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

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

rust-lang#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 rust-lang#63770)
cc @Centril

Author: Centril

src/libcore/lib.rs

@@ -70,7 +70,6 @@
 #![feature(bound_cloned)]
 #![feature(cfg_target_has_atomic)]
 #![feature(concat_idents)]
-#![feature(const_fn)]
 #![feature(const_if_match)]
 #![feature(const_panic)]
 #![feature(const_fn_union)]

src/libcore/tests/lib.rs

@@ -31,7 +31,6 @@
 #![feature(slice_internals)]
 #![feature(slice_partition_dedup)]
 #![feature(int_error_matching)]
-#![feature(const_fn)]
 #![feature(array_value_iter)]
 #![feature(iter_partition_in_place)]
 #![feature(iter_is_partitioned)]

src/libproc_macro/lib.rs

@@ -21,7 +21,6 @@
 #![feature(nll)]
 #![feature(staged_api)]
 #![feature(allow_internal_unstable)]
-#![feature(const_fn)]
 #![feature(decl_macro)]
 #![feature(extern_types)]
 #![feature(in_band_lifetimes)]

src/librustc/lib.rs

@@ -31,7 +31,6 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(const_fn)]
 #![feature(const_transmute)]
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]

src/librustc_hir/lib.rs

@@ -3,7 +3,7 @@
 //! [rustc guide]: https://rust-lang.github.io/rustc-guide/hir.html
 
 #![feature(crate_visibility_modifier)]
-#![feature(const_fn)]
+#![feature(const_fn)] // For the unsizing cast on `&[]`
 #![feature(in_band_lifetimes)]
 #![feature(specialization)]
 #![recursion_limit = "256"]

src/librustc_mir/lib.rs

@@ -13,7 +13,6 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(box_syntax)]
 #![feature(crate_visibility_modifier)]
 #![feature(core_intrinsics)]
-#![feature(const_fn)]
 #![feature(decl_macro)]
 #![feature(drain_filter)]
 #![feature(exhaustive_patterns)]

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -281,8 +281,22 @@ fn check_place(
     Ok(())
 }
 
-/// Returns whether `allow_internal_unstable(..., <feature_gate>, ...)` is present.
+/// Returns `true` if the given feature gate is allowed within the function with the given `DefId`.
 fn feature_allowed(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: Symbol) -> bool {
+    // All features require that the corresponding gate be enabled,
+    // even if the function has `#[allow_internal_unstable(the_gate)]`.
+    if !tcx.features().enabled(feature_gate) {
+        return false;
+    }
+
+    // If this crate is not using stability attributes, or this function is not claiming to be a
+    // stable `const fn`, that is all that is required.
+    if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
+        return true;
+    }
+
+    // However, we cannot allow stable `const fn`s to use unstable features without an explicit
+    // opt-in via `allow_internal_unstable`.
     attr::allow_internal_unstable(&tcx.get_attrs(def_id), &tcx.sess.diagnostic())
         .map_or(false, |mut features| features.any(|name| name == feature_gate))
 }

src/librustc_span/lib.rs

@@ -5,7 +5,6 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/")]
-#![feature(const_fn)]
 #![feature(crate_visibility_modifier)]
 #![feature(nll)]
 #![feature(optin_builtin_traits)]

src/librustdoc/lib.rs

@@ -12,7 +12,6 @@
 #![feature(test)]
 #![feature(ptr_offset_from)]
 #![feature(crate_visibility_modifier)]
-#![feature(const_fn)]
 #![feature(drain_filter)]
 #![feature(never_type)]
 #![feature(unicode_internals)]

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)]