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 #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

Author: bors

src/liballoc/slice.rs

@@ -123,12 +123,12 @@ pub use core::slice::{RChunks, RChunksMut, RChunksExact, RChunksExactMut};
 ////////////////////////////////////////////////////////////////////////////////
 
 // HACK(japaric) needed for the implementation of `vec!` macro during testing
-// NB see the hack module in this file for more details
+// N.B., see the `hack` module in this file for more details.
 #[cfg(test)]
 pub use hack::into_vec;
 
 // HACK(japaric) needed for the implementation of `Vec::clone` during testing
-// NB see the hack module in this file for more details
+// N.B., see the `hack` module in this file for more details.
 #[cfg(test)]
 pub use hack::to_vec;
 
@@ -376,7 +376,7 @@ impl<T> [T] {
     pub fn to_vec(&self) -> Vec<T>
         where T: Clone
     {
-        // NB see hack module in this file
+        // N.B., see the `hack` module in this file for more details.
         hack::to_vec(self)
     }
 
@@ -397,7 +397,7 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn into_vec(self: Box<Self>) -> Vec<T> {
-        // NB see hack module in this file
+        // N.B., see the `hack` module in this file for more details.
         hack::into_vec(self)
     }
 

src/librustc/hir/map/mod.rs

@@ -957,7 +957,7 @@ impl<'hir> Map<'hir> {
         }
     }
 
-    /// Returns the name associated with the given NodeId's AST.
+    /// Returns the name associated with the given `NodeId`'s AST.
     pub fn name(&self, id: NodeId) -> Name {
         let hir_id = self.node_to_hir_id(id);
         self.name_by_hir_id(hir_id)

src/librustc/hir/mod.rs

@@ -2143,11 +2143,11 @@ pub enum UseKind {
     ListStem,
 }
 
-/// TraitRef's appear in impls.
+/// References to traits in impls.
 ///
-/// resolve maps each TraitRef's ref_id to its defining trait; that's all
-/// that the ref_id is for. Note that ref_id's value is not the NodeId of the
-/// trait being referred to but just a unique NodeId that serves as a key
+/// `resolve` maps each `TraitRef`'s `ref_id` to its defining trait; that's all
+/// that the `ref_id` is for. Note that `ref_id`'s value is not the `NodeId` of the
+/// trait being referred to but just a unique `NodeId` that serves as a key
 /// within the resolution map.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub struct TraitRef {

src/librustc/query/mod.rs

@@ -168,7 +168,7 @@ rustc_queries! {
         query predicates_defined_on(_: DefId)
             -> Lrc<ty::GenericPredicates<'tcx>> {}
 
-        /// Returns the predicates written explicit by the user.
+        /// Returns the predicates written explicitly by the user.
         query explicit_predicates_of(_: DefId)
             -> Lrc<ty::GenericPredicates<'tcx>> {}
 
@@ -216,17 +216,17 @@ rustc_queries! {
             _: DefId
         ) -> Result<DtorckConstraint<'tcx>, NoSolution> {}
 
-        /// True if this is a const fn, use the `is_const_fn` to know whether your crate actually
-        /// sees it as const fn (e.g., the const-fn-ness might be unstable and you might not have
-        /// the feature gate active)
+        /// Returns `true` if this is a const fn, use the `is_const_fn` to know whether your crate
+        /// actually sees it as const fn (e.g., the const-fn-ness might be unstable and you might
+        /// not have the feature gate active).
         ///
         /// **Do not call this function manually.** It is only meant to cache the base data for the
         /// `is_const_fn` function.
         query is_const_fn_raw(key: DefId) -> bool {
             desc { |tcx| "checking if item is const fn: `{}`", tcx.def_path_str(key) }
         }
 
-        /// Returns true if calls to the function may be promoted
+        /// Returns `true` if calls to the function may be promoted.
         ///
         /// This is either because the function is e.g., a tuple-struct or tuple-variant
         /// constructor, or because it has the `#[rustc_promotable]` attribute. The attribute should
@@ -237,36 +237,34 @@ rustc_queries! {
 
         query const_fn_is_allowed_fn_ptr(_: DefId) -> bool {}
 
-        /// True if this is a foreign item (i.e., linked via `extern { ... }`).
+        /// Returns `true` if this is a foreign item (i.e., linked via `extern { ... }`).
         query is_foreign_item(_: DefId) -> bool {}
 
         /// Returns `Some(mutability)` if the node pointed to by `def_id` is a static item.
         query static_mutability(_: DefId) -> Option<hir::Mutability> {}
 
-        /// Get a map with the variance of every item; use `item_variance`
-        /// instead.
+        /// Gets a map with the variance of every item; use `item_variance` instead.
         query crate_variances(_: CrateNum) -> Lrc<ty::CrateVariancesMap<'tcx>> {
             desc { "computing the variances for items in this crate" }
         }
 
-        /// Maps from def-id of a type or region parameter to its
-        /// (inferred) variance.
+        /// Maps from the `DefId` of a type or region parameter to its (inferred) variance.
         query variances_of(_: DefId) -> &'tcx [ty::Variance] {}
     }
 
     TypeChecking {
-        /// Maps from def-id of a type to its (inferred) outlives.
+        /// Maps from thee `DefId` of a type to its (inferred) outlives.
         query inferred_outlives_crate(_: CrateNum)
             -> Lrc<ty::CratePredicatesMap<'tcx>> {
             desc { "computing the inferred outlives predicates for items in this crate" }
         }
     }
 
