allow_internal_unstable attr only allowed once per function

[tesuji]

Discussed in zulip: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Using.20const.20unreachable.20in.20libcore/near/210673352

The problem is that compiler doesn't accept code like this:

    #[allow_internal_unstable(const_fn)]
    #[allow_internal_unstable(const_unreachable_unchecked)]
    pub const fn len(&self) -> usize {
        // SAFETY: this is safe because `&[T]` and `FatPtr<T>` have the same layout.
        // Only `std` can make this guarantee.
        let raw_len = unsafe { crate::ptr::Repr { rust: self }.raw.len };

        match mem::size_of::<T>() {
            0 => {}
            // SAFETY: this branch taken if mathematically `isize::MAX < raw_len * N`.
            // But references must point to one allocation with size at most isize::MAX.
            N if (isize::MAX as usize) / N < raw_len => unsafe { hint::unreachable_unchecked() },
            _ => {}
        }

        raw_len
    }

But it accepts code when merging mutiple allow_internal_unstable annotations into one: #77023

Meta

rustc --version --verbose: Version 1.48.0-nightly (0da5800 2020-09-22)

cc #76807 which modified some const stability checks.

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[ecstatic-morse]

#76807 just refactored a few individual calls to rustc_attr::allow_internal_unstable into a single helper function.

rust/compiler/rustc_mir/src/transform/check_consts/mod.rs

Lines 61 to 65 in 6d3acf5

	pub fn allow_internal_unstable(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: Symbol) -> bool {
	let attrs = tcx.get_attrs(def_id);
	attr::allow_internal_unstable(&tcx.sess, attrs)
	.map_or(false, |mut features| features.any(|name| name == feature_gate))
	}

rustc_attr::allow_internal_unstable only looks for the first attribute it seems. Shouldn't be hard to fix.

rust/compiler/rustc_attr/src/builtin.rs

Lines 1023 to 1041 in 6d3acf5

	pub fn allow_internal_unstable<'a>(
	sess: &'a Session,
	attrs: &[Attribute],
	) -> Option<impl Iterator<Item = Symbol> + 'a> {
	let attr = sess.find_by_name(attrs, sym::allow_internal_unstable)?;
	let list = attr.meta_item_list().or_else(|| {
	sess.diagnostic()
	.span_err(attr.span, "allow_internal_unstable expects list of feature names");
	None
	})?;
	Some(list.into_iter().filter_map(move |it| {
	let name = it.ident().map(|ident| ident.name);
	if name.is_none() {
	sess.diagnostic()
	.span_err(it.span(), "`allow_internal_unstable` expects feature names");
	}
	name
	}))
	}

[tesuji]

@rustbot assign @bugadani

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[bugadani]

It doesn't hurt to double check, just takes some time :)