add #[rustc_non_const_trait_method]

Author: fee1-dead

compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs

@@ -320,3 +320,15 @@ impl<S: Stage> NoArgsAttributeParser<S> for RustcHasIncoherentInherentImplsParse
     ]);
     const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::RustcHasIncoherentInherentImpls;
 }
+
+pub(crate) struct RustcNonConstTraitMethodParser;
+
+impl<S: Stage> NoArgsAttributeParser<S> for RustcNonConstTraitMethodParser {
+    const PATH: &'static [Symbol] = &[sym::rustc_non_const_trait_method];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
+        Allow(Target::Method(MethodKind::Trait { body: true })),
+        Allow(Target::Method(MethodKind::Trait { body: false })),
+    ]);
+    const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::RustcNonConstTraitMethod;
+}

compiler/rustc_attr_parsing/src/context.rs

@@ -74,7 +74,8 @@ use crate::attributes::rustc_internal::{
     RustcLintDiagnosticsParser, RustcLintOptDenyFieldAccessParser, RustcLintOptTyParser,
     RustcLintQueryInstabilityParser, RustcLintUntrackedQueryInformationParser, RustcMainParser,
     RustcMustImplementOneOfParser, RustcNeverReturnsNullPointerParser,
-    RustcNoImplicitAutorefsParser, RustcObjectLifetimeDefaultParser, RustcScalableVectorParser,
+    RustcNoImplicitAutorefsParser, RustcNonConstTraitMethodParser,
+    RustcObjectLifetimeDefaultParser, RustcScalableVectorParser,
     RustcSimdMonomorphizeLaneLimitParser,
 };
 use crate::attributes::semantics::MayDangleParser;
@@ -286,6 +287,7 @@ attribute_parsers!(
         Single<WithoutArgs<RustcMainParser>>,
         Single<WithoutArgs<RustcNeverReturnsNullPointerParser>>,
         Single<WithoutArgs<RustcNoImplicitAutorefsParser>>,
+        Single<WithoutArgs<RustcNonConstTraitMethodParser>>,
         Single<WithoutArgs<RustcPassIndirectlyInNonRusticAbisParser>>,
         Single<WithoutArgs<RustcShouldNotBeCalledOnConstItems>>,
         Single<WithoutArgs<SpecializationTraitParser>>,

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -778,12 +778,12 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     // than usual.
 
                     trace!("attempting to call a trait method");
-                    let trait_is_const = tcx.is_const_trait(trait_did);
+                    let is_const = tcx.constness(callee) == hir::Constness::Const;
 
                     // Only consider a trait to be const if the const conditions hold.
                     // Otherwise, it's really misleading to call something "conditionally"
                     // const when it's very obviously not conditionally const.
-                    if trait_is_const && has_const_conditions == Some(ConstConditionsHold::Yes) {
+                    if is_const && has_const_conditions == Some(ConstConditionsHold::Yes) {
                         // Trait calls are always conditionally-const.
                         self.check_op(ops::ConditionallyConstCall {
                             callee,

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -1,4 +1,6 @@
+use rustc_hir::attrs::AttributeKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::find_attr;
 use rustc_hir::{
     Constness, ExprKind, ForeignItemKind, ImplItem, ImplItemImplKind, ImplItemKind, Item, ItemKind,
     Node, TraitItem, TraitItemKind, VariantData,
@@ -36,7 +38,13 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Constness {
                 Constness::NotConst => tcx.constness(tcx.local_parent(def_id)),
             }
         }
-        Node::TraitItem(TraitItem { kind: TraitItemKind::Fn(..), .. }) => tcx.trait_def(tcx.local_parent(def_id)).constness,
+        Node::TraitItem(ti @ TraitItem { kind: TraitItemKind::Fn(..), .. }) => {
+            if find_attr!(tcx.hir_attrs(ti.hir_id()), AttributeKind::RustcNonConstTraitMethod) {
+                Constness::NotConst
+            } else {
+                tcx.trait_def(tcx.local_parent(def_id)).constness
+            }
+        }
         _ => {
             tcx.dcx().span_bug(
                 tcx.def_span(def_id),

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1335,6 +1335,12 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "`#[rustc_has_incoherent_inherent_impls]` allows the addition of incoherent inherent impls for \
          the given type by annotating all impl items with `#[rustc_allow_incoherent_impl]`."
     ),
+    rustc_attr!(
+        rustc_non_const_trait_method, AttributeType::Normal, template!(Word),
+        ErrorFollowing, EncodeCrossCrate::No,
+        "`#[rustc_non_const_trait_method]` should only used by the standard library to mark trait methods \
+        as non-const to allow large traits to easier transition to const"
+    ),
 
     BuiltinAttribute {
         name: sym::rustc_diagnostic_item,

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -963,6 +963,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_no_implicit_autorefs]`
     RustcNoImplicitAutorefs,
 
+    /// Represents `#[rustc_non_const_trait_method]`.
+    RustcNonConstTraitMethod,
+
     /// Represents `#[rustc_object_lifetime_default]`.
     RustcObjectLifetimeDefault,
 

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -116,6 +116,7 @@ impl AttributeKind {
             RustcMustImplementOneOf { .. } => No,
             RustcNeverReturnsNullPointer => Yes,
             RustcNoImplicitAutorefs => Yes,
+            RustcNonConstTraitMethod => No, // should be reported via other queries like `constness`
             RustcObjectLifetimeDefault => No,
             RustcPassIndirectlyInNonRusticAbis(..) => No,
             RustcScalableVector { .. } => Yes,

compiler/rustc_middle/src/ty/mod.rs

@@ -2180,8 +2180,11 @@ impl<'tcx> TyCtxt<'tcx> {
                     DefKind::Impl { of_trait: false } => {
                         self.constness(def_id) == hir::Constness::Const
                     }
-                    DefKind::Impl { of_trait: true } | DefKind::Trait => {
-                        self.is_conditionally_const(parent_def_id)
+                    DefKind::Impl { of_trait: true } => {
+                        self.constness(self.trait_item_of(def_id).unwrap()) == hir::Constness::Const && self.is_conditionally_const(parent_def_id)
+                    }
+                    DefKind::Trait => {
+                        self.constness(def_id) == hir::Constness::Const && self.is_conditionally_const(parent_def_id)
                     }
                     _ => bug!("unexpected parent item of associated fn: {parent_def_id:?}"),
                 }

compiler/rustc_passes/src/check_attr.rs

@@ -315,6 +315,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     | AttributeKind::RustcDumpPredicates
                     | AttributeKind::RustcDumpDefParents
                     | AttributeKind::RustcDumpVtable(..)
+                    | AttributeKind::RustcNonConstTraitMethod
                 ) => { /* do nothing  */ }
                 Attribute::Unparsed(attr_item) => {
                     style = Some(attr_item.style);

compiler/rustc_span/src/symbol.rs

@@ -1990,6 +1990,7 @@ symbols! {
         rustc_no_implicit_bounds,
         rustc_no_mir_inline,
         rustc_nonnull_optimization_guaranteed,
+        rustc_non_const_trait_method,
         rustc_nounwind,
         rustc_objc_class,
         rustc_objc_selector,