Rollup merge of rust-lang#103525 - oli-obk:const_impl_on_non_const_tr…

…ait, r=lcnr

Move a wf-check into the site where the value is instantiated

r? `@lcnr`

compiler/rustc_error_messages/locales/en-US/hir_analysis.ftl

@@ -137,3 +137,12 @@ hir_analysis_expected_used_symbol = expected `used`, `used(compiler)` or `used(l
 hir_analysis_missing_parentheses_in_range = can't call method `{$method_name}` on type `{$ty_str}`
 
 hir_analysis_add_missing_parentheses_in_range = you must surround the range in parentheses to call its `{$func_name}` function
+
+hir_analysis_const_impl_for_non_const_trait =
+    const `impl` for trait `{$trait_name}` which is not marked with `#[const_trait]`
+    .suggestion = mark `{$trait_name}` as const
+    .note = marking a trait with `#[const_trait]` ensures all default method bodies are `const`
+    .adding = adding a non-const method body in the future would be a breaking change
+
+hir_analysis_const_bound_for_non_const_trait =
+    ~const can only be applied to `#[const_trait]` traits

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -36,7 +36,7 @@ use rustc_session::lint::builtin::{AMBIGUOUS_ASSOCIATED_ITEMS, BARE_TRAIT_OBJECT
 use rustc_span::edition::Edition;
 use rustc_span::lev_distance::find_best_match_for_name;
 use rustc_span::symbol::{kw, Ident, Symbol};
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 use rustc_target::spec::abi;
 use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::astconv_object_safety_violations;
@@ -275,6 +275,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             item_segment.args(),
             item_segment.infer_args,
             None,
+            None,
         );
         if let Some(b) = item_segment.args().bindings.first() {
             Self::prohibit_assoc_ty_binding(self.tcx(), b.span);
@@ -324,6 +325,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         generic_args: &'a hir::GenericArgs<'_>,
         infer_args: bool,
         self_ty: Option<Ty<'tcx>>,
+        constness: Option<ty::BoundConstness>,
     ) -> (SubstsRef<'tcx>, GenericArgCountResult) {
         // If the type is parameterized by this region, then replace this
         // region with the current anon region binding (in other words,
@@ -534,6 +536,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             &mut substs_ctx,
         );
 
+        if let Some(ty::BoundConstness::ConstIfConst) = constness
+            && generics.has_self && !tcx.has_attr(def_id, sym::const_trait)
+        {
+            tcx.sess.emit_err(crate::errors::ConstBoundForNonConstTrait { span } );
+        }
+
         (substs, arg_count)
     }
 
@@ -601,6 +609,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             item_segment.args(),
             item_segment.infer_args,
             None,
+            None,
         );
 
         if let Some(b) = item_segment.args().bindings.first() {
@@ -620,6 +629,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         &self,
         trait_ref: &hir::TraitRef<'_>,
         self_ty: Ty<'tcx>,
+        constness: ty::BoundConstness,
     ) -> ty::TraitRef<'tcx> {
         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1.iter(), |_| {});
 
@@ -629,6 +639,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             self_ty,
             trait_ref.path.segments.last().unwrap(),
             true,
+            Some(constness),
         )
     }
 
@@ -655,6 +666,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             args,
             infer_args,
             Some(self_ty),
+            Some(constness),
         );
 
         let tcx = self.tcx();
@@ -680,6 +692,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 speculative,
                 &mut dup_bindings,
                 binding_span.unwrap_or(binding.span),
+                constness,
             );
             // Okay to ignore `Err` because of `ErrorGuaranteed` (see above).
         }
@@ -783,13 +796,15 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         self_ty: Ty<'tcx>,
         trait_segment: &hir::PathSegment<'_>,
         is_impl: bool,
+        constness: Option<ty::BoundConstness>,
     ) -> ty::TraitRef<'tcx> {
         let (substs, _) = self.create_substs_for_ast_trait_ref(
             span,
             trait_def_id,
             self_ty,
             trait_segment,
             is_impl,
+            constness,
         );
         if let Some(b) = trait_segment.args().bindings.first() {
             Self::prohibit_assoc_ty_binding(self.tcx(), b.span);
@@ -805,6 +820,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         self_ty: Ty<'tcx>,
         trait_segment: &'a hir::PathSegment<'a>,
         is_impl: bool,
+        constness: Option<ty::BoundConstness>,
     ) -> (SubstsRef<'tcx>, GenericArgCountResult) {
         self.complain_about_internal_fn_trait(span, trait_def_id, trait_segment, is_impl);
 
@@ -816,6 +832,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             trait_segment.args(),
             trait_segment.infer_args,
             Some(self_ty),
+            constness,
         )
     }
 
@@ -1027,6 +1044,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         speculative: bool,
         dup_bindings: &mut FxHashMap<DefId, Span>,
         path_span: Span,
+        constness: ty::BoundConstness,
     ) -> Result<(), ErrorGuaranteed> {
         // Given something like `U: SomeTrait<T = X>`, we want to produce a
         // predicate like `<U as SomeTrait>::T = X`. This is somewhat
@@ -1122,10 +1140,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 trait_ref.substs,
             );
 
