tcx.is_const_fn doesn't work the way it is described, remove it

Then we can rename the _raw functions to drop their suffix, and instead
explicitly use is_stable_const_fn for the few cases where that is really what
you want.

Author: RalfJung

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -731,7 +731,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 }
 
                 // Trait functions are not `const fn` so we have to skip them here.
-                if !tcx.is_const_fn_raw(callee) && !is_trait {
+                if !tcx.is_const_fn(callee) && !is_trait {
                     self.check_op(ops::FnCallNonConst {
                         caller,
                         callee,

compiler/rustc_const_eval/src/check_consts/mod.rs

@@ -112,7 +112,7 @@ pub fn is_safe_to_expose_on_stable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> b
     }
 
     // Const-stability is only relevant for `const fn`.
-    assert!(tcx.is_const_fn_raw(def_id));
+    assert!(tcx.is_const_fn(def_id));
 
     match tcx.lookup_const_stability(def_id) {
         None => {

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -122,7 +122,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
 
                     if let Ok(Some(ImplSource::UserDefined(data))) = implsrc {
                         // FIXME(effects) revisit this
-                        if !tcx.is_const_trait_impl_raw(data.impl_def_id) {
+                        if !tcx.is_const_trait_impl(data.impl_def_id) {
                             let span = tcx.def_span(data.impl_def_id);
                             err.subdiagnostic(errors::NonConstImplNote { span });
                         }
@@ -174,7 +174,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                 let note = match self_ty.kind() {
                     FnDef(def_id, ..) => {
                         let span = tcx.def_span(*def_id);
-                        if ccx.tcx.is_const_fn_raw(*def_id) {
+                        if ccx.tcx.is_const_fn(*def_id) {
                             span_bug!(span, "calling const FnDef errored when it shouldn't");
                         }
 

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -431,8 +431,8 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
             // sensitive check here. But we can at least rule out functions that are not const at
             // all. That said, we have to allow calling functions inside a trait marked with
             // #[const_trait]. These *are* const-checked!
-            // FIXME: why does `is_const_fn_raw` not classify them as const?
-            if (!ecx.tcx.is_const_fn_raw(def) && !ecx.tcx.is_const_default_method(def))
+            // FIXME(effects): why does `is_const_fn` not classify them as const?
+            if (!ecx.tcx.is_const_fn(def) && !ecx.tcx.is_const_default_method(def))
                 || ecx.tcx.has_attr(def, sym::rustc_do_not_const_check)
             {
                 // We certainly do *not* want to actually call the fn

compiler/rustc_hir_analysis/src/collect.rs

@@ -1597,7 +1597,7 @@ 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_raw(def_id.to_def_id()), ast_trait_ref);
+        check_impl_constness(tcx, tcx.is_const_trait_impl(def_id.to_def_id()), ast_trait_ref);
 
         let trait_ref = icx.lowerer().lower_impl_trait_ref(ast_trait_ref, selfty);
 

compiler/rustc_hir_typeck/src/callee.rs

@@ -537,7 +537,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 //
                 // This check is here because there is currently no way to express a trait bound for `FnDef` types only.
                 if let ty::FnDef(def_id, _args) = *arg_ty.kind() {
-                    if idx == 0 && !self.tcx.is_const_fn_raw(def_id) {
+                    if idx == 0 && !self.tcx.is_const_fn(def_id) {
                         self.dcx().emit_err(errors::ConstSelectMustBeConst { span });
                     }
                 } else {

compiler/rustc_hir_typeck/src/expr.rs

@@ -1751,7 +1751,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // to tell them that in the diagnostic. Does not affect typeck.
         let is_constable = match element.kind {
             hir::ExprKind::Call(func, _args) => match *self.node_ty(func.hir_id).kind() {
-                ty::FnDef(def_id, _) if tcx.is_const_fn(def_id) => traits::IsConstable::Fn,
+                ty::FnDef(def_id, _) if tcx.is_stable_const_fn(def_id) => traits::IsConstable::Fn,
                 _ => traits::IsConstable::No,
             },
             hir::ExprKind::Path(qpath) => {

