Auto merge of rust-lang#119825 - compiler-errors:elaborate-dyn, r=<try>

Eagerly elaborate auto-trait supertraits into `dyn Trait`

When we have a trait like:
```
trait Foo: Send {}
```

Then the `dyn Foo` type is elaborated to include `Send` as part of its auto-traits list. That means it is the *same* type as `dyn Foo + Send`.

## Why?

When a auto trait is implied as part of the supertraits of the trait object's principal, the user should *never* care about whether they explicitly write `+ AutoTrait` or have it implied from the supertrait bounds. This PR removes this subtle distinction.

IMO, It *never* makes sense for `dyn Foo` to be a distinct type from `dyn Foo + Send`, since both implement `Send`, just via different code paths in the trait solver.

---

Alternative approach to rust-lang#119338.

r? lcnr

Author: bors

compiler/rustc_hir_analysis/src/astconv/object_safety.rs

@@ -1,7 +1,7 @@
 use crate::astconv::{GenericArgCountMismatch, GenericArgCountResult, OnlySelfBounds};
 use crate::bounds::Bounds;
 use crate::errors::TraitObjectDeclaredWithNoTraits;
-use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
+use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
@@ -83,7 +83,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let expanded_traits =
             traits::expand_trait_aliases(tcx, trait_bounds.iter().map(|&(a, b)| (a, b)));
 
-        let (mut auto_traits, regular_traits): (Vec<_>, Vec<_>) = expanded_traits
+        let (auto_traits, regular_traits): (Vec<_>, Vec<_>) = expanded_traits
             .filter(|i| i.trait_ref().self_ty().skip_binder() == dummy_self)
             .partition(|i| tcx.trait_is_auto(i.trait_ref().def_id()));
         if regular_traits.len() > 1 {
@@ -238,14 +238,14 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             hir_trait_bounds,
         );
 
-        // De-duplicate auto traits so that, e.g., `dyn Trait + Send + Send` is the same as
-        // `dyn Trait + Send`.
-        // We remove duplicates by inserting into a `FxHashSet` to avoid re-ordering
-        // the bounds
-        let mut duplicates = FxHashSet::default();
-        auto_traits.retain(|i| duplicates.insert(i.trait_ref().def_id()));
-        debug!("regular_traits: {:?}", regular_traits);
-        debug!("auto_traits: {:?}", auto_traits);
+        let auto_traits: FxIndexSet<DefId> = auto_traits
+            .into_iter()
+            .map(|info| info.trait_ref().def_id())
+            .chain(regular_traits.iter().flat_map(|info| {
+                traits::supertrait_def_ids(tcx, info.trait_ref().def_id())
+                    .filter(|def_id| tcx.trait_is_auto(*def_id))
+            }))
+            .collect();
 
         // Erase the `dummy_self` (`trait_object_dummy_self`) used above.
         let existential_trait_refs = regular_traits.iter().map(|i| {
@@ -351,9 +351,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         let regular_trait_predicates = existential_trait_refs
             .map(|trait_ref| trait_ref.map_bound(ty::ExistentialPredicate::Trait));
-        let auto_trait_predicates = auto_traits.into_iter().map(|trait_ref| {
-            ty::Binder::dummy(ty::ExistentialPredicate::AutoTrait(trait_ref.trait_ref().def_id()))
-        });
+        let auto_trait_predicates = auto_traits
+            .into_iter()
+            .map(|def_id| ty::Binder::dummy(ty::ExistentialPredicate::AutoTrait(def_id)));
         // N.b. principal, projections, auto traits
         // FIXME: This is actually wrong with multiple principals in regards to symbol mangling
         let mut v = regular_trait_predicates

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

@@ -1293,6 +1293,19 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
         // FIXME(eddyb) avoid printing twice (needed to ensure
         // that the auto traits are sorted *and* printed via cx).
         let mut auto_traits: Vec<_> = predicates.auto_traits().collect();
+        let principal_super_traits: FxHashSet<_> = predicates
+            .principal()
+            .into_iter()
+            .flat_map(|principal| {
+                supertraits_for_pretty_printing(
+                    self.tcx(),
+                    principal.with_self_ty(self.tcx(), self.tcx().types.trait_object_dummy_self),
+                )
+                .map(|trait_ref| trait_ref.def_id())
+            })
+            .collect();
+
+        auto_traits.retain(|def_id| !principal_super_traits.contains(def_id));
 