-            debug!(
-                "add_predicates_for_ast_type_binding: substs for trait-ref and assoc_item: {:?}",
-                substs_trait_ref_and_assoc_item
-            );
+            debug!(?substs_trait_ref_and_assoc_item);
 
             ty::ProjectionTy {
                 item_def_id: assoc_item.def_id,
@@ -1146,8 +1161,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     tcx.collect_constrained_late_bound_regions(&projection_ty);
                 let late_bound_in_ty =
                     tcx.collect_referenced_late_bound_regions(&trait_ref.rebind(ty));
-                debug!("late_bound_in_trait_ref = {:?}", late_bound_in_trait_ref);
-                debug!("late_bound_in_ty = {:?}", late_bound_in_ty);
+                debug!(?late_bound_in_trait_ref);
+                debug!(?late_bound_in_ty);
 
                 // FIXME: point at the type params that don't have appropriate lifetimes:
                 // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
@@ -1648,6 +1663,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
     // Checks that `bounds` contains exactly one element and reports appropriate
     // errors otherwise.
+    #[instrument(level = "debug", skip(self, all_candidates, ty_param_name, is_equality), ret)]
     fn one_bound_for_assoc_type<I>(
         &self,
         all_candidates: impl Fn() -> I,
@@ -1677,10 +1693,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 return Err(reported);
             }
         };
-        debug!("one_bound_for_assoc_type: bound = {:?}", bound);
+        debug!(?bound);
 
         if let Some(bound2) = next_cand {
-            debug!("one_bound_for_assoc_type: bound2 = {:?}", bound2);
+            debug!(?bound2);
 
             let is_equality = is_equality();
             let bounds = IntoIterator::into_iter([bound, bound2]).chain(matching_candidates);
@@ -1776,6 +1792,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     // parameter or `Self`.
     // NOTE: When this function starts resolving `Trait::AssocTy` successfully
     // it should also start reporting the `BARE_TRAIT_OBJECTS` lint.
+    #[instrument(level = "debug", skip(self, hir_ref_id, span, qself, assoc_segment), fields(assoc_ident=?assoc_segment.ident), ret)]
     pub fn associated_path_to_ty(
         &self,
         hir_ref_id: hir::HirId,
@@ -1793,8 +1810,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             Res::Err
         };
 
-        debug!("associated_path_to_ty: {:?}::{}", qself_ty, assoc_ident);
-
         // Check if we have an enum variant.
         let mut variant_resolution = None;
         if let ty::Adt(adt_def, _) = qself_ty.kind() {
@@ -2050,6 +2065,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         item_def_id: DefId,
         trait_segment: &hir::PathSegment<'_>,
         item_segment: &hir::PathSegment<'_>,
+        constness: ty::BoundConstness,
     ) -> Ty<'tcx> {
         let tcx = self.tcx();
 
@@ -2094,8 +2110,14 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         debug!("qpath_to_ty: self_type={:?}", self_ty);
 
-        let trait_ref =
-            self.ast_path_to_mono_trait_ref(span, trait_def_id, self_ty, trait_segment, false);
+        let trait_ref = self.ast_path_to_mono_trait_ref(
+            span,
+            trait_def_id,
+            self_ty,
+            trait_segment,
+            false,
+            Some(constness),
+        );
 
         let item_substs = self.create_substs_for_associated_item(
             span,
@@ -2534,12 +2556,19 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             Res::Def(DefKind::AssocTy, def_id) => {
                 debug_assert!(path.segments.len() >= 2);
                 self.prohibit_generics(path.segments[..path.segments.len() - 2].iter(), |_| {});
+                // HACK: until we support `<Type as ~const Trait>`, assume all of them are.
+                let constness = if tcx.has_attr(tcx.parent(def_id), sym::const_trait) {
+                    ty::BoundConstness::ConstIfConst
+                } else {
+                    ty::BoundConstness::NotConst
+                };
                 self.qpath_to_ty(
                     span,
                     opt_self_ty,
                     def_id,
                     &path.segments[path.segments.len() - 2],
                     path.segments.last().unwrap(),
+                    constness,
                 )
             }
             Res::PrimTy(prim_ty) => {
@@ -2658,6 +2687,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     &GenericArgs::none(),
                     true,
                     None,
+                    None,
                 );
                 EarlyBinder(self.normalize_ty(span, tcx.at(span).type_of(def_id)))
                     .subst(tcx, substs)
@@ -2766,6 +2796,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         }
     }
 
+    #[instrument(level = "debug", skip(self, hir_id, unsafety, abi, decl, generics, hir_ty), ret)]
     pub fn ty_of_fn(
         &self,
         hir_id: hir::HirId,
@@ -2775,8 +2806,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         generics: Option<&hir::Generics<'_>>,
         hir_ty: Option<&hir::Ty<'_>>,
     ) -> ty::PolyFnSig<'tcx> {
-        debug!("ty_of_fn");
-
         let tcx = self.tcx();
         let bound_vars = tcx.late_bound_vars(hir_id);
         debug!(?bound_vars);
@@ -2826,7 +2855,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             hir::FnRetTy::DefaultReturn(..) => tcx.mk_unit(),
         };
 
