When obligation is a sized predicate, prefer projection or object candidates instead of param_env candidates

Author: jackh726

compiler/rustc_infer/src/traits/util.rs

@@ -152,7 +152,7 @@ impl Elaborator<'tcx> {
                         obligation.cause.clone(),
                     )
                 });
-                debug!("super_predicates: data={:?}", data);
+                debug!(?data, ?obligations, "super_predicates");
 
                 // Only keep those bounds that we haven't already seen.
                 // This is necessary to prevent infinite recursion in some

compiler/rustc_middle/src/ty/print/mod.rs

@@ -188,6 +188,11 @@ pub trait Printer<'tcx>: Sized {
             own_params.start = 1;
         }
 
+        // If we're in verbose mode, then print default-equal args too
+        if self.tcx().sess.verbose() {
+            return &substs[own_params];
+        }
+
         // Don't print args that are the defaults of their respective parameters.
         own_params.end -= generics
             .params

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1776,10 +1776,11 @@ impl<F: fmt::Write> Printer<'tcx> for FmtPrinter<'_, 'tcx, F> {
         self = print_prefix(self)?;
 
         // Don't print `'_` if there's no unerased regions.
-        let print_regions = args.iter().any(|arg| match arg.unpack() {
-            GenericArgKind::Lifetime(r) => *r != ty::ReErased,
-            _ => false,
-        });
+        let print_regions = self.tcx.sess.verbose()
+            || args.iter().any(|arg| match arg.unpack() {
+                GenericArgKind::Lifetime(r) => *r != ty::ReErased,
+                _ => false,
+            });
         let args = args.iter().cloned().filter(|arg| match arg.unpack() {
             GenericArgKind::Lifetime(_) => print_regions,
             _ => true,

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1242,6 +1242,10 @@ fn assemble_candidates_from_object_ty<'cx, 'tcx>(
     );
 }
 
+#[tracing::instrument(
+    level = "debug",
+    skip(selcx, candidate_set, ctor, env_predicates, potentially_unnormalized_candidates)
+)]
 fn assemble_candidates_from_predicates<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
@@ -1250,8 +1254,6 @@ fn assemble_candidates_from_predicates<'cx, 'tcx>(
     env_predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
     potentially_unnormalized_candidates: bool,
 ) {
-    debug!(?obligation, "assemble_candidates_from_predicates");
-
     let infcx = selcx.infcx();
     for predicate in env_predicates {
         debug!(?predicate);
@@ -1287,13 +1289,12 @@ fn assemble_candidates_from_predicates<'cx, 'tcx>(
     }
 }
 
+#[tracing::instrument(level = "debug", skip(selcx, obligation, candidate_set))]
 fn assemble_candidates_from_impls<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 ) {
-    debug!("assemble_candidates_from_impls");
-
     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
     // start out by selecting the predicate `T as TraitRef<...>`:
     let poly_trait_ref = ty::Binder::dummy(obligation.predicate.trait_ref(selcx.tcx()));

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

@@ -173,6 +173,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let needs_infer = stack.obligation.predicate.has_infer_types_or_consts();
 
+        let sized_predicate = self.tcx().lang_items().sized_trait()
+            == Some(stack.obligation.predicate.skip_binder().def_id());
+
         // If there are STILL multiple candidates, we can further
         // reduce the list by dropping duplicates -- including
         // resolving specializations.
@@ -181,6 +184,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             while i < candidates.len() {
                 let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
                     self.candidate_should_be_dropped_in_favor_of(
+                        sized_predicate,
                         &candidates[i],
                         &candidates[j],
                         needs_infer,
@@ -338,13 +342,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(candidates)
     }
 
+    #[tracing::instrument(level = "debug", skip(self, candidates))]
     fn assemble_candidates_from_projected_tys(
         &mut self,
         obligation: &TraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
-        debug!(?obligation, "assemble_candidates_from_projected_tys");
-
         // Before we go into the whole placeholder thing, just
         // quickly check if the self-type is a projection at all.
         match obligation.predicate.skip_binder().trait_ref.self_ty().kind() {
@@ -369,12 +372,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// supplied to find out whether it is listed among them.
     ///
     /// Never affects the inference environment.
+    #[tracing::instrument(level = "debug", skip(self, stack, candidates))]
     fn assemble_candidates_from_caller_bounds<'o>(
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) -> Result<(), SelectionError<'tcx>> {
-        debug!(?stack.obligation, "assemble_candidates_from_caller_bounds");
+        debug!(?stack.obligation);
 
         let all_bounds = stack
             .obligation
@@ -876,14 +880,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
     }
 
+    #[tracing::instrument(level = "debug", skip(self, obligation, candidates))]
     fn assemble_candidates_for_trait_alias(
         &mut self,
         obligation: &TraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         // Okay to skip binder here because the tests we do below do not involve bound regions.
         let self_ty = obligation.self_ty().skip_binder();
-        debug!(?self_ty, "assemble_candidates_for_trait_alias");
+        debug!(?self_ty);
 
         let def_id = obligation.predicate.def_id();
 
@@ -894,21 +899,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     /// Assembles the trait which are built-in to the language itself:
     /// `Copy`, `Clone` and `Sized`.
+    #[tracing::instrument(level = "debug", skip(self, candidates))]
     fn assemble_builtin_bound_candidates(
         &mut self,
         conditions: BuiltinImplConditions<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         match conditions {
             BuiltinImplConditions::Where(nested) => {
-                debug!(?nested, "builtin_bound");
                 candidates
                     .vec
                     .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
             }
             BuiltinImplConditions::None => {}
             BuiltinImplConditions::Ambiguous => {
-                debug!("assemble_builtin_bound_candidates: ambiguous builtin");
                 candidates.ambiguous = true;
             }
         }

