Auto merge of #59445 - alexreg:ban-multi-trait-objects-via-aliases, r=oli-obk

Ban multi-trait objects via trait aliases

Obviously, multi-trait objects are not normally supported, so they should not be supported via trait aliases.

This has been factored out from the previous PR https://github.com/rust-lang/rust/pull/55994 (see point 1).

r? @Centril

CC @nikomatsakis

------------------

### RELNOTES:

We now allow `dyn Send + fmt::Debug` with equivalent semantics to `dyn fmt::Debug + Send`.
That is, the order of the mentioned traits does not matter wrt. principal/not-principal traits.
This is a small change that might deserve a mention in the blog post because it is a language change but most likely not.

See https://github.com/rust-lang/rust/blob/ce2ee305f9165c037ecddddb5792588a15ff6c37/src/test/ui/traits/wf-trait-object-reverse-order.rs.

// @Centril

2 files changed, 8 insertions, 9 deletions

diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index b60ef557cb8..4d242265e61 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -1075,25 +1075,25 @@ impl<'a, 'gcx, 'tcx> GenericPredicates<'tcx> {
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, HashStable)]
 pub enum Predicate<'tcx> {
-    /// Corresponds to `where Foo: Bar<A,B,C>`. `Foo` here would be
+    /// Corresponds to `where Foo: Bar<A, B, C>`. `Foo` here would be
     /// the `Self` type of the trait reference and `A`, `B`, and `C`
     /// would be the type parameters.
     Trait(PolyTraitPredicate<'tcx>),
 
-    /// where `'a: 'b`
+    /// `where 'a: 'b`
     RegionOutlives(PolyRegionOutlivesPredicate<'tcx>),
 
-    /// where `T: 'a`
+    /// `where T: 'a`
     TypeOutlives(PolyTypeOutlivesPredicate<'tcx>),
 
-    /// where `<T as TraitRef>::Name == X`, approximately.
+    /// `where <T as TraitRef>::Name == X`, approximately.
     /// See the `ProjectionPredicate` struct for details.
     Projection(PolyProjectionPredicate<'tcx>),
 
     /// no syntax: `T` well-formed
     WellFormed(Ty<'tcx>),
 
-    /// trait must be object-safe
+    /// Trait must be object-safe.
     ObjectSafe(DefId),
 
     /// No direct syntax. May be thought of as `where T: FnFoo<...>`
@@ -1234,7 +1234,7 @@ impl<'tcx> TraitPredicate<'tcx> {
         self.trait_ref.def_id
     }
 
-    pub fn input_types<'a>(&'a self) -> impl DoubleEndedIterator<Item=Ty<'tcx>> + 'a {
+    pub fn input_types<'a>(&'a self) -> impl DoubleEndedIterator<Item = Ty<'tcx>> + 'a {
         self.trait_ref.input_types()
     }
 
@@ -2400,7 +2400,7 @@ impl<'a, 'gcx, 'tcx> AdtDef {
     pub fn discriminants(
         &'a self,
         tcx: TyCtxt<'a, 'gcx, 'tcx>,
-    ) -> impl Iterator<Item=(VariantIdx, Discr<'tcx>)> + Captures<'gcx> + 'a {
+    ) -> impl Iterator<Item = (VariantIdx, Discr<'tcx>)> + Captures<'gcx> + 'a {
         let repr_type = self.repr.discr_type();
         let initial = repr_type.initial_discriminant(tcx.global_tcx());
         let mut prev_discr = None::<Discr<'tcx>>;
diff --git a/src/librustc/ty/query/mod.rs b/src/librustc/ty/query/mod.rs
index 18d575f7364..be52b7e6451 100644
--- a/src/librustc/ty/query/mod.rs
+++ b/src/librustc/ty/query/mod.rs
@@ -100,8 +100,7 @@ pub use self::on_disk_cache::OnDiskCache;
 
 rustc_query_append! { [define_queries!][ <'tcx>
     Other {
-        /// Run analysis passes on the crate
+        /// Runs analysis passes on the crate.
         [] fn analysis: Analysis(CrateNum) -> Result<(), ErrorReported>,
-
     },
 ]}