-        debug!("ty_of_fn: output_ty={:?}", output_ty);
+        debug!(?output_ty);
 
         let fn_ty = tcx.mk_fn_sig(input_tys.into_iter(), output_ty, decl.c_variadic, unsafety, abi);
         let bare_fn_ty = ty::Binder::bind_with_vars(fn_ty, bound_vars);
@@ -2903,8 +2932,11 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(i), .. }) =
                 hir.get(hir.get_parent_node(fn_hir_id)) else { bug!("ImplItem should have Impl parent") };
 
-        let trait_ref =
-            self.instantiate_mono_trait_ref(i.of_trait.as_ref()?, self.ast_ty_to_ty(i.self_ty));
+        let trait_ref = self.instantiate_mono_trait_ref(
+            i.of_trait.as_ref()?,
+            self.ast_ty_to_ty(i.self_ty),
+            ty::BoundConstness::NotConst,
+        );
 
         let assoc = tcx.associated_items(trait_ref.def_id).find_by_name_and_kind(
             tcx,

compiler/rustc_hir_analysis/src/collect.rs

@@ -1143,7 +1143,7 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
         }
 
         ImplItem(hir::ImplItem { kind: ImplItemKind::Fn(sig, _), generics, .. }) => {
-            // Do not try to inference the return type for a impl method coming from a trait
+            // Do not try to infer the return type for a impl method coming from a trait
             if let Item(hir::Item { kind: ItemKind::Impl(i), .. }) =
                 tcx.hir().get(tcx.hir().get_parent_node(hir_id))
                 && i.of_trait.is_some()
@@ -1286,15 +1286,46 @@ fn infer_return_ty_for_fn_sig<'tcx>(
 
 fn impl_trait_ref(tcx: TyCtxt<'_>, def_id: DefId) -> Option<ty::TraitRef<'_>> {
     let icx = ItemCtxt::new(tcx, def_id);
-    match tcx.hir().expect_item(def_id.expect_local()).kind {
+    let item = tcx.hir().expect_item(def_id.expect_local());
+    match item.kind {
         hir::ItemKind::Impl(ref impl_) => impl_.of_trait.as_ref().map(|ast_trait_ref| {
             let selfty = tcx.type_of(def_id);
-            <dyn AstConv<'_>>::instantiate_mono_trait_ref(&icx, ast_trait_ref, selfty)
+            <dyn AstConv<'_>>::instantiate_mono_trait_ref(
+                &icx,
+                ast_trait_ref,
+                selfty,
+                check_impl_constness(tcx, impl_.constness, ast_trait_ref),
+            )
         }),
         _ => bug!(),
     }
 }
 
+fn check_impl_constness(
+    tcx: TyCtxt<'_>,
+    constness: hir::Constness,
+    ast_trait_ref: &hir::TraitRef<'_>,
+) -> ty::BoundConstness {
+    match constness {
+        hir::Constness::Const => {
+            if let Some(trait_def_id) = ast_trait_ref.trait_def_id() && !tcx.has_attr(trait_def_id, sym::const_trait) {
+                let trait_name = tcx.item_name(trait_def_id).to_string();
+                tcx.sess.emit_err(errors::ConstImplForNonConstTrait {
+                    trait_ref_span: ast_trait_ref.path.span,
+                    trait_name,
+                    local_trait_span: trait_def_id.as_local().map(|_| tcx.def_span(trait_def_id).shrink_to_lo()),
+                    marking: (),
+                    adding: (),
+                });
+                ty::BoundConstness::NotConst
+            } else {
+                ty::BoundConstness::ConstIfConst
+            }
+        },
+        hir::Constness::NotConst => ty::BoundConstness::NotConst,
+    }
+}
+
 fn impl_polarity(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ImplPolarity {
     let is_rustc_reservation = tcx.has_attr(def_id, sym::rustc_reservation_impl);
     let item = tcx.hir().expect_item(def_id.expect_local());

compiler/rustc_hir_analysis/src/errors.rs

@@ -249,3 +249,24 @@ pub struct ExpectedUsedSymbol {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_const_impl_for_non_const_trait)]
+pub struct ConstImplForNonConstTrait {
+    #[primary_span]
+    pub trait_ref_span: Span,
+    pub trait_name: String,
+    #[suggestion(applicability = "machine-applicable", code = "#[const_trait]")]
+    pub local_trait_span: Option<Span>,
+    #[note]
+    pub marking: (),
+    #[note(adding)]
+    pub adding: (),
+}
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_const_bound_for_non_const_trait)]
+pub struct ConstBoundForNonConstTrait {
+    #[primary_span]
+    pub span: Span,
+}