Move impl constness into impl trait header

Author: oli-obk

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -4,10 +4,15 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 
-fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
+fn parent_impl_constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
     let parent_id = tcx.local_parent(def_id);
-    matches!(tcx.def_kind(parent_id), DefKind::Impl { .. })
-        && tcx.constness(parent_id) == hir::Constness::Const
+    if matches!(tcx.def_kind(parent_id), DefKind::Impl { .. })
+        && let Some(header) = tcx.impl_trait_header(parent_id)
+    {
+        header.constness
+    } else {
+        hir::Constness::NotConst
+    }
 }
 
 /// Checks whether an item is considered to be `const`. If it is a constructor, anonymous const,
@@ -18,13 +23,10 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
     let node = tcx.hir_node_by_def_id(def_id);
 
     match node {
-        hir::Node::Ctor(_)
-        | hir::Node::AnonConst(_)
-        | hir::Node::ConstBlock(_)
+        hir::Node::Ctor(hir::VariantData::Tuple(..))
         | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Const(..), .. }) => {
             hir::Constness::Const
         }
-        hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(impl_), .. }) => impl_.constness,
         hir::Node::ForeignItem(_) => {
             // Foreign items cannot be evaluated at compile-time.
             hir::Constness::NotConst
@@ -38,10 +40,12 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
 
                 // If the function itself is not annotated with `const`, it may still be a `const fn`
                 // if it resides in a const trait impl.
-                let is_const = is_parent_const_impl_raw(tcx, def_id);
-                if is_const { hir::Constness::Const } else { hir::Constness::NotConst }
+                parent_impl_constness(tcx, def_id)
             } else {
-                hir::Constness::NotConst
+                tcx.dcx().span_bug(
+                    tcx.def_span(def_id),
+                    format!("should not be requesting the constness of items that can't be const: {node:#?}: {:?}", tcx.def_kind(def_id))
+                )
             }
         }
     }

compiler/rustc_hir/src/hir.rs

@@ -4044,9 +4044,7 @@ impl<'hir> Node<'hir> {
                 _ => None,
             },
             Node::TraitItem(ti) => match ti.kind {
-                TraitItemKind::Fn(ref sig, TraitFn::Provided(_)) => {
-                    Some(FnKind::Method(ti.ident, sig))
-                }
+                TraitItemKind::Fn(ref sig, _) => Some(FnKind::Method(ti.ident, sig)),
                 _ => None,
             },
             Node::ImplItem(ii) => match ii.kind {

compiler/rustc_hir_analysis/src/collect.rs

@@ -1611,41 +1611,43 @@ fn impl_trait_header(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::ImplTrai
     impl_.of_trait.as_ref().map(|ast_trait_ref| {
         let selfty = tcx.type_of(def_id).instantiate_identity();
 
-        check_impl_constness(tcx, tcx.is_const_trait_impl(def_id.to_def_id()), ast_trait_ref);
+        check_impl_constness(tcx, impl_.constness, ast_trait_ref);
 
         let trait_ref = icx.lowerer().lower_impl_trait_ref(ast_trait_ref, selfty);
 
         ty::ImplTraitHeader {
             trait_ref: ty::EarlyBinder::bind(trait_ref),
             safety: impl_.safety,
             polarity: polarity_of_impl(tcx, def_id, impl_, item.span),
+            constness: impl_.constness,
         }
     })
 }
 
 fn check_impl_constness(
     tcx: TyCtxt<'_>,
-    is_const: bool,
+    constness: hir::Constness,
     hir_trait_ref: &hir::TraitRef<'_>,
-) -> Option<ErrorGuaranteed> {
-    if !is_const {
-        return None;
+) {
+    if let hir::Constness::NotConst = constness {
+        return;
     }
 
-    let trait_def_id = hir_trait_ref.trait_def_id()?;
+    let Some(trait_def_id) = hir_trait_ref.trait_def_id() else { return };
     if tcx.is_const_trait(trait_def_id) {
-        return None;
+        return;
     }
 
     let trait_name = tcx.item_name(trait_def_id).to_string();
-    Some(tcx.dcx().emit_err(errors::ConstImplForNonConstTrait {
+    tcx.dcx().emit_err(errors::ConstImplForNonConstTrait {
         trait_ref_span: hir_trait_ref.path.span,
         trait_name,
-        local_trait_span:
-            trait_def_id.as_local().map(|_| tcx.def_span(trait_def_id).shrink_to_lo()),
+        local_trait_span: trait_def_id
+            .as_local()
+            .map(|_| tcx.def_span(trait_def_id).shrink_to_lo()),
         marking: (),
         adding: (),
-    }))
+    });
 }
 
 fn polarity_of_impl(

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1264,12 +1264,7 @@ fn should_encode_fn_sig(def_kind: DefKind) -> bool {
 
 fn should_encode_constness(def_kind: DefKind) -> bool {
     match def_kind {
-        DefKind::Fn
-        | DefKind::AssocFn
-        | DefKind::Closure
-        | DefKind::Impl { of_trait: true }
-        | DefKind::Variant
-        | DefKind::Ctor(..) => true,
+        DefKind::Fn | DefKind::AssocFn | DefKind::Closure | DefKind::Ctor(_, CtorKind::Fn) => true,
 
