Auto merge of #127200 - fee1-dead-contrib:trait_def_const_trait, r=compiler-errors

Add `constness` to `TraitDef`

Second attempt at fixing the regression @ https://github.com/rust-lang/rust/pull/120639#issuecomment-2198373716

r? project-const-traits

compiler/rustc_ast_lowering/src/item.rs

@@ -625,9 +625,12 @@ fn lower_assoc_item(
                             _ => Const::No,
                         }
                     } else {
-                        self.tcx
+                        if self.tcx.is_const_trait(def_id) {
-                            .get_attr(def_id, sym::const_trait)
+                            // FIXME(effects) span
-                            .map_or(Const::No, |attr| Const::Yes(attr.span))
+                            Const::Yes(self.tcx.def_ident_span(def_id).unwrap())
+                        } else {
+                            Const::No
+                        }
                     }
                 } else {
                     Const::No

compiler/rustc_feature/src/builtin_attrs.rs

@@ -556,7 +556,7 @@ pub struct BuiltinAttribute {
     ),
     // RFC 2632
     gated!(
-        const_trait, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::Yes, const_trait_impl,
+        const_trait, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No, const_trait_impl,
         "`const_trait` is a temporary placeholder for marking a trait that is suitable for `const` \
         `impls` and all default bodies as `const`, which may be removed or renamed in the \
         future."

compiler/rustc_hir_analysis/src/bounds.rs

@@ -7,7 +7,7 @@
 use rustc_middle::ty::fold::FnMutDelegate;
 use rustc_middle::ty::{self, Ty, TyCtxt, Upcast};
 use rustc_span::def_id::DefId;
-use rustc_span::{sym, Span};
+use rustc_span::Span;
 
 /// Collects together a list of type bounds. These lists of bounds occur in many places
 /// in Rust's syntax:
@@ -80,7 +80,7 @@ pub fn push_trait_bound(
             }
 
             (_, ty::BoundConstness::NotConst) => {
-                if !tcx.has_attr(bound_trait_ref.def_id(), sym::const_trait) {
+                if !tcx.is_const_trait(bound_trait_ref.def_id()) {
                     return;
                 }
                 tcx.consts.true_

compiler/rustc_hir_analysis/src/collect.rs

@@ -1194,6 +1194,11 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
         _ => span_bug!(item.span, "trait_def_of_item invoked on non-trait"),
     };
 
+    let constness = if tcx.has_attr(def_id, sym::const_trait) {
+        hir::Constness::Const
+    } else {
+        hir::Constness::NotConst
+    };
     let paren_sugar = tcx.has_attr(def_id, sym::rustc_paren_sugar);
     if paren_sugar && !tcx.features().unboxed_closures {
         tcx.dcx().emit_err(errors::ParenSugarAttribute { span: item.span });
@@ -1349,6 +1354,7 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
     ty::TraitDef {
         def_id: def_id.to_def_id(),
         safety,
+        constness,
         paren_sugar,
         has_auto_impl: is_auto,
         is_marker,
@@ -1682,7 +1688,7 @@ fn check_impl_constness(
     }
 
     let trait_def_id = hir_trait_ref.trait_def_id()?;
-    if tcx.has_attr(trait_def_id, sym::const_trait) {
+    if tcx.is_const_trait(trait_def_id) {
         return None;
     }
 

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -47,7 +47,7 @@
 use rustc_session::lint::builtin::AMBIGUOUS_ASSOCIATED_ITEMS;
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::symbol::{kw, Ident, Symbol};
-use rustc_span::{sym, Span, DUMMY_SP};
+use rustc_span::{Span, DUMMY_SP};
 use rustc_target::spec::abi;
 use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::wf::object_region_bounds;
@@ -560,7 +560,7 @@ fn inferred_kind(
         }
         if let ty::BoundConstness::Const | ty::BoundConstness::ConstIfConst = constness
             && generics.has_self
-            && !tcx.has_attr(def_id, sym::const_trait)
+            && !tcx.is_const_trait(def_id)
         {
             let reported = tcx.dcx().emit_err(crate::errors::ConstBoundForNonConstTrait {
                 span,
@@ -1848,19 +1848,13 @@ pub fn lower_path(
                     path.segments[..path.segments.len() - 2].iter(),
                     GenericsArgsErrExtend::None,
                 );
-                // 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.lower_qpath(
                     span,
                     opt_self_ty,
                     def_id,
                     &path.segments[path.segments.len() - 2],
                     path.segments.last().unwrap(),
-                    constness,
+                    ty::BoundConstness::NotConst,
                 )
             }
             Res::PrimTy(prim_ty) => {

