Rollup merge of #78208 - liketechnik:issue-69399, r=oli-obk

replace `#[allow_internal_unstable]` with `#[rustc_allow_const_fn_unstable]` for `const fn`s

`#[allow_internal_unstable]` is currently used to side-step feature gate and stability checks.
While it was originally only meant to be used only on macros, its use was expanded to `const fn`s.

This pr adds stricter checks for the usage of `#[allow_internal_unstable]` (only on macros) and introduces the `#[rustc_allow_const_fn_unstable]` attribute for usage on `const fn`s.

This pr does not change any of the functionality associated with the use of `#[allow_internal_unstable]` on macros or the usage of `#[rustc_allow_const_fn_unstable]` (instead of `#[allow_internal_unstable]`) on `const fn`s (see https://github.com/rust-lang/rust/issues/69399#issuecomment-712911540).

Note: The check for `#[rustc_allow_const_fn_unstable]` currently only validates that the attribute is used on a function, because I don't know how I would check if the function is a `const fn` at the place of the check. I therefore openend this as a 'draft pull request'.

Closes rust-lang/rust#69399

r? @oli-obk

compiler/rustc_passes/src/check_attr.rs

@@ -85,6 +85,10 @@ impl CheckAttrVisitor<'tcx> {
                 self.check_export_name(&attr, span, target)
             } else if self.tcx.sess.check_name(attr, sym::rustc_args_required_const) {
                 self.check_rustc_args_required_const(&attr, span, target, item)
+            } else if self.tcx.sess.check_name(attr, sym::allow_internal_unstable) {
+                self.check_allow_internal_unstable(&attr, span, target, &attrs)
+            } else if self.tcx.sess.check_name(attr, sym::rustc_allow_const_fn_unstable) {
+                self.check_rustc_allow_const_fn_unstable(hir_id, &attr, span, target)
             } else {
                 // lint-only checks
                 if self.tcx.sess.check_name(attr, sym::cold) {
@@ -719,6 +723,55 @@ impl CheckAttrVisitor<'tcx> {
             }
         }
     }
+
+    /// Outputs an error for `#[allow_internal_unstable]` which can only be applied to macros.
+    /// (Allows proc_macro functions)
+    fn check_allow_internal_unstable(
+        &self,
+        attr: &Attribute,
+        span: &Span,
+        target: Target,
+        attrs: &[Attribute],
+    ) -> bool {
+        debug!("Checking target: {:?}", target);
+        if target == Target::Fn {
+            for attr in attrs {
+                if self.tcx.sess.is_proc_macro_attr(attr) {
+                    debug!("Is proc macro attr");
+                    return true;
+                }
+            }
+            debug!("Is not proc macro attr");
+        }
+        self.tcx
+            .sess
+            .struct_span_err(attr.span, "attribute should be applied to a macro")
+            .span_label(*span, "not a macro")
+            .emit();
+        false
+    }
+
+    /// Outputs an error for `#[allow_internal_unstable]` which can only be applied to macros.
+    /// (Allows proc_macro functions)
+    fn check_rustc_allow_const_fn_unstable(
+        &self,
+        hir_id: HirId,
+        attr: &Attribute,
+        span: &Span,
+        target: Target,
+    ) -> bool {
+        if let Target::Fn | Target::Method(_) = target {
+            if self.tcx.is_const_fn_raw(self.tcx.hir().local_def_id(hir_id)) {
+                return true;
+            }
+        }
+        self.tcx
+            .sess
+            .struct_span_err(attr.span, "attribute should be applied to `const fn`")
+            .span_label(*span, "not a `const fn`")
+            .emit();
+        false
+    }
 }
 
 impl Visitor<'tcx> for CheckAttrVisitor<'tcx> {

compiler/rustc_passes/src/check_const.rs

@@ -87,7 +87,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
 
         let is_feature_allowed = |feature_gate| {
             // All features require that the corresponding gate be enabled,
-            // even if the function has `#[allow_internal_unstable(the_gate)]`.
+            // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
             if !tcx.features().enabled(feature_gate) {
                 return false;
             }
@@ -105,8 +105,8 @@ impl<'tcx> CheckConstVisitor<'tcx> {
             }
 
             // 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.sess, &tcx.get_attrs(def_id))
+            // opt-in via `rustc_allow_const_fn_unstable`.
+            attr::rustc_allow_const_fn_unstable(&tcx.sess, &tcx.get_attrs(def_id))
                 .map_or(false, |mut features| features.any(|name| name == feature_gate))
         };