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1081,7 +1081,7 @@ fn should_encode_mir(
                     && (generics.requires_monomorphization(tcx)
                         || tcx.cross_crate_inlinable(def_id)));
             // The function has a `const` modifier or is in a `#[const_trait]`.
-            let is_const_fn = tcx.is_const_fn_raw(def_id.to_def_id())
+            let is_const_fn = tcx.is_const_fn(def_id.to_def_id())
                 || tcx.is_const_default_method(def_id.to_def_id());
             (is_const_fn, opt)
         }

compiler/rustc_middle/src/hir/map/mod.rs

@@ -329,7 +329,7 @@ impl<'hir> Map<'hir> {
             BodyOwnerKind::Static(mutability) => ConstContext::Static(mutability),
 
             BodyOwnerKind::Fn if self.tcx.is_constructor(def_id) => return None,
-            BodyOwnerKind::Fn | BodyOwnerKind::Closure if self.tcx.is_const_fn_raw(def_id) => {
+            BodyOwnerKind::Fn | BodyOwnerKind::Closure if self.tcx.is_const_fn(def_id) => {
                 ConstContext::ConstFn
             }
             BodyOwnerKind::Fn if self.tcx.is_const_default_method(def_id) => ConstContext::ConstFn,

compiler/rustc_middle/src/mir/graphviz.rs

@@ -17,7 +17,7 @@ where
     let mirs = def_ids
         .iter()
         .flat_map(|def_id| {
-            if tcx.is_const_fn_raw(*def_id) {
+            if tcx.is_const_fn(*def_id) {
                 vec![tcx.optimized_mir(*def_id), tcx.mir_for_ctfe(*def_id)]
             } else {
                 vec![tcx.instance_mir(ty::InstanceKind::Item(*def_id))]

compiler/rustc_middle/src/mir/pretty.rs

@@ -317,7 +317,7 @@ pub fn write_mir_pretty<'tcx>(
         };
 
         // For `const fn` we want to render both the optimized MIR and the MIR for ctfe.
-        if tcx.is_const_fn_raw(def_id) {
+        if tcx.is_const_fn(def_id) {
             render_body(w, tcx.optimized_mir(def_id))?;
             writeln!(w)?;
             writeln!(w, "// MIR FOR CTFE")?;

compiler/rustc_middle/src/query/mod.rs

@@ -741,12 +741,11 @@ rustc_queries! {
         desc { |tcx| "computing drop-check constraints for `{}`", tcx.def_path_str(key) }
     }
 
-    /// Returns `true` if this is a const fn, use the `is_const_fn` to know whether your crate
-    /// actually sees it as const fn (e.g., the const-fn-ness might be unstable and you might
-    /// not have the feature gate active).
+    /// Returns `true` if this is a const fn / const impl.
     ///
     /// **Do not call this function manually.** It is only meant to cache the base data for the
-    /// `is_const_fn` function. Consider using `is_const_fn` or `is_const_fn_raw` instead.
+    /// higher-level functions. Consider using `is_const_fn` or `is_const_trait_impl` instead.
+    /// Also note that neither of them takes into account feature gates and stability.
     query constness(key: DefId) -> hir::Constness {
         desc { |tcx| "checking if item is const: `{}`", tcx.def_path_str(key) }
         separate_provide_extern

compiler/rustc_middle/src/ty/context.rs

@@ -3120,39 +3120,24 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
-    /// Whether the `def_id` counts as const fn in the current crate, considering all active
-    /// feature gates
-    pub fn is_const_fn(self, def_id: DefId) -> bool {
-        if self.is_const_fn_raw(def_id) {
-            match self.lookup_const_stability(def_id) {
-                Some(stability) if stability.is_const_unstable() => {
-                    // has a `rustc_const_unstable` attribute, check whether the user enabled the
-                    // corresponding feature gate.
-                    stability.feature.is_some_and(|f| self.features().enabled(f))
-                }
-                // functions without const stability are either stable user written
-                // const fn or the user is using feature gates and we thus don't
-                // care what they do
-                _ => true,
+    /// Whether `def_id` is a stable const fn (i.e., doesn't need any feature gates to be called).
+    ///
+    /// When this is `false`, the function may still be callable as a `const fn` due to features
+    /// being enabled!
+    pub fn is_stable_const_fn(self, def_id: DefId) -> bool {
+        self.is_const_fn(def_id)
+            && match self.lookup_const_stability(def_id) {
+                None => true, // a fn in a non-staged_api crate
+                Some(stability) if stability.is_const_stable() => true,
+                _ => false,
             }
-        } else {
-            false
-        }
     }
 