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -21,7 +21,7 @@
 use rustc_middle::ty::{GenericArgs, GenericArgsRef};
 use rustc_middle::{bug, span_bug};
 use rustc_span::symbol::Ident;
-use rustc_span::{sym, Span};
+use rustc_span::Span;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{self, NormalizeExt};
 
@@ -359,7 +359,7 @@ pub(super) fn lookup_method_in_trait(
         // FIXME(effects) find a better way to do this
         // Operators don't have generic methods, but making them `#[const_trait]` gives them
         // `const host: bool`.
-        let args = if self.tcx.has_attr(trait_def_id, sym::const_trait) {
+        let args = if self.tcx.is_const_trait(trait_def_id) {
             self.tcx.mk_args_from_iter(
                 args.iter()
                     .chain([self.tcx.expected_host_effect_param_for_body(self.body_id).into()]),

compiler/rustc_middle/src/ty/mod.rs

@@ -1967,9 +1967,14 @@ pub fn is_const_fn_raw(self, def_id: DefId) -> bool {
         ) && self.constness(def_id) == hir::Constness::Const
     }
 
+    #[inline]
+    pub fn is_const_trait(self, def_id: DefId) -> bool {
+        self.trait_def(def_id).constness == hir::Constness::Const
+    }
+
     #[inline]
     pub fn is_const_default_method(self, def_id: DefId) -> bool {
-        matches!(self.trait_of_item(def_id), Some(trait_id) if self.has_attr(trait_id, sym::const_trait))
+        matches!(self.trait_of_item(def_id), Some(trait_id) if self.is_const_trait(trait_id))
     }
 
     pub fn impl_method_has_trait_impl_trait_tys(self, def_id: DefId) -> bool {

compiler/rustc_middle/src/ty/trait_def.rs

@@ -18,6 +18,9 @@ pub struct TraitDef {
 
     pub safety: hir::Safety,
 
+    /// Whether this trait has been annotated with `#[const_trait]`.
+    pub constness: hir::Constness,
+
     /// If `true`, then this trait had the `#[rustc_paren_sugar]`
     /// attribute, indicating that it should be used with `Foo()`
     /// sugar. This is a temporary thing -- eventually any trait will

tests/ui/traits/default-method/rustc_must_implement_one_of_misuse.stderr

@@ -22,18 +22,6 @@ LL |
 LL | struct Struct {}
    | ---------------- not a trait
 
-error: function not found in this trait
-  --> $DIR/rustc_must_implement_one_of_misuse.rs:3:31
-   |
-LL | #[rustc_must_implement_one_of(a, b)]
-   |                               ^
-
-error: function not found in this trait
-  --> $DIR/rustc_must_implement_one_of_misuse.rs:3:34
-   |
-LL | #[rustc_must_implement_one_of(a, b)]
-   |                                  ^
-
 error: function not found in this trait
   --> $DIR/rustc_must_implement_one_of_misuse.rs:8:34
    |
@@ -46,6 +34,18 @@ error: the `#[rustc_must_implement_one_of]` attribute must be used with at least
 LL | #[rustc_must_implement_one_of(a)]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: function not found in this trait
+  --> $DIR/rustc_must_implement_one_of_misuse.rs:3:31
+   |
+LL | #[rustc_must_implement_one_of(a, b)]
+   |                               ^
+
+error: function not found in this trait
+  --> $DIR/rustc_must_implement_one_of_misuse.rs:3:34
+   |
+LL | #[rustc_must_implement_one_of(a, b)]
+   |                                  ^
+
 error: not a function
   --> $DIR/rustc_must_implement_one_of_misuse.rs:26:5
    |