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

@@ -200,6 +200,7 @@ struct EvaluatedCandidate<'tcx> {
 }
 
 /// When does the builtin impl for `T: Trait` apply?
+#[derive(Debug)]
 enum BuiltinImplConditions<'tcx> {
     /// The impl is conditional on `T1, T2, ...: Trait`.
     Where(ty::Binder<'tcx, Vec<Ty<'tcx>>>),
@@ -343,7 +344,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
             Err(e) => Err(e),
             Ok(candidate) => {
-                debug!(?candidate);
+                debug!(?candidate, "confirmed");
                 Ok(Some(candidate))
             }
         }
@@ -1476,6 +1477,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// See the comment for "SelectionCandidate" for more details.
     fn candidate_should_be_dropped_in_favor_of(
         &mut self,
+        sized_predicate: bool,
         victim: &EvaluatedCandidate<'tcx>,
         other: &EvaluatedCandidate<'tcx>,
         needs_infer: bool,
@@ -1547,6 +1549,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             // Drop otherwise equivalent non-const fn pointer candidates
             (FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => true,
 
+            // If obligation is a sized predicate or the where-clause bound is
+            // global, prefer the projection or object candidate. See issue
+            // #50825 and #89352.
+            (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
+                sized_predicate || is_global(cand)
+            }
+            (ParamCandidate(ref cand), ObjectCandidate(_) | ProjectionCandidate(_)) => {
+                !(sized_predicate || is_global(cand))
+            }
+
             // Global bounds from the where clause should be ignored
             // here (see issue #50825). Otherwise, we have a where
             // clause so don't go around looking for impls.
@@ -1562,15 +1574,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
-                | TraitAliasCandidate(..)
-                | ObjectCandidate(_)
-                | ProjectionCandidate(_),
+                | TraitAliasCandidate(..),
             ) => !is_global(cand),
-            (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
-                // Prefer these to a global where-clause bound
-                // (see issue #50825).
-                is_global(cand)
-            }
             (
                 ImplCandidate(_)
                 | ClosureCandidate

compiler/rustc_typeck/src/astconv/mod.rs

@@ -286,7 +286,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     /// Given the type/lifetime/const arguments provided to some path (along with
     /// an implicit `Self`, if this is a trait reference), returns the complete
     /// set of substitutions. This may involve applying defaulted type parameters.
-    /// Also returns back constraints on associated types.
+    /// Constraints on associated typess are created from `create_assoc_bindings_for_generic_args`.
     ///
     /// Example:
     ///
@@ -300,7 +300,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     ///    which will have been resolved to a `def_id`
     /// 3. The `generic_args` contains info on the `<...>` contents. The `usize` type
     ///    parameters are returned in the `SubstsRef`, the associated type bindings like
-    ///    `Output = u32` are returned in the `Vec<ConvertedBinding...>` result.
+    ///    `Output = u32` are returned from `create_assoc_bindings_for_generic_args`.
     ///
     /// Note that the type listing given here is *exactly* what the user provided.
     ///

compiler/rustc_typeck/src/check/method/mod.rs

@@ -360,6 +360,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let (obligation, substs) =
             self.obligation_for_method(span, trait_def_id, self_ty, opt_input_types);
 
+        debug!(?obligation);
+
         // Now we want to know if this can be matched
         if !self.predicate_may_hold(&obligation) {
             debug!("--> Cannot match obligation");

src/test/ui/associated-types/substs-ppaux.rs

@@ -25,7 +25,7 @@ fn foo<'z>() where &'z (): Sized {
     let x: () = <i8 as Foo<'static, 'static,  u32>>::bar::<'static, char>;
     //[verbose]~^ ERROR mismatched types
     //[verbose]~| expected unit type `()`
-    //[verbose]~| found fn item `fn() {<i8 as Foo<ReStatic, ReStatic>>::bar::<ReStatic, char>}`
+    //[verbose]~| found fn item `fn() {<i8 as Foo<ReStatic, ReStatic, u32>>::bar::<ReStatic, char>}`
     //[normal]~^^^^ ERROR mismatched types
     //[normal]~| expected unit type `()`
     //[normal]~| found fn item `fn() {<i8 as Foo<'static, 'static>>::bar::<'static, char>}`

src/test/ui/associated-types/substs-ppaux.verbose.stderr

@@ -20,15 +20,15 @@ error[E0308]: mismatched types
   --> $DIR/substs-ppaux.rs:25:17
    |
 LL |     fn bar<'a, T>() where T: 'a {}
-   |     --------------------------- fn() {<i8 as Foo<ReStatic, ReStatic>>::bar::<ReStatic, char>} defined here
+   |     --------------------------- fn() {<i8 as Foo<ReStatic, ReStatic, u32>>::bar::<ReStatic, char>} defined here
 ...
 LL |     let x: () = <i8 as Foo<'static, 'static,  u32>>::bar::<'static, char>;
    |            --   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found fn item
    |            |
    |            expected due to this
    |
    = note: expected unit type `()`
-                found fn item `fn() {<i8 as Foo<ReStatic, ReStatic>>::bar::<ReStatic, char>}`
+                found fn item `fn() {<i8 as Foo<ReStatic, ReStatic, u32>>::bar::<ReStatic, char>}`
 help: use parentheses to call this function
    |
 LL |     let x: () = <i8 as Foo<'static, 'static,  u32>>::bar::<'static, char>();