Rollup of 4 pull requests

compiler/rustc_ast_passes/src/errors.rs

@@ -950,6 +950,16 @@ pub(crate) struct IncompatibleFeatures {
     pub f2: Symbol,
 }
 
+#[derive(Diagnostic)]
+#[diag("`{$parent}` requires {$missing} to be enabled")]
+#[help("enable all of these features")]
+pub(crate) struct MissingDependentFeatures {
+    #[primary_span]
+    pub parent_span: Span,
+    pub parent: Symbol,
+    pub missing: String,
+}
+
 #[derive(Diagnostic)]
 #[diag("negative bounds are not supported")]
 pub(crate) struct NegativeBoundUnsupported {

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -441,6 +441,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     maybe_stage_features(sess, features, krate);
     check_incompatible_features(sess, features);
+    check_dependent_features(sess, features);
     check_new_solver_banned_features(sess, features);
 
     let mut visitor = PostExpansionVisitor { sess, features };
@@ -649,6 +650,27 @@ fn check_incompatible_features(sess: &Session, features: &Features) {
     }
 }
 
+fn check_dependent_features(sess: &Session, features: &Features) {
+    for &(parent, children) in
+        rustc_feature::DEPENDENT_FEATURES.iter().filter(|(parent, _)| features.enabled(*parent))
+    {
+        if children.iter().any(|f| !features.enabled(*f)) {
+            let parent_span = features
+                .enabled_features_iter_stable_order()
+                .find_map(|(name, span)| (name == parent).then_some(span))
+                .unwrap();
+            // FIXME: should probably format this in fluent instead of here
+            let missing = children
+                .iter()
+                .filter(|f| !features.enabled(**f))
+                .map(|s| format!("`{}`", s.as_str()))
+                .intersperse(String::from(", "))
+                .collect();
+            sess.dcx().emit_err(errors::MissingDependentFeatures { parent_span, parent, missing });
+        }
+    }
+}
+
 fn check_new_solver_banned_features(sess: &Session, features: &Features) {
     if !sess.opts.unstable_opts.next_solver.globally {
         return;

compiler/rustc_ast_passes/src/lib.rs

@@ -5,6 +5,7 @@
 // tidy-alphabetical-start
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
+#![feature(iter_intersperse)]
 #![feature(iter_is_partitioned)]
 // tidy-alphabetical-end
 

compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs

@@ -739,6 +739,19 @@ impl<S: Stage> CombineAttributeParser<S> for RustcThenThisWouldNeedParser {
     }
 }
 
+pub(crate) struct RustcInsignificantDtorParser;
+
+impl<S: Stage> NoArgsAttributeParser<S> for RustcInsignificantDtorParser {
+    const PATH: &[Symbol] = &[sym::rustc_insignificant_dtor];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
+        Allow(Target::Enum),
+        Allow(Target::Struct),
+        Allow(Target::ForeignTy),
+    ]);
+    const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::RustcInsignificantDtor;
+}
+
 pub(crate) struct RustcEffectiveVisibilityParser;
 
 impl<S: Stage> NoArgsAttributeParser<S> for RustcEffectiveVisibilityParser {
@@ -845,3 +858,18 @@ impl<S: Stage> NoArgsAttributeParser<S> for RustcIntrinsicConstStableIndirectPar
     const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[Allow(Target::Fn)]);
     const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::RustcIntrinsicConstStableIndirect;
 }
+
+pub(crate) struct RustcStrictCoherenceParser;
+
+impl<S: Stage> NoArgsAttributeParser<S> for RustcStrictCoherenceParser {
+    const PATH: &[Symbol] = &[sym::rustc_strict_coherence];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
+        Allow(Target::Trait),
+        Allow(Target::Struct),
+        Allow(Target::Enum),
+        Allow(Target::Union),
+        Allow(Target::ForeignTy),
+    ]);
+    const CREATE: fn(Span) -> AttributeKind = AttributeKind::RustcStrictCoherence;
+}

compiler/rustc_attr_parsing/src/context.rs

@@ -272,6 +272,7 @@ attribute_parsers!(
         Single<WithoutArgs<RustcEvaluateWhereClausesParser>>,
         Single<WithoutArgs<RustcHasIncoherentInherentImplsParser>>,
         Single<WithoutArgs<RustcHiddenTypeOfOpaquesParser>>,
+        Single<WithoutArgs<RustcInsignificantDtorParser>>,
         Single<WithoutArgs<RustcIntrinsicConstStableIndirectParser>>,
         Single<WithoutArgs<RustcIntrinsicParser>>,
         Single<WithoutArgs<RustcLintOptTyParser>>,
@@ -289,6 +290,7 @@ attribute_parsers!(
         Single<WithoutArgs<RustcReallocatorParser>>,
         Single<WithoutArgs<RustcRegionsParser>>,
         Single<WithoutArgs<RustcShouldNotBeCalledOnConstItems>>,
+        Single<WithoutArgs<RustcStrictCoherenceParser>>,
         Single<WithoutArgs<RustcVarianceOfOpaquesParser>>,
         Single<WithoutArgs<RustcVarianceParser>>,
         Single<WithoutArgs<SpecializationTraitParser>>,

