Rollup merge of #132492 - RalfJung:const-intrinsics, r=compiler-errors

remove support for extern-block const intrinsics

This converts all const-callable intrinsics into the "new" form of a regular `fn` with `#[rustc_intrinsic]` attribute. That simplifies some of the logic since those functions can be marked `const fn` like regular functions, so intrinsics no longer need a special case to be considered const-callable at all.

I also added a new attribute `#[rustc_const_stable_intrinsic]` to mark an intrinsic as being ready to be exposed on stable. Previously we used the `#[rustc_const_stable_indirect]` attribute for that, but that attribute had a dual role -- when used on a regular function, it is an entirely safe marker to make this function part of recursive const stability, but on an intrinsic it is a trusted marker requiring special care. It's not great for the same attribute to be sometimes fully checked and safe, and sometimes trusted and requiring special care, so I split this into two attributes.

This also fixes #122652 by accepting intrinsics as const-stable if they have a fallback body that is recursively const-stable.

The library changes are best reviewed with whitespace hidden.

r? `@compiler-errors`

Author: workingjubilee

compiler/rustc_attr/src/builtin.rs

@@ -273,8 +273,7 @@ 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.
 ///
-/// `is_const_fn` indicates whether this is a function marked as `const`. It will always
-/// be false for intrinsics in an `extern` block!
+/// `is_const_fn` indicates whether this is a function marked as `const`.
 pub fn find_const_stability(
     sess: &Session,
     attrs: &[Attribute],
@@ -330,7 +329,7 @@ pub fn find_const_stability(
         }
     }
 
-    // Merge promotable and not_exposed_on_stable into stability info
+    // Merge promotable and const_stable_indirect into stability info
     if promotable {
         match &mut const_stab {
             Some((stab, _)) => stab.promotable = promotable,
@@ -352,10 +351,7 @@ pub fn find_const_stability(
                     })
                 }
             }
-            _ => {
-                // We ignore the `#[rustc_const_stable_indirect]` here, it should be picked up by
-                // the `default_const_unstable` logic.
-            }
+            _ => {}
         }
     }
     // Make sure if `const_stable_indirect` is present, that is recorded. Also make sure all `const

compiler/rustc_const_eval/messages.ftl

@@ -399,7 +399,7 @@ const_eval_uninhabited_enum_variant_written =
 const_eval_unmarked_const_fn_exposed = `{$def_path}` cannot be (indirectly) exposed to stable
     .help = either mark the callee as `#[rustc_const_stable_indirect]`, or the caller as `#[rustc_const_unstable]`
 const_eval_unmarked_intrinsic_exposed = intrinsic `{$def_path}` cannot be (indirectly) exposed to stable
-    .help = mark the caller as `#[rustc_const_unstable]`, or mark the intrinsic `#[rustc_const_stable_indirect]` (but this requires team approval)
+    .help = mark the caller as `#[rustc_const_unstable]`, or mark the intrinsic `#[rustc_const_stable_intrinsic]` (but this requires team approval)
 
 const_eval_unreachable = entering unreachable code
 const_eval_unreachable_unwind =

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -736,16 +736,25 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
 