     Other {
-        /// Maps from an impl/trait def-id to a list of the def-ids of its items
+        /// Maps from an impl/trait `DefId to a list of the `DefId`s of its items.
         query associated_item_def_ids(_: DefId) -> Lrc<Vec<DefId>> {}
 
-        /// Maps from a trait item to the trait item "descriptor"
+        /// Maps from a trait item to the trait item "descriptor".
         query associated_item(_: DefId) -> ty::AssociatedItem {}
 
         query impl_trait_ref(_: DefId) -> Option<ty::TraitRef<'tcx>> {}
@@ -276,7 +274,7 @@ rustc_queries! {
     }
 
     TypeChecking {
-        /// Maps a DefId of a type to a list of its inherent impls.
+        /// Maps a `DefId` of a type to a list of its inherent impls.
         /// Contains implementations of methods that are inherent to a type.
         /// Methods in these implementations don't need to be exported.
         query inherent_impls(_: DefId) -> Lrc<Vec<DefId>> {
@@ -300,7 +298,7 @@ rustc_queries! {
             desc { |tcx| "linting {}", key.describe_as_module(tcx) }
         }
 
-        /// Checks the attributes in the module
+        /// Checks the attributes in the module.
         query check_mod_attrs(key: DefId) -> () {
             desc { |tcx| "checking attributes in {}", key.describe_as_module(tcx) }
         }
@@ -309,7 +307,7 @@ rustc_queries! {
             desc { |tcx| "checking for unstable API usage in {}", key.describe_as_module(tcx) }
         }
 
-        /// Checks the loops in the module
+        /// Checks the loops in the module.
         query check_mod_loops(key: DefId) -> () {
             desc { |tcx| "checking loops in {}", key.describe_as_module(tcx) }
         }
@@ -338,7 +336,7 @@ rustc_queries! {
             desc { |tcx| "collecting item types in {}", key.describe_as_module(tcx) }
         }
 
-        /// Caches CoerceUnsized kinds for impls on custom types.
+        /// Caches `CoerceUnsized` kinds for impls on custom types.
         query coerce_unsized_info(_: DefId)
             -> ty::adjustment::CoerceUnsizedInfo {}
     }
@@ -375,7 +373,7 @@ rustc_queries! {
     BorrowChecking {
         query borrowck(_: DefId) -> Lrc<BorrowCheckResult> {}
 
-        /// Borrow checks the function body. If this is a closure, returns
+        /// Borrow-checks the function body. If this is a closure, returns
         /// additional requirements that the closure's creator must verify.
         query mir_borrowck(_: DefId) -> mir::BorrowCheckResult<'tcx> {}
     }
@@ -401,11 +399,11 @@ rustc_queries! {
     }
 
     Other {
-        /// Evaluate a constant without running sanity checks
+        /// Evaluates a constant without running sanity checks.
         ///
         /// **Do not use this** outside const eval. Const eval uses this to break query cycles
         /// during validation. Please add a comment to every use site explaining why using
-        /// `const_eval` isn't sufficient
+        /// `const_eval` isn't sufficient.
         query const_eval_raw(key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>)
             -> ConstEvalRawResult<'tcx> {
             no_force
@@ -660,12 +658,12 @@ rustc_queries! {
     }
 
     Linking {
-        // The DefIds of all non-generic functions and statics in the given crate
+        // The `DefId`s of all non-generic functions and statics in the given crate
         // that can be reached from outside the crate.
         //
         // We expect this items to be available for being linked to.
         //
-        // This query can also be called for LOCAL_CRATE. In this case it will
+        // This query can also be called for `LOCAL_CRATE`. In this case it will
         // compute which items will be reachable to other crates, taking into account
         // the kind of crate that is currently compiled. Crates with only a
         // C interface have fewer reachable things.

src/librustc/traits/mod.rs

@@ -60,8 +60,10 @@ pub use self::specialize::specialization_graph::FutureCompatOverlapError;
 pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
 pub use self::engine::{TraitEngine, TraitEngineExt};
 pub use self::util::{elaborate_predicates, elaborate_trait_ref, elaborate_trait_refs};
-pub use self::util::{supertraits, supertrait_def_ids, transitive_bounds,
-                     Supertraits, SupertraitDefIds};
+pub use self::util::{
+    supertraits, supertrait_def_ids, transitive_bounds, Supertraits, SupertraitDefIds,
+};
+pub use self::util::{expand_trait_aliases, TraitAliasExpander};
 
 pub use self::chalk_fulfill::{
     CanonicalGoal as ChalkCanonicalGoal,
@@ -1043,7 +1045,7 @@ fn vtable_methods<'a, 'tcx>(
     )
 }
 
-impl<'tcx,O> Obligation<'tcx,O> {
+impl<'tcx, O> Obligation<'tcx, O> {
     pub fn new(cause: ObligationCause<'tcx>,
                param_env: ty::ParamEnv<'tcx>,
                predicate: O)

src/librustc/traits/select.rs

@@ -1772,7 +1772,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             bounds
         );
 
-        let matching_bound = util::elaborate_predicates(self.tcx(), bounds.predicates)
+        let elaborated_predicates = util::elaborate_predicates(self.tcx(), bounds.predicates);
+        let matching_bound = elaborated_predicates
             .filter_to_traits()
             .find(|bound| {
                 self.infcx.probe(|_| {