         DefKind::Struct
         | DefKind::Union
@@ -1281,7 +1276,7 @@ fn should_encode_constness(def_kind: DefKind) -> bool {
         | DefKind::Static { .. }
         | DefKind::TyAlias
         | DefKind::OpaqueTy
-        | DefKind::Impl { of_trait: false }
+        | DefKind::Impl { .. }
         | DefKind::ForeignTy
         | DefKind::ConstParam
         | DefKind::InlineConst
@@ -1296,6 +1291,8 @@ fn should_encode_constness(def_kind: DefKind) -> bool {
         | DefKind::LifetimeParam
         | DefKind::GlobalAsm
         | DefKind::ExternCrate
+        | DefKind::Ctor(_, CtorKind::Const)
+        | DefKind::Variant
         | DefKind::SyntheticCoroutineBody => false,
     }
 }

compiler/rustc_middle/src/query/mod.rs

@@ -746,7 +746,8 @@ rustc_queries! {
         desc { |tcx| "computing drop-check constraints for `{}`", tcx.def_path_str(key) }
     }
 
-    /// Returns `true` if this is a const fn / const impl.
+    /// Returns the constness of function-like things (tuple struct/variant constructors, functions,
+    /// methods)
     ///
     /// **Do not call this function manually.** It is only meant to cache the base data for the
     /// higher-level functions. Consider using `is_const_fn` or `is_const_trait_impl` instead.

compiler/rustc_middle/src/ty/context.rs

@@ -3141,7 +3141,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Whether the trait impl is marked const. This does not consider stability or feature gates.
     pub fn is_const_trait_impl(self, def_id: DefId) -> bool {
         self.def_kind(def_id) == DefKind::Impl { of_trait: true }
-            && self.constness(def_id) == hir::Constness::Const
+            && self.impl_trait_header(def_id).unwrap().constness == hir::Constness::Const
     }
 
     pub fn intrinsic(self, def_id: impl IntoQueryParam<DefId> + Copy) -> Option<ty::IntrinsicDef> {

compiler/rustc_middle/src/ty/mod.rs

@@ -253,6 +253,7 @@ pub struct ImplTraitHeader<'tcx> {
     pub trait_ref: ty::EarlyBinder<'tcx, ty::TraitRef<'tcx>>,
     pub polarity: ImplPolarity,
     pub safety: hir::Safety,
+    pub constness: hir::Constness,
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, TypeFoldable, TypeVisitable)]
@@ -2004,17 +2005,25 @@ impl<'tcx> TyCtxt<'tcx> {
         let def_id: DefId = def_id.into();
         match self.def_kind(def_id) {
             DefKind::Impl { of_trait: true } => {
-                self.constness(def_id) == hir::Constness::Const
-                    && self.is_const_trait(
-                        self.trait_id_of_impl(def_id)
-                            .expect("expected trait for trait implementation"),
-                    )
+                let header = self.impl_trait_header(def_id).unwrap();
+                header.constness == hir::Constness::Const
+                    && self.is_const_trait(header.trait_ref.skip_binder().def_id)
             }
             DefKind::Fn | DefKind::Ctor(_, CtorKind::Fn) => {
                 self.constness(def_id) == hir::Constness::Const
             }
             DefKind::Trait => self.is_const_trait(def_id),
-            DefKind::AssocTy | DefKind::AssocFn => {
+            DefKind::AssocTy => {
+                let parent_def_id = self.parent(def_id);
+                match self.def_kind(parent_def_id) {
+                    DefKind::Impl { of_trait: false } => false,
+                    DefKind::Impl { of_trait: true } | DefKind::Trait => {
+                        self.is_conditionally_const(parent_def_id)
+                    }
+                    _ => bug!("unexpected parent item of associated type: {parent_def_id:?}"),
+                }
+            }
+            DefKind::AssocFn => {
                 let parent_def_id = self.parent(def_id);
                 match self.def_kind(parent_def_id) {
                     DefKind::Impl { of_trait: false } => {
@@ -2023,7 +2032,7 @@ impl<'tcx> TyCtxt<'tcx> {
                     DefKind::Impl { of_trait: true } | DefKind::Trait => {
                         self.is_conditionally_const(parent_def_id)
                     }
-                    _ => bug!("unexpected parent item of associated item: {parent_def_id:?}"),
+                    _ => bug!("unexpected parent item of associated fn: {parent_def_id:?}"),
                 }
             }
             DefKind::OpaqueTy => match self.opaque_ty_origin(def_id) {

compiler/rustc_middle/src/ty/util.rs

@@ -389,7 +389,7 @@ impl<'tcx> TyCtxt<'tcx> {
                     .delay_as_bug();
             }
 
-            dtor_candidate = Some((*item_id, self.constness(impl_did)));
+            dtor_candidate = Some((*item_id, self.impl_trait_header(impl_did).unwrap().constness));
         });
 
         let (did, constness) = dtor_candidate?;

compiler/rustc_trait_selection/src/traits/effects.rs

@@ -128,7 +128,9 @@ fn evaluate_host_effect_from_selection_candiate<'tcx>(
             Err(_) => Err(EvaluationFailure::NoSolution),
             Ok(Some(source)) => match source {
                 ImplSource::UserDefined(impl_) => {
-                    if tcx.constness(impl_.impl_def_id) != hir::Constness::Const {
+                    if tcx.impl_trait_header(impl_.impl_def_id).unwrap().constness
+                        != hir::Constness::Const
+                    {
                         return Err(EvaluationFailure::NoSolution);
                     }