Lines 238 to 261 in fb76025

	macro or_malformed($($code:tt)*) {{
	let Some(ret) = (||{
	Some($($code)*)
	})() else {
	malformed!()
	};
	ret
	}}
	macro duplicate($name: ident, $($first_span:tt)*) {{
	let first_span = $($first_span)*;
	cx.emit_dyn_lint(
	MALFORMED_DIAGNOSTIC_ATTRIBUTES,
	move |dcx, level| IgnoredDiagnosticOption {
	first_span,
	later_span: span,
	option_name: $name,
	}.into_diag(dcx, level),
	span,
	);
	}}
	let item: &MetaItemParser = or_malformed!(item.meta_item()?);
	let name = or_malformed!(item.ident()?).name;

View changes since the review

[scrabsha]

(||{ Some($($code)*) })()

... wow

[mejrs]

... wow

A poor man's try block 😢

[JonathanBrouwer]

This problem is already fixed by my pr: #155193
(Which has been in the crater queue for like half a month at this point, lmao)

[scrabsha]

oof, you have been more thorough than me hehe. closing my PR then - good luck for 155193 😭