Rollup merge of #68114 - ecstatic-morse:fix-feature-gating, r=Centril

Don't require `allow_internal_unstable` unless `staged_api` is enabled.

#63770 changed `qualify_min_const_fn` to require `allow_internal_unstable` for *all* crates that used an unstable feature, regardless of whether `staged_api` was enabled or the `fn` that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added `#![feature(const_fn)]`, which skips `qualify_min_const_fn` entirely.

After this PR, crates that do not have `#![feature(staged_api)]` will only need to enable the feature they are interested in. For example, `#![feature(const_if_match)]` will be enough to enable `if` and `match` in constants. Crates with `staged_api` (e.g., `libstd`) require `#[allow_internal_unstable]` to be added to a function if it uses nightly features unless that function is also marked `#[rustc_const_unstable]`. This prevents proliferation of `#[allow_internal_unstable]` into functions that are not callable in a `const` context on stable.

r? @oli-obk (author of #63770)
cc @Centril

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index a96fee0cf8f..0184a3214b5 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -7,7 +7,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/", test(attr(deny(warnings))))]
 #![feature(bool_to_option)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
+#![feature(const_fn)] // For the `transmute` in `P::new`
 #![feature(const_transmute)]
 #![feature(crate_visibility_modifier)]
 #![feature(label_break_value)]