                 // Intrinsics are language primitives, not regular calls, so treat them separately.
                 if let Some(intrinsic) = tcx.intrinsic(callee) {
+                    // We use `intrinsic.const_stable` to determine if this can be safely exposed to
+                    // stable code, rather than `const_stable_indirect`. This is to make
+                    // `#[rustc_const_stable_indirect]` an attribute that is always safe to add.
+                    // We also ask is_safe_to_expose_on_stable_const_fn; this determines whether the intrinsic
+                    // fallback body is safe to expose on stable.
+                    let is_const_stable = intrinsic.const_stable
+                        || (!intrinsic.must_be_overridden
+                            && tcx.is_const_fn(callee)
+                            && is_safe_to_expose_on_stable_const_fn(tcx, callee));
                     match tcx.lookup_const_stability(callee) {
                         None => {
                             // Non-const intrinsic.
                             self.check_op(ops::IntrinsicNonConst { name: intrinsic.name });
                         }
-                        Some(ConstStability { feature: None, const_stable_indirect, .. }) => {
+                        Some(ConstStability { feature: None, .. }) => {
                             // Intrinsic does not need a separate feature gate (we rely on the
                             // regular stability checker). However, we have to worry about recursive
                             // const stability.
-                            if !const_stable_indirect && self.enforce_recursive_const_stability() {
+                            if !is_const_stable && self.enforce_recursive_const_stability() {
                                 self.dcx().emit_err(errors::UnmarkedIntrinsicExposed {
                                     span: self.span,
                                     def_path: self.tcx.def_path_str(callee),
@@ -755,17 +764,19 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                         Some(ConstStability {
                             feature: Some(feature),
                             level: StabilityLevel::Unstable { .. },
-                            const_stable_indirect,
                             ..
                         }) => {
                             self.check_op(ops::IntrinsicUnstable {
                                 name: intrinsic.name,
                                 feature,
-                                const_stable_indirect,
+                                const_stable: is_const_stable,
                             });
                         }
                         Some(ConstStability { level: StabilityLevel::Stable { .. }, .. }) => {
-                            // All good.
+                            // All good. Note that a `#[rustc_const_stable]` intrinsic (meaning it
+                            // can be *directly* invoked from stable const code) does not always
+                            // have the `#[rustc_const_stable_intrinsic]` attribute (which controls
+                            // exposing an intrinsic indirectly); we accept this call anyway.
                         }
                     }
                     // This completes the checks for intrinsics.

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -354,14 +354,14 @@ impl<'tcx> NonConstOp<'tcx> for IntrinsicNonConst {
 pub(crate) struct IntrinsicUnstable {
     pub name: Symbol,
     pub feature: Symbol,
-    pub const_stable_indirect: bool,
+    pub const_stable: bool,
 }
 
 impl<'tcx> NonConstOp<'tcx> for IntrinsicUnstable {
     fn status_in_item(&self, _ccx: &ConstCx<'_, 'tcx>) -> Status {
         Status::Unstable {
             gate: self.feature,
-            safe_to_expose_on_stable: self.const_stable_indirect,
+            safe_to_expose_on_stable: self.const_stable,
             // We do *not* want to suggest to mark the intrinsic as `const_stable_indirect`,
             // that's not a trivial change!
             is_function_call: false,

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -25,15 +25,9 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
             hir::Constness::Const
         }
         hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(impl_), .. }) => impl_.constness,
-        hir::Node::ForeignItem(hir::ForeignItem { kind: hir::ForeignItemKind::Fn(..), .. }) => {
-            // Intrinsics use `rustc_const_{un,}stable` attributes to indicate constness. All other
-            // foreign items cannot be evaluated at compile-time.
-            let is_const = if tcx.intrinsic(def_id).is_some() {
-                tcx.lookup_const_stability(def_id).is_some()
-            } else {
-                false
-            };
-            if is_const { hir::Constness::Const } else { hir::Constness::NotConst }
+        hir::Node::ForeignItem(_) => {
+            // Foreign items cannot be evaluated at compile-time.
+            hir::Constness::NotConst
         }
         hir::Node::Expr(e) if let hir::ExprKind::Closure(c) = e.kind => c.constness,
         _ => {

compiler/rustc_feature/src/builtin_attrs.rs

@@ -837,6 +837,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_const_stable_indirect, Normal,
         template!(Word), WarnFollowing, EncodeCrossCrate::No, IMPL_DETAIL,
     ),
+    rustc_attr!(
+        rustc_const_stable_intrinsic, Normal,
+        template!(Word), WarnFollowing, EncodeCrossCrate::No, IMPL_DETAIL,
+    ),
     gated!(
         rustc_allow_const_fn_unstable, Normal,
         template!(Word, List: "feat1, feat2, ..."), DuplicatesOk, EncodeCrossCrate::No,