compiler/rustc_feature/src/lib.rs

@@ -136,5 +136,6 @@ pub use builtin_attrs::{
 };
 pub use removed::REMOVED_LANG_FEATURES;
 pub use unstable::{
-    EnabledLangFeature, EnabledLibFeature, Features, INCOMPATIBLE_FEATURES, UNSTABLE_LANG_FEATURES,
+    DEPENDENT_FEATURES, EnabledLangFeature, EnabledLibFeature, Features, INCOMPATIBLE_FEATURES,
+    UNSTABLE_LANG_FEATURES,
 };

compiler/rustc_feature/src/unstable.rs

@@ -605,6 +605,8 @@ declare_features! (
     (unstable, offset_of_enum, "1.75.0", Some(120141)),
     /// Allows using fields with slice type in offset_of!
     (unstable, offset_of_slice, "1.81.0", Some(126151)),
+    /// Allows using generics in more complex const expressions, based on definitional equality.
+    (unstable, opaque_generic_const_args, "CURRENT_RUSTC_VERSION", Some(151972)),
     /// Allows using `#[optimize(X)]`.
     (unstable, optimize_attribute, "1.34.0", Some(54882)),
     /// Allows specifying nop padding on functions for dynamic patching.
@@ -782,3 +784,9 @@ pub const INCOMPATIBLE_FEATURES: &[(Symbol, Symbol)] = &[
     // boolean logic required to tell which typing rules to use.
     (sym::ref_pat_eat_one_layer_2024, sym::ref_pat_eat_one_layer_2024_structural),
 ];
+
+/// Some features require one or more other features to be enabled.
+pub const DEPENDENT_FEATURES: &[(Symbol, &[Symbol])] = &[
+    (sym::opaque_generic_const_args, &[sym::min_generic_const_args]),
+    (sym::unsized_const_params, &[sym::adt_const_params]),
+];

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -1126,6 +1126,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_if_this_changed]`
     RustcIfThisChanged(Span, Option<Symbol>),
 
+    /// Represents `#[rustc_insignificant_dtor]`
+    RustcInsignificantDtor,
+
     /// Represents `#[rustc_intrinsic]`
     RustcIntrinsic,
 
@@ -1239,6 +1242,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_std_internal_symbol]`.
     RustcStdInternalSymbol(Span),
 
+    /// Represents `#[rustc_strict_coherence]`.
+    RustcStrictCoherence(Span),
+
     /// Represents `#[rustc_symbol_name]`
     RustcSymbolName(Span),
 
@@ -1275,6 +1281,7 @@ pub enum AttributeKind {
         /// Span of the attribute.
         span: Span,
     },
+
     /// Represents `#[target_feature(enable = "...")]` and
     /// `#[unsafe(force_target_feature(enable = "...")]`.
     TargetFeature { features: ThinVec<(Symbol, Span)>, attr_span: Span, was_forced: bool },

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -120,6 +120,7 @@ impl AttributeKind {
             RustcHasIncoherentInherentImpls => Yes,
             RustcHiddenTypeOfOpaques => No,
             RustcIfThisChanged(..) => No,
+            RustcInsignificantDtor => Yes,
             RustcIntrinsic => Yes,
             RustcIntrinsicConstStableIndirect => No,
             RustcLayout(..) => No,
@@ -156,6 +157,7 @@ impl AttributeKind {
             RustcSkipDuringMethodDispatch { .. } => No,
             RustcSpecializationTrait(..) => No,
             RustcStdInternalSymbol(..) => No,
+            RustcStrictCoherence(..) => Yes,
             RustcSymbolName(..) => Yes,
             RustcThenThisWouldNeed(..) => No,
             RustcUnsafeSpecializationMarker(..) => No,

compiler/rustc_hir_analysis/src/collect.rs

@@ -16,7 +16,7 @@
 
 use std::cell::Cell;
 use std::iter;
-use std::ops::Bound;
+use std::ops::{Bound, ControlFlow};
 
