only use rustc_const_stable on functions that are actually stable, and inherit const feature gate from other unstable gates

With this, we need fewer rustc_const_(un)stable annotations

Author: RalfJung

Diff for: compiler/rustc_attr/src/builtin.rs

@@ -274,10 +274,14 @@ pub fn find_stability(
 
 /// Collects stability info from `rustc_const_stable`/`rustc_const_unstable`/`rustc_promotable`
 /// attributes in `attrs`. Returns `None` if no stability attributes are found.
+///
+/// `inherited_feature_gate` says which feature gate this function should be under if it doesn't
+/// declare a gate itself, but has `#[rustc_const_stable_indirect]`.
 pub fn find_const_stability(
     sess: &Session,
     attrs: &[Attribute],
     item_sp: Span,
+    inherited_feature_gate: Option<Symbol>,
 ) -> Option<(ConstStability, Span)> {
     let mut const_stab: Option<(ConstStability, Span)> = None;
     let mut promotable = false;
@@ -351,8 +355,11 @@ pub fn find_const_stability(
                 }
             }
             _ => {
+                // `#[rustc_const_stable_indirect]` implicitly makes the function unstably const,
+                // inheriting the feature gate from `#[unstable]` if it xists, or without any
+                // feature gate otherwise.
                 let c = ConstStability {
-                    feature: None,
+                    feature: inherited_feature_gate,
                     safe_to_expose_on_stable: true,
                     promotable: false,
                     level: StabilityLevel::Unstable {

Diff for: compiler/rustc_expand/src/base.rs

@@ -865,7 +865,11 @@ impl SyntaxExtension {
             })
             .unwrap_or_else(|| (None, helper_attrs));
         let stability = attr::find_stability(sess, attrs, span);
-        let const_stability = attr::find_const_stability(sess, attrs, span);
+        // FIXME: this will give a different result than the normal stability computation, since we
+        // don't inherit stability from the parent. But that's true even for `stability` above so
+        // it's probably okay?
+        let const_stability =
+            attr::find_const_stability(sess, attrs, span, stability.map(|(s, _)| s.feature));
         let body_stability = attr::find_body_stability(sess, attrs);
         if let Some((_, sp)) = const_stability {
             sess.dcx().emit_err(errors::MacroConstStability {

Diff for: compiler/rustc_passes/messages.ftl

@@ -450,13 +450,17 @@ passes_may_dangle =
 passes_maybe_string_interpolation = you might have meant to use string interpolation in this string literal
 
 passes_missing_const_err =
-    attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+    attributes `#[rustc_const_unstable]`, `#[rustc_const_stable]` and `#[rustc_const_stable_indirect]` require the function or method to be `const`
     .help = make the function or method const
     .label = attribute specified here
 
 passes_missing_const_stab_attr =
     {$descr} has missing const stability attribute
 
+passes_const_stable_not_stable =
+    attribute `#[rustc_const_stable]` can only be applied to functions that are declared `#[stable]`
+    .label = attribute specified here
+
 passes_missing_lang_item =
     lang item required, but not found: `{$name}`
     .note = this can occur when a binary crate with `#![no_std]` is compiled for a target where `{$name}` is defined in the standard library

Diff for: compiler/rustc_passes/src/errors.rs

@@ -1544,6 +1544,7 @@ pub(crate) struct DuplicateFeatureErr {
     pub span: Span,
     pub feature: Symbol,
 }
+
 #[derive(Diagnostic)]
 #[diag(passes_missing_const_err)]
 pub(crate) struct MissingConstErr {
@@ -1554,6 +1555,15 @@ pub(crate) struct MissingConstErr {
     pub const_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_const_stable_not_stable)]
+pub(crate) struct ConstStableNotStable {
+    #[primary_span]
+    pub fn_sig_span: Span,
+    #[label]
+    pub const_span: Span,
+}
+
 #[derive(LintDiagnostic)]
 pub(crate) enum MultipleDeadCodes<'tcx> {
     #[diag(passes_dead_codes)]

Diff for: compiler/rustc_passes/src/stability.rs

@@ -162,19 +162,32 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         }
 
         let stab = attr::find_stability(self.tcx.sess, attrs, item_sp);
-        let const_stab = attr::find_const_stability(self.tcx.sess, attrs, item_sp);
+        let const_stab = attr::find_const_stability(
+            self.tcx.sess,
+            attrs,
+            item_sp,
+            // Compute the feature gate we inherit if this
+            // doesn't have its own feature gate.
+            self.parent_const_stab.and_then(|c| c.feature).or_else(|| {
+                // Infer the const feature gate from the regular feature gate,
+                // but only if that regular gate is unstable.
+                if let Some((s, _)) = stab {
+                    s.is_unstable().then_some(s.feature)
+                } else if inherit_deprecation.yes()
+                    && let Some(parent_stab) = self.parent_stab
+                    && parent_stab.is_unstable()
+                {
+                    Some(parent_stab.feature)
+                } else {
+                    None
+                }
+            }),
+        );
         let body_stab = attr::find_body_stability(self.tcx.sess, attrs);
-        let mut const_span = None;
-
-        let const_stab = const_stab.map(|(const_stab, const_span_node)| {
-            self.index.const_stab_map.insert(def_id, const_stab);
-            const_span = Some(const_span_node);
-            const_stab
-        });
 