     // FIXME(effects): Please remove this. It's a footgun.
     /// Whether the trait impl is marked const. This does not consider stability or feature gates.
-    pub fn is_const_trait_impl_raw(self, def_id: DefId) -> bool {
-        let Some(local_def_id) = def_id.as_local() else { return false };
-        let node = self.hir_node_by_def_id(local_def_id);
-
-        matches!(
-            node,
-            hir::Node::Item(hir::Item {
-                kind: hir::ItemKind::Impl(hir::Impl { constness, .. }),
-                ..
-            }) if matches!(constness, hir::Constness::Const)
-        )
+    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
     }
 
     pub fn intrinsic(self, def_id: impl IntoQueryParam<DefId> + Copy) -> Option<ty::IntrinsicDef> {

compiler/rustc_middle/src/ty/mod.rs

@@ -1995,8 +1995,11 @@ impl<'tcx> TyCtxt<'tcx> {
         (ident, scope)
     }
 
+    /// Checks whether this is a `const fn`. Returns `false` for non-functions.
+    ///
+    /// Even if this returns `true`, constness may still be unstable!
     #[inline]
-    pub fn is_const_fn_raw(self, def_id: DefId) -> bool {
+    pub fn is_const_fn(self, def_id: DefId) -> bool {
         matches!(
             self.def_kind(def_id),
             DefKind::Fn | DefKind::AssocFn | DefKind::Ctor(_, CtorKind::Fn) | DefKind::Closure

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -673,7 +673,7 @@ impl<'tcx> Validator<'_, 'tcx> {
         }
         // Make sure the callee is a `const fn`.
         let is_const_fn = match *fn_ty.kind() {
-            ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id),
+            ty::FnDef(def_id, _) => self.tcx.is_const_fn(def_id),
             _ => false,
         };
         if !is_const_fn {

compiler/rustc_passes/src/check_attr.rs

@@ -1997,7 +1997,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
     ) {
         match target {
             Target::Fn | Target::Method(_)
-                if self.tcx.is_const_fn_raw(hir_id.expect_owner().to_def_id()) => {}
+                if self.tcx.is_const_fn(hir_id.expect_owner().to_def_id()) => {}
             // FIXME(#80564): We permit struct fields and match arms to have an
             // `#[allow_internal_unstable]` attribute with just a lint, because we previously
             // erroneously allowed it and some crates used it accidentally, to be compatible

compiler/rustc_passes/src/stability.rs

@@ -179,7 +179,7 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             // their ABI; `fn_sig.abi` is *not* correct for foreign functions.
             && !is_foreign_item
             && const_stab.is_some()
-            && (!self.in_trait_impl || !self.tcx.is_const_fn_raw(def_id.to_def_id()))
+            && (!self.in_trait_impl || !self.tcx.is_const_fn(def_id.to_def_id()))
         {
             self.tcx.dcx().emit_err(errors::MissingConstErr { fn_sig_span: fn_sig.span });
         }
@@ -597,8 +597,8 @@ impl<'tcx> MissingStabilityAnnotations<'tcx> {
             return;
         }
 
-        let is_const = self.tcx.is_const_fn_raw(def_id.to_def_id())
-            || self.tcx.is_const_trait_impl_raw(def_id.to_def_id());
+        let is_const = self.tcx.is_const_fn(def_id.to_def_id())
+            || self.tcx.is_const_trait_impl(def_id.to_def_id());
         let is_stable =
             self.tcx.lookup_stability(def_id).is_some_and(|stability| stability.level.is_stable());
         let missing_const_stability_attribute =
@@ -820,7 +820,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'tcx> {
                     // `#![feature(const_trait_impl)]` is unstable, so any impl declared stable
                     // needs to have an error emitted.
                     if features.const_trait_impl()
-                        && self.tcx.is_const_trait_impl_raw(item.owner_id.to_def_id())
+                        && self.tcx.is_const_trait_impl(item.owner_id.to_def_id())
                         && const_stab.is_some_and(|(stab, _)| stab.is_const_stable())
                     {
                         self.tcx.dcx().emit_err(errors::TraitImplConstStable { span: item.span });

