Auto merge of #134122 - oli-obk:push-zqnyznxtpnll, r=petrochenkov

Move impl constness into impl trait header

This PR is kind of the opposite of the rejected https://github.com/rust-lang/rust/pull/134114

Instead of moving more things into the `constness` query, we want to keep them where their corresponding hir nodes are lowered. So I gave this a spin for impls, which have an obvious place to be (the impl trait header). And surprisingly it's also a perf improvement (likely just slightly better query & cache usage).

The issue was that removing anything from the `constness` query makes it just return `NotConst`, which is wrong. So I had to change it to `bug!` out if used wrongly, and only then remove the impl blocks from the `constness` query. I think this change is good in general, because it makes using `constness` more robust (as can be seen by how few sites that had to be changed, so it was almost solely used specifically for the purpose of asking for functions' constness). The main thing where this change was not great was in clippy, which was using the `constness` query as a general DefId -> constness map. I added a `DefKind` filter in front of that. If it becomes a more common pattern we can always move that helper into rustc.

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -4,27 +4,29 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 
-pub 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 { .. })
+    if matches!(tcx.def_kind(parent_id), DefKind::Impl { .. })
-        && tcx.constness(parent_id) == hir::Constness::Const
+        && 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,
+/// Checks whether an item is considered to be `const`. If it is a constructor, it is const.
-/// const block, const item or associated const, it is const. If it is a trait impl/function,
+/// If it is an assoc method or function,
 /// return if it has a `const` modifier. If it is an intrinsic, report whether said intrinsic
-/// has a `rustc_const_{un,}stable` attribute. Otherwise, return `Constness::NotConst`.
+/// has a `rustc_const_{un,}stable` attribute. Otherwise, panic.
 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::Ctor(hir::VariantData::Tuple(..))
-        | hir::Node::AnonConst(_)
-        | hir::Node::ConstBlock(_)
         | 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);
+                parent_impl_constness(tcx, def_id)
-                if is_const { hir::Constness::Const } else { hir::Constness::NotConst }
             } 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

@@ -4042,9 +4042,7 @@ impl<'hir> Node<'hir> {
                 _ => None,
             },
             Node::TraitItem(ti) => match ti.kind {
-                TraitItemKind::Fn(ref sig, TraitFn::Provided(_)) => {
+                TraitItemKind::Fn(ref sig, _) => Some(FnKind::Method(ti.ident, sig)),
-                    Some(FnKind::Method(ti.ident, sig))
-                }
                 _ => None,
             },
             Node::ImplItem(ii) => match ii.kind {

compiler/rustc_hir_analysis/src/collect.rs

@@ -1611,7 +1611,7 @@ 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);
 
@@ -1619,22 +1619,23 @@ fn impl_trait_header(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::ImplTrai
             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 {
+    if let hir::Constness::NotConst = constness {
-        return None;
+        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();
@@ -1650,14 +1651,14 @@ fn check_impl_constness(
             ),
             (false, _) | (_, false) => (None, ""),
         };
-    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,
         suggestion_pre,
         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::Fn | DefKind::AssocFn | DefKind::Closure | DefKind::Ctor(_, CtorKind::Fn) => true,
-        | DefKind::AssocFn
-        | DefKind::Closure
-        | DefKind::Impl { of_trait: true }
-        | DefKind::Variant
-        | DefKind::Ctor(..) => 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 +746,10 @@ 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)
+    ///
+    /// Will ICE if used on things that are always const or never const.
     ///
     /// **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

@@ -254,6 +254,7 @@ 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)]
@@ -2005,17 +2006,25 @@ 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
+                let header = self.impl_trait_header(def_id).unwrap();
-                    && self.is_const_trait(
+                header.constness == hir::Constness::Const
-                        self.trait_id_of_impl(def_id)
+                    && self.is_const_trait(header.trait_ref.skip_binder().def_id)
-                            .expect("expected trait for trait implementation"),
-                    )
             }
             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 } => {
@@ -2024,7 +2033,7 @@ 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);
                     }
 

src/tools/clippy/clippy_lints/src/manual_float_methods.rs

@@ -6,8 +6,11 @@ use clippy_utils::source::SpanRangeExt;
 use clippy_utils::{is_from_proc_macro, path_to_local};
 use rustc_errors::Applicability;
 use rustc_hir::{BinOpKind, Constness, Expr, ExprKind};
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::DefId;
 use rustc_lint::{LateContext, LateLintPass, Lint, LintContext};
 use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::TyCtxt;
 use rustc_session::impl_lint_pass;
 
 declare_clippy_lint! {
@@ -94,6 +97,44 @@ impl ManualFloatMethods {
     }
 }
 
+fn is_not_const(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
+    match tcx.def_kind(def_id) {
+        DefKind::Mod
+        | DefKind::Struct
+        | DefKind::Union
+        | DefKind::Enum
+        | DefKind::Variant
+        | DefKind::Trait
+        | DefKind::TyAlias
+        | DefKind::ForeignTy
+        | DefKind::TraitAlias
+        | DefKind::AssocTy
+        | DefKind::Macro(..)
+        | DefKind::Field
+        | DefKind::LifetimeParam
+        | DefKind::ExternCrate
+        | DefKind::Use
+        | DefKind::ForeignMod
+        | DefKind::GlobalAsm
+        | DefKind::Impl { .. }
+        | DefKind::OpaqueTy
+        | DefKind::SyntheticCoroutineBody
+        | DefKind::TyParam => true,
+
+        DefKind::AnonConst
+        | DefKind::InlineConst
+        | DefKind::Const
+        | DefKind::ConstParam
+        | DefKind::Static { .. }
+        | DefKind::Ctor(..)
+        | DefKind::AssocConst => false,
+
+        DefKind::Fn
+        | DefKind::AssocFn
+        | DefKind::Closure => tcx.constness(def_id) == Constness::NotConst,
+    }
+}
+
 impl<'tcx> LateLintPass<'tcx> for ManualFloatMethods {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
         if let ExprKind::Binary(kind, lhs, rhs) = expr.kind
@@ -105,7 +146,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualFloatMethods {
             && exprs.iter_mut().partition_in_place(|i| path_to_local(i).is_some()) == 2
             && !in_external_macro(cx.sess(), expr.span)
             && (
-                matches!(cx.tcx.constness(cx.tcx.hir().enclosing_body_owner(expr.hir_id)), Constness::NotConst)
+                is_not_const(cx.tcx, cx.tcx.hir().enclosing_body_owner(expr.hir_id).into())
                     || self.msrv.meets(msrvs::CONST_FLOAT_CLASSIFY)
             )
             && let [first, second, const_1, const_2] = exprs