Require the constness query to only be invoked on things that can have constness

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -18,9 +18,7 @@ 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
         }
@@ -41,7 +39,10 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
                 let is_const = is_parent_const_impl_raw(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

@@ -4044,9 +4044,7 @@ 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_metadata/src/rmeta/encoder.rs

@@ -1268,8 +1268,7 @@ fn should_encode_constness(def_kind: DefKind) -> bool {
         | DefKind::AssocFn
         | DefKind::Closure
         | DefKind::Impl { of_trait: true }
-        | DefKind::Variant
+        | DefKind::Ctor(_, CtorKind::Fn) => true,
-        | DefKind::Ctor(..) => true,
 
         DefKind::Struct
         | DefKind::Union
@@ -1296,6 +1295,8 @@ 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 +746,9 @@ 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 functions and impls.
+    ///
+    /// 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/mod.rs

@@ -2014,7 +2014,17 @@ impl<'tcx> TyCtxt<'tcx> {
                 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 +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) {

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