compiler/rustc_middle/src/ty/intrinsic.rs

@@ -9,6 +9,8 @@ pub struct IntrinsicDef {
     pub name: Symbol,
     /// Whether the intrinsic has no meaningful body and all backends need to shim all calls to it.
     pub must_be_overridden: bool,
+    /// Whether the intrinsic can be invoked from stable const fn
+    pub const_stable: bool,
 }
 
 impl TyCtxt<'_> {

compiler/rustc_middle/src/ty/util.rs

@@ -1789,6 +1789,7 @@ pub fn intrinsic_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::Intrinsi
         Some(ty::IntrinsicDef {
             name: tcx.item_name(def_id.into()),
             must_be_overridden: tcx.has_attr(def_id, sym::rustc_intrinsic_must_be_overridden),
+            const_stable: tcx.has_attr(def_id, sym::rustc_const_stable_intrinsic),
         })
     } else {
         None

compiler/rustc_passes/src/stability.rs

@@ -106,7 +106,6 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         def_id: LocalDefId,
         item_sp: Span,
         fn_sig: Option<&'tcx hir::FnSig<'tcx>>,
-        is_foreign_item: bool,
         kind: AnnotationKind,
         inherit_deprecation: InheritDeprecation,
         inherit_const_stability: InheritConstStability,
@@ -175,11 +174,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
         // implied), check if the function/method is const or the parent impl block is const.
         if let Some(fn_sig) = fn_sig
             && !fn_sig.header.is_const()
-            // We have to exclude foreign items as they might be intrinsics. Sadly we can't check
-            // their ABI; `fn_sig.abi` is *not* correct for foreign functions.
-            && !is_foreign_item
             && const_stab.is_some()
-            && (!self.in_trait_impl || !self.tcx.is_const_fn(def_id.to_def_id()))
         {
             self.tcx.dcx().emit_err(errors::MissingConstErr { fn_sig_span: fn_sig.span });
         }
@@ -398,7 +393,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
                         ctor_def_id,
                         i.span,
                         None,
-                        /* is_foreign_item */ false,
                         AnnotationKind::Required,
                         InheritDeprecation::Yes,
                         InheritConstStability::No,
@@ -417,7 +411,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
             i.owner_id.def_id,
             i.span,
             fn_sig,
-            /* is_foreign_item */ false,
             kind,
             InheritDeprecation::Yes,
             const_stab_inherit,
@@ -437,7 +430,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
             ti.owner_id.def_id,
             ti.span,
             fn_sig,
-            /* is_foreign_item */ false,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -461,7 +453,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
             ii.owner_id.def_id,
             ii.span,
             fn_sig,
-            /* is_foreign_item */ false,
             kind,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -477,7 +468,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
             var.def_id,
             var.span,
             None,
-            /* is_foreign_item */ false,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -488,7 +478,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
                         ctor_def_id,
                         var.span,
                         None,
-                        /* is_foreign_item */ false,
                         AnnotationKind::Required,
                         InheritDeprecation::Yes,
                         InheritConstStability::No,
@@ -507,7 +496,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
             s.def_id,
             s.span,
             None,
-            /* is_foreign_item */ false,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -527,7 +515,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
             i.owner_id.def_id,
             i.span,
             fn_sig,
-            /* is_foreign_item */ true,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,
@@ -550,7 +537,6 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
             p.def_id,
             p.span,
             None,
-            /* is_foreign_item */ false,
             kind,
             InheritDeprecation::No,
             InheritConstStability::No,
@@ -712,7 +698,6 @@ fn stability_index(tcx: TyCtxt<'_>, (): ()) -> Index {
             CRATE_DEF_ID,
             tcx.hir().span(CRATE_HIR_ID),
             None,
-            /* is_foreign_item */ false,
             AnnotationKind::Required,
             InheritDeprecation::Yes,
             InheritConstStability::No,

compiler/rustc_span/src/symbol.rs

@@ -1663,6 +1663,7 @@ symbols! {
         rustc_const_panic_str,
         rustc_const_stable,
         rustc_const_stable_indirect,
+        rustc_const_stable_intrinsic,
         rustc_const_unstable,
         rustc_conversion_suggestion,
         rustc_deallocator,