         // If the current node is a function, has const stability attributes and if it doesn not have an intrinsic ABI,
         // check if the function/method is const or the parent impl block is const
-        if let (Some(const_span), Some(fn_sig)) = (const_span, fn_sig)
+        if let (Some((_, const_span)), Some(fn_sig)) = (const_stab, fn_sig)
             && fn_sig.header.abi != Abi::RustIntrinsic
             && !fn_sig.header.is_const()
             && (!self.in_trait_impl || !self.tcx.is_const_fn_raw(def_id.to_def_id()))
@@ -184,6 +197,22 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
                 .emit_err(errors::MissingConstErr { fn_sig_span: fn_sig.span, const_span });
         }
 
+        // If this is marked const *stable*, it must also be regular-stable.
+        if let Some((const_stab, const_span)) = const_stab
+            && let Some(fn_sig) = fn_sig
+            && const_stab.is_const_stable()
+            && !stab.is_some_and(|(s, _)| s.is_stable())
+        {
+            self.tcx
+                .dcx()
+                .emit_err(errors::ConstStableNotStable { fn_sig_span: fn_sig.span, const_span });
+        }
+
+        let const_stab = const_stab.map(|(const_stab, _span)| {
+            self.index.const_stab_map.insert(def_id, const_stab);
+            const_stab
+        });
+
         // `impl const Trait for Type` items forward their const stability to their
         // immediate children.
         if const_stab.is_none() {
@@ -274,8 +303,6 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             }
         }
 
-        // Every `const fn` that has stability should also have const-stability.
-
         self.recurse_with_stability_attrs(
             depr.map(|(d, _)| DeprecationEntry::local(d, def_id)),
             stab,
@@ -724,7 +751,15 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                 if features.staged_api {
                     let attrs = self.tcx.hir().attrs(item.hir_id());
                     let stab = attr::find_stability(self.tcx.sess, attrs, item.span);
-                    let const_stab = attr::find_const_stability(self.tcx.sess, attrs, item.span);
+                    // FIXME: this will give a different result than the normal stability
+                    // computation, since we don't inherit stability from the parent. But that's
+                    // true even for `stab` above so it's probably okay?
+                    let const_stab = attr::find_const_stability(
+                        self.tcx.sess,
+                        attrs,
+                        item.span,
+                        stab.map(|(s, _)| s.feature),
+                    );
 
                     // If this impl block has an #[unstable] attribute, give an
                     // error if all involved types and traits are stable, because

Diff for: library/core/src/alloc/layout.rs

@@ -302,7 +302,6 @@ impl Layout {
     /// This can return at most `Alignment::MAX` (aka `isize::MAX + 1`)
     /// because the original size is at most `isize::MAX`.
     #[inline]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     const fn size_rounded_up_to_custom_align(&self, align: Alignment) -> usize {
         // SAFETY:
         // Rounded up value is:
@@ -507,7 +506,6 @@ impl Layout {
         return inner(T::LAYOUT, n);
 
         #[inline]
-        #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
         const fn inner(element_layout: Layout, n: usize) -> Result<Layout, LayoutError> {
             let Layout { size: element_size, align } = element_layout;
 

Diff for: library/core/src/ffi/c_str.rs

@@ -137,11 +137,11 @@ enum FromBytesWithNulErrorKind {
 
 // FIXME: const stability attributes should not be required here, I think
 impl FromBytesWithNulError {
-    #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
+    #[cfg_attr(bootstrap, rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0"))]
     const fn interior_nul(pos: usize) -> FromBytesWithNulError {
         FromBytesWithNulError { kind: FromBytesWithNulErrorKind::InteriorNul(pos) }
     }
-    #[rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0")]
+    #[cfg_attr(bootstrap, rustc_const_stable(feature = "const_cstr_methods", since = "1.72.0"))]
     const fn not_nul_terminated() -> FromBytesWithNulError {
         FromBytesWithNulError { kind: FromBytesWithNulErrorKind::NotNulTerminated }
     }
@@ -730,7 +730,7 @@ impl AsRef<CStr> for CStr {
 /// located within `isize::MAX` from `ptr`.
 #[inline]
 #[unstable(feature = "cstr_internals", issue = "none")]
-#[rustc_const_stable(feature = "const_cstr_from_ptr", since = "1.81.0")]
+#[cfg_attr(bootstrap, rustc_const_stable(feature = "const_cstr_from_ptr", since = "1.81.0"))]
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 const unsafe fn strlen(ptr: *const c_char) -> usize {
     const fn strlen_ct(s: *const c_char) -> usize {