         // The auto traits come ordered by `DefPathHash`. While
         // `DefPathHash` is *stable* in the sense that it depends on

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -853,6 +853,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             let principal_trait_ref = principal.with_self_ty(tcx, self_ty);
             self.walk_vtable(principal_trait_ref, |ecx, assumption, vtable_base, _| {
                 match G::consider_object_bound_candidate(ecx, goal, assumption.to_predicate(tcx)) {
+                    // FIXME: We could skip auto traits here, since they're eagerly elaborated in
+                    // <https://github.com/rust-lang/rust/pull/119825>.
                     Ok(result) => candidates.push(Candidate {
                         source: CandidateSource::BuiltinImpl(BuiltinImplSource::Object {
                             vtable_base,

tests/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs

@@ -12,8 +12,11 @@ auto trait Marker2 {}
 trait Object: Marker1 {}
 
 // A supertrait marker is illegal...
-impl !Marker1 for dyn Object + Marker2 {} //~ ERROR E0371
-                                          //~^ ERROR 0321
+impl !Marker1 for dyn Object + Marker2 {}
+//~^ ERROR E0371
+//~| ERROR E0321
+//~| ERROR E0371
+
 // ...and also a direct component.
 impl !Marker2 for dyn Object + Marker2 {} //~ ERROR E0371
                                           //~^ ERROR 0321

tests/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr

@@ -4,6 +4,14 @@ error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically i
 LL | impl !Marker1 for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
 
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:15:1
+   |
+LL | impl !Marker1 for dyn Object + Marker2 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 error[E0321]: traits with a default impl, like `Marker1`, cannot be implemented for trait object `(dyn Object + Marker2 + 'static)`
   --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:15:1
    |
@@ -13,29 +21,29 @@ LL | impl !Marker1 for dyn Object + Marker2 {}
    = note: a trait object implements `Marker1` if and only if `Marker1` is one of the trait object's trait bounds
 
 error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:18:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:21:1
    |
 LL | impl !Marker2 for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2`
 
 error[E0321]: traits with a default impl, like `Marker2`, cannot be implemented for trait object `(dyn Object + Marker2 + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:18:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:21:1
    |
 LL | impl !Marker2 for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: a trait object implements `Marker2` if and only if `Marker2` is one of the trait object's trait bounds
 
 error[E0321]: traits with a default impl, like `Marker2`, cannot be implemented for trait object `(dyn Object + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:26:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:29:1
    |
 LL | impl !Marker2 for dyn Object {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: a trait object implements `Marker2` if and only if `Marker2` is one of the trait object's trait bounds
 
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:22:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:25:1
    |
 LL | impl !Send for dyn Marker2 {}
    | ^^^^^^^^^^^^^^^-----------
@@ -46,26 +54,26 @@ LL | impl !Send for dyn Marker2 {}
    = note: define and implement a trait or new type instead
 
 error[E0321]: cross-crate traits with a default impl, like `Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:27:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:30:1
    |
 LL | impl !Send for dyn Object {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
 
 error[E0321]: cross-crate traits with a default impl, like `Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:28:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:31:1
    |
 LL | impl !Send for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
 
 error[E0321]: traits with a default impl, like `Marker3`, cannot be implemented for generic type `T`
-  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:32:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:35:1
    |
 LL | impl<T: ?Sized> !Marker3 for T {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: a trait object implements `Marker3` if and only if `Marker3` is one of the trait object's trait bounds
 
-error: aborting due to 9 previous errors
+error: aborting due to 10 previous errors
 
 Some errors have detailed explanations: E0117, E0321, E0371.
 For more information about an error, try `rustc --explain E0117`.