 use rustc_abi::{ExternAbi, Size};
 use rustc_ast::Recovered;
@@ -26,12 +26,13 @@ use rustc_errors::{
     Applicability, Diag, DiagCtxtHandle, E0228, ErrorGuaranteed, StashKey, struct_span_code_err,
 };
 use rustc_hir::attrs::AttributeKind;
-use rustc_hir::def::DefKind;
+use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_hir::intravisit::{InferKind, Visitor, VisitorExt};
+use rustc_hir::intravisit::{self, InferKind, Visitor, VisitorExt};
 use rustc_hir::{self as hir, GenericParamKind, HirId, Node, PreciseCapturingArgKind, find_attr};
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::{DynCompatibilityViolation, ObligationCause};
+use rustc_middle::hir::nested_filter;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
 use rustc_middle::ty::{
@@ -1511,6 +1512,20 @@ fn anon_const_kind<'tcx>(tcx: TyCtxt<'tcx>, def: LocalDefId) -> ty::AnonConstKin
             let parent_hir_node = tcx.hir_node(tcx.parent_hir_id(const_arg_id));
             if tcx.features().generic_const_exprs() {
                 ty::AnonConstKind::GCE
+            } else if tcx.features().opaque_generic_const_args() {
+                // Only anon consts that are the RHS of a const item can be OGCA.
+                // Note: We can't just check tcx.parent because it needs to be EXACTLY
+                // the RHS, not just part of the RHS.
+                if !is_anon_const_rhs_of_const_item(tcx, def) {
+                    return ty::AnonConstKind::MCG;
+                }
+
+                let body = tcx.hir_body_owned_by(def);
+                let mut visitor = OGCAParamVisitor(tcx);
+                match visitor.visit_body(body) {
+                    ControlFlow::Break(UsesParam) => ty::AnonConstKind::OGCA,
+                    ControlFlow::Continue(()) => ty::AnonConstKind::MCG,
+                }
             } else if tcx.features().min_generic_const_args() {
                 ty::AnonConstKind::MCG
             } else if let hir::Node::Expr(hir::Expr {
@@ -1528,6 +1543,49 @@ fn anon_const_kind<'tcx>(tcx: TyCtxt<'tcx>, def: LocalDefId) -> ty::AnonConstKin
     }
 }
 
+fn is_anon_const_rhs_of_const_item<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> bool {
+    let hir_id = tcx.local_def_id_to_hir_id(def_id);
+    let Some((_, grandparent_node)) = tcx.hir_parent_iter(hir_id).nth(1) else { return false };
+    let (Node::Item(hir::Item { kind: hir::ItemKind::Const(_, _, _, ct_rhs), .. })
+    | Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Const(_, ct_rhs), .. })
+    | Node::TraitItem(hir::TraitItem {
+        kind: hir::TraitItemKind::Const(_, Some(ct_rhs)), ..
+    })) = grandparent_node
+    else {
+        return false;
+    };
+    let hir::ConstItemRhs::TypeConst(hir::ConstArg {
+        kind: hir::ConstArgKind::Anon(rhs_anon), ..
+    }) = ct_rhs
+    else {
+        return false;
+    };
+    def_id == rhs_anon.def_id
+}
+
+struct OGCAParamVisitor<'tcx>(TyCtxt<'tcx>);
+
+struct UsesParam;
+
+impl<'tcx> Visitor<'tcx> for OGCAParamVisitor<'tcx> {
+    type NestedFilter = nested_filter::OnlyBodies;
+    type Result = ControlFlow<UsesParam>;
+
+    fn maybe_tcx(&mut self) -> TyCtxt<'tcx> {
+        self.0
+    }
+
+    fn visit_path(&mut self, path: &hir::Path<'tcx>, _id: HirId) -> ControlFlow<UsesParam> {
+        if let Res::Def(DefKind::TyParam | DefKind::ConstParam | DefKind::LifetimeParam, _) =
+            path.res
+        {
+            return ControlFlow::Break(UsesParam);
+        }
+
+        intravisit::walk_path(self, path)
+    }
+}
+
 #[instrument(level = "debug", skip(tcx), ret)]
 fn const_of_item<'tcx>(
     tcx: TyCtxt<'tcx>,

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -92,6 +92,8 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
             match tcx.anon_const_kind(def_id) {
                 // Stable: anon consts are not able to use any generic parameters...
                 ty::AnonConstKind::MCG => None,
+                // OGCA anon consts inherit their parent's generics.
+                ty::AnonConstKind::OGCA => Some(parent_did),
                 // we provide generics to repeat expr counts as a backwards compatibility hack. #76200
                 ty::AnonConstKind::RepeatExprCount => Some(parent_did),
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -404,6 +404,11 @@ impl<'tcx> ForbidMCGParamUsesFolder<'tcx> {
                 diag.span_note(impl_.self_ty.span, "not a concrete type");
             }
         }
+        if self.tcx.features().min_generic_const_args()
+            && !self.tcx.features().opaque_generic_const_args()
+        {
+            diag.help("add `#![feature(opaque_generic_const_args)]` to allow generic expressions as the RHS of const items");
+        }
         diag.emit()
     }
 }

compiler/rustc_middle/src/error.rs

@@ -93,7 +93,7 @@ pub(crate) struct StrictCoherenceNeedsNegativeCoherence {
     #[primary_span]
     pub span: Span,
     #[label("due to this attribute")]
-    pub attr_span: Option<Span>,
+    pub attr_span: Span,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_middle/src/traits/specialization_graph.rs