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -393,7 +393,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let self_ty = obligation.self_ty().skip_binder();
         match *self_ty.kind() {
             ty::Closure(def_id, _) => {
-                let is_const = self.tcx().is_const_fn_raw(def_id);
+                let is_const = self.tcx().is_const_fn(def_id);
                 debug!(?kind, ?obligation, "assemble_unboxed_candidates");
                 match self.infcx.closure_kind(self_ty) {
                     Some(closure_kind) => {
@@ -413,7 +413,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
             ty::CoroutineClosure(def_id, args) => {
                 let args = args.as_coroutine_closure();
-                let is_const = self.tcx().is_const_fn_raw(def_id);
+                let is_const = self.tcx().is_const_fn(def_id);
                 if let Some(closure_kind) = self.infcx.closure_kind(self_ty)
                     // Ambiguity if upvars haven't been constrained yet
                     && !args.tupled_upvars_ty().is_ty_var()

src/librustdoc/clean/types.rs

@@ -640,7 +640,7 @@ impl Item {
             asyncness: ty::Asyncness,
         ) -> hir::FnHeader {
             let sig = tcx.fn_sig(def_id).skip_binder();
-            let constness = if tcx.is_const_fn_raw(def_id) {
+            let constness = if tcx.is_const_fn(def_id) {
                 hir::Constness::Const
             } else {
                 hir::Constness::NotConst
@@ -662,7 +662,7 @@ impl Item {
                         safety
                     },
                     abi,
-                    constness: if tcx.is_const_fn_raw(def_id) {
+                    constness: if tcx.is_const_fn(def_id) {
                         hir::Constness::Const
                     } else {
                         hir::Constness::NotConst

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -334,7 +334,7 @@ fn check_terminator<'tcx>(
         | TerminatorKind::TailCall { func, args, fn_span: _ } => {
             let fn_ty = func.ty(body, tcx);
             if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
-                if !is_const_fn(tcx, fn_def_id, msrv) {
+                if !is_stable_const_fn(tcx, fn_def_id, msrv) {
                     return Err((
                         span,
                         format!(
@@ -377,12 +377,12 @@ fn check_terminator<'tcx>(
     }
 }
 
-fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: &Msrv) -> bool {
+fn is_stable_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: &Msrv) -> bool {
     tcx.is_const_fn(def_id)
-        && tcx.lookup_const_stability(def_id).map_or(true, |const_stab| {
+        && tcx.lookup_const_stability(def_id).is_none_or(|const_stab| {
             if let rustc_attr::StabilityLevel::Stable { since, .. } = const_stab.level {
                 // Checking MSRV is manually necessary because `rustc` has no such concept. This entire
-                // function could be removed if `rustc` provided a MSRV-aware version of `is_const_fn`.
+                // function could be removed if `rustc` provided a MSRV-aware version of `is_stable_const_fn`.
                 // as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
 
                 let const_stab_rust_version = match since {
@@ -393,8 +393,12 @@ fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: &Msrv) -> bool {
 
                 msrv.meets(const_stab_rust_version)
             } else {
-                // Unstable const fn with the feature enabled.
-                msrv.current().is_none()
+                // Unstable const fn, check if the feature is enabled. We need both the regular stability
+                // feature and (if set) the const stability feature to const-call this function.
+                let stab = tcx.lookup_stability(def_id);
+                let is_enabled = stab.is_some_and(|s| s.is_stable() || tcx.features().enabled(s.feature))
+                    && const_stab.feature.is_none_or(|f| tcx.features().enabled(f));
+                is_enabled && msrv.current().is_none()
             }
         })
 }

src/tools/clippy/clippy_utils/src/visitors.rs

@@ -346,13 +346,13 @@ pub fn is_const_evaluatable<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) ->
                     .cx
                     .qpath_res(p, hir_id)
                     .opt_def_id()
-                    .map_or(false, |id| self.cx.tcx.is_const_fn_raw(id)) => {},
+                    .map_or(false, |id| self.cx.tcx.is_const_fn(id)) => {},
                 ExprKind::MethodCall(..)
                     if self
                         .cx
                         .typeck_results()
                         .type_dependent_def_id(e.hir_id)
-                        .map_or(false, |id| self.cx.tcx.is_const_fn_raw(id)) => {},
+                        .map_or(false, |id| self.cx.tcx.is_const_fn(id)) => {},
                 ExprKind::Binary(_, lhs, rhs)
                     if self.cx.typeck_results().expr_ty(lhs).peel_refs().is_primitive_ty()
                         && self.cx.typeck_results().expr_ty(rhs).peel_refs().is_primitive_ty() => {},