tests/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs

@@ -12,8 +12,10 @@ auto trait Marker2 {}
 trait Object: Marker1 {}
 
 // A supertrait marker is illegal...
-impl Marker1 for dyn Object + Marker2 {} //~ ERROR E0371
-                                         //~^ ERROR E0321
+impl Marker1 for dyn Object + Marker2 {}
+//~^ ERROR E0371
+//~| ERROR E0321
+//~| ERROR E0371
 // ...and also a direct component.
 impl Marker2 for dyn Object + Marker2 {} //~ ERROR E0371
                                          //~^ ERROR E0321

tests/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr

@@ -4,6 +4,14 @@ error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically i
 LL | impl Marker1 for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
 
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:15:1
+   |
+LL | impl Marker1 for dyn Object + Marker2 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
 error[E0321]: traits with a default impl, like `Marker1`, cannot be implemented for trait object `(dyn Object + Marker2 + 'static)`
   --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:15:1
    |
@@ -13,29 +21,29 @@ LL | impl Marker1 for dyn Object + Marker2 {}
    = note: a trait object implements `Marker1` if and only if `Marker1` is one of the trait object's trait bounds
 
 error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:18:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:20:1
    |
 LL | impl Marker2 for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2`
 
 error[E0321]: traits with a default impl, like `Marker2`, cannot be implemented for trait object `(dyn Object + Marker2 + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:18:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:20:1
    |
 LL | impl Marker2 for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: a trait object implements `Marker2` if and only if `Marker2` is one of the trait object's trait bounds
 
 error[E0321]: traits with a default impl, like `Marker2`, cannot be implemented for trait object `(dyn Object + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:26:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:28:1
    |
 LL | impl Marker2 for dyn Object {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: a trait object implements `Marker2` if and only if `Marker2` is one of the trait object's trait bounds
 
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:22:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:24:1
    |
 LL | unsafe impl Send for dyn Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^-----------
@@ -46,26 +54,26 @@ LL | unsafe impl Send for dyn Marker2 {}
    = note: define and implement a trait or new type instead
 
 error[E0321]: cross-crate traits with a default impl, like `Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:27:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:29:1
    |
 LL | unsafe impl Send for dyn Object {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
 
 error[E0321]: cross-crate traits with a default impl, like `Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:28:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:30:1
    |
 LL | unsafe impl Send for dyn Object + Marker2 {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
 
 error[E0321]: traits with a default impl, like `Marker3`, cannot be implemented for generic type `T`
-  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:32:1
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:34:1
    |
 LL | impl<T: ?Sized> Marker3 for T {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: a trait object implements `Marker3` if and only if `Marker3` is one of the trait object's trait bounds
 
-error: aborting due to 9 previous errors
+error: aborting due to 10 previous errors
 
 Some errors have detailed explanations: E0117, E0321, E0371.
 For more information about an error, try `rustc --explain E0117`.

tests/ui/traits/object/coherence-with-implied-auto-traits.rs

@@ -0,0 +1,9 @@
+trait Foo: Send {}
+
+trait Bar {}
+
+impl Bar for dyn Foo {}
+impl Bar for dyn Foo + Send {}
+//~^ ERROR conflicting implementations of trait `Bar` for type `(dyn Foo + 'static)`
+
+fn main() {}

tests/ui/traits/object/coherence-with-implied-auto-traits.stderr

@@ -0,0 +1,11 @@
+error[E0119]: conflicting implementations of trait `Bar` for type `(dyn Foo + 'static)`
+  --> $DIR/coherence-with-implied-auto-traits.rs:6:1
+   |
+LL | impl Bar for dyn Foo {}
+   | -------------------- first implementation here
+LL | impl Bar for dyn Foo + Send {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn Foo + 'static)`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0119`.

tests/ui/traits/object/upcasting-with-implied-auto-traits.rs

@@ -0,0 +1,13 @@
+// check-pass
+// revisions: current next
+//[next] compile-flags: -Znext-solver
+
+trait Target {}
+trait Source: Send + Target {}
+
+fn upcast(x: &dyn Source) -> &(dyn Target + Send) { x }
+
+fn same(x: &dyn Source) -> &(dyn Source + Send) { x }
+// ^ This isn't upcasting, just passing dyn through unchanged.
+
+fn main() {}