@@ -1,8 +1,9 @@
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::ErrorGuaranteed;
+use rustc_hir::attrs::AttributeKind;
 use rustc_hir::def_id::{DefId, DefIdMap};
+use rustc_hir::find_attr;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable};
-use rustc_span::sym;
 
 use crate::error::StrictCoherenceNeedsNegativeCoherence;
 use crate::ty::fast_reject::SimplifiedType;
@@ -61,23 +62,15 @@ pub enum OverlapMode {
 impl OverlapMode {
     pub fn get(tcx: TyCtxt<'_>, trait_id: DefId) -> OverlapMode {
         let with_negative_coherence = tcx.features().with_negative_coherence();
-        let strict_coherence = tcx.has_attr(trait_id, sym::rustc_strict_coherence);
+        let strict_coherence = find_attr!(tcx.get_all_attrs(trait_id), AttributeKind::RustcStrictCoherence(span) => *span);
 
         if with_negative_coherence {
-            if strict_coherence { OverlapMode::Strict } else { OverlapMode::WithNegative }
+            if strict_coherence.is_some() { OverlapMode::Strict } else { OverlapMode::WithNegative }
         } else {
-            if strict_coherence {
-                let attr_span = trait_id
-                    .as_local()
-                    .into_iter()
-                    .flat_map(|local_def_id| {
-                        tcx.hir_attrs(tcx.local_def_id_to_hir_id(local_def_id))
-                    })
-                    .find(|attr| attr.has_name(sym::rustc_strict_coherence))
-                    .map(|attr| attr.span());
+            if let Some(span) = strict_coherence {
                 tcx.dcx().emit_err(StrictCoherenceNeedsNegativeCoherence {
                     span: tcx.def_span(trait_id),
-                    attr_span,
+                    attr_span: span,
                 });
             }
             OverlapMode::Stable

compiler/rustc_middle/src/ty/consts.rs

@@ -2,7 +2,7 @@ use std::borrow::Cow;
 
 use rustc_data_structures::intern::Interned;
 use rustc_error_messages::MultiSpan;
-use rustc_macros::{HashStable, TyDecodable, TyEncodable};
+use rustc_macros::HashStable;
 use rustc_type_ir::walk::TypeWalker;
 use rustc_type_ir::{self as ir, TypeFlags, WithCachedTypeInfo};
 
@@ -335,16 +335,3 @@ impl<'tcx> Const<'tcx> {
         TypeWalker::new(self.into())
     }
 }
-
-#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, TyEncodable, TyDecodable, HashStable)]
-pub enum AnonConstKind {
-    /// `feature(generic_const_exprs)` anon consts are allowed to use arbitrary generic parameters in scope
-    GCE,
-    /// stable `min_const_generics` anon consts are not allowed to use any generic parameters
-    MCG,
-    /// anon consts used as the length of a repeat expr are syntactically allowed to use generic parameters
-    /// but must not depend on the actual instantiation. See #76200 for more information
-    RepeatExprCount,
-    /// anon consts outside of the type system, e.g. enum discriminants
-    NonTypeSystem,
-}

compiler/rustc_middle/src/ty/context.rs

@@ -238,6 +238,9 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     fn const_of_item(self, def_id: DefId) -> ty::EarlyBinder<'tcx, Const<'tcx>> {
         self.const_of_item(def_id)
     }
+    fn anon_const_kind(self, def_id: DefId) -> ty::AnonConstKind {
+        self.anon_const_kind(def_id)
+    }
 
     type AdtDef = ty::AdtDef<'tcx>;
     fn adt_def(self, adt_def_id: DefId) -> Self::AdtDef {

compiler/rustc_middle/src/ty/mod.rs

@@ -76,8 +76,8 @@ pub use self::closure::{
     place_to_string_for_capture,
 };
 pub use self::consts::{
-    AnonConstKind, AtomicOrdering, Const, ConstInt, ConstKind, ConstToValTreeResult, Expr,
-    ExprKind, ScalarInt, SimdAlign, UnevaluatedConst, ValTree, ValTreeKindExt, Value,
+    AtomicOrdering, Const, ConstInt, ConstKind, ConstToValTreeResult, Expr, ExprKind, ScalarInt,
+    SimdAlign, UnevaluatedConst, ValTree, ValTreeKindExt, Value,
 };
 pub use self::context::{
     CtxtInterners, CurrentGcx, Feed, FreeRegionInfo, GlobalCtxt, Lift, TyCtxt, TyCtxtFeed, tls,