Auto merge of #45047 - durka:trait-alias, r=petrochenkov

trait alias infrastructure

This will be an implementation of trait aliases (RFC 1733, #41517).

Progress so far:

- [x] Feature gate
- [x] Add to parser
  - [x] `where` clauses
    - [x] prohibit LHS type parameter bounds via AST validation #45047 (comment)
- [x] Add to AST and HIR
  - [x] make a separate PathSource for trait alias contexts #45047 (comment)
- [x] Stub out enough of typeck and resolve to just barely not ICE

Postponed:

- [ ] Actually implement the alias part
- [ ] #21903
- [ ] #24010

I need some pointers on where to start with that last one. The test currently does this:

```
error[E0283]: type annotations required: cannot resolve `_: CD`
  --> src/test/run-pass/trait-alias.rs:34:16
   |
34 |     let both = foo();
   |                ^^^
   |
   = note: required by `foo`
```

Author: bors

src/librustc/hir/def.rs

@@ -37,6 +37,7 @@ pub enum Def {
     Trait(DefId),
     TyAlias(DefId),
     TyForeign(DefId),
+    TraitAlias(DefId),
     AssociatedTy(DefId),
     PrimTy(hir::PrimTy),
     TyParam(DefId),
@@ -155,7 +156,8 @@ impl Def {
     pub fn def_id(&self) -> DefId {
         match *self {
             Def::Fn(id) | Def::Mod(id) | Def::Static(id, _) |
-            Def::Variant(id) | Def::VariantCtor(id, ..) | Def::Enum(id) | Def::TyAlias(id) |
+            Def::Variant(id) | Def::VariantCtor(id, ..) | Def::Enum(id) |
+            Def::TyAlias(id) | Def::TraitAlias(id) |
             Def::AssociatedTy(id) | Def::TyParam(id) | Def::Struct(id) | Def::StructCtor(id, ..) |
             Def::Union(id) | Def::Trait(id) | Def::Method(id) | Def::Const(id) |
             Def::AssociatedConst(id) | Def::Macro(id, ..) |
@@ -186,6 +188,7 @@ impl Def {
             Def::VariantCtor(.., CtorKind::Fictive) => "struct variant",
             Def::Enum(..) => "enum",
             Def::TyAlias(..) => "type alias",
+            Def::TraitAlias(..) => "trait alias",
             Def::AssociatedTy(..) => "associated type",
             Def::Struct(..) => "struct",
             Def::StructCtor(.., CtorKind::Fn) => "tuple struct",

src/librustc/hir/intravisit.rs

@@ -526,6 +526,11 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item) {
             walk_list!(visitor, visit_ty_param_bound, bounds);
             walk_list!(visitor, visit_trait_item_ref, trait_item_refs);
         }
+        ItemTraitAlias(ref generics, ref bounds) => {
+            visitor.visit_id(item.id);
+            visitor.visit_generics(generics);
+            walk_list!(visitor, visit_ty_param_bound, bounds);
+        }
     }
     walk_list!(visitor, visit_attribute, &item.attrs);
 }

src/librustc/hir/lowering.rs

@@ -1924,9 +1924,11 @@ impl<'a> LoweringContext<'a> {
                                bounds,
                                items)
             }
-            ItemKind::MacroDef(..) | ItemKind::Mac(..) => {
-                panic!("Shouldn't still be around")
+            ItemKind::TraitAlias(ref generics, ref bounds) => {
+                hir::ItemTraitAlias(self.lower_generics(generics),
+                                    self.lower_bounds(bounds, ImplTraitContext::Disallowed))
             }
+            ItemKind::MacroDef(..) | ItemKind::Mac(..) => panic!("Shouldn't still be around"),
         }
 
         // [1] `defaultness.has_value()` is never called for an `impl`, always `true` in order to

src/librustc/hir/map/def_collector.rs

@@ -106,7 +106,8 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
         let def_data = match i.node {
             ItemKind::AutoImpl(..) | ItemKind::Impl(..) =>
                 DefPathData::Impl,
-            ItemKind::Enum(..) | ItemKind::Struct(..) | ItemKind::Union(..) | ItemKind::Trait(..) |
+            ItemKind::Enum(..) | ItemKind::Struct(..) | ItemKind::Union(..) |
+            ItemKind::Trait(..) | ItemKind::TraitAlias(..) |
             ItemKind::ExternCrate(..) | ItemKind::ForeignMod(..) | ItemKind::Ty(..) =>
                 DefPathData::TypeNs(i.ident.name.as_str()),
             ItemKind::Mod(..) if i.ident == keywords::Invalid.ident() => {

src/librustc/hir/map/mod.rs

@@ -1185,6 +1185,7 @@ fn node_id_to_string(map: &Map, id: NodeId, include_id: bool) -> String {
                 ItemStruct(..) => "struct",
                 ItemUnion(..) => "union",
                 ItemTrait(..) => "trait",
+                ItemTraitAlias(..) => "trait alias",
                 ItemImpl(..) => "impl",
                 ItemAutoImpl(..) => "default impl",
             };

src/librustc/hir/mod.rs

@@ -1888,6 +1888,8 @@ pub enum Item_ {
     ItemUnion(VariantData, Generics),
     /// Represents a Trait Declaration
     ItemTrait(IsAuto, Unsafety, Generics, TyParamBounds, HirVec<TraitItemRef>),
+    /// Represents a Trait Alias Declaration
+    ItemTraitAlias(Generics, TyParamBounds),
 
     /// Auto trait implementations
     ///
@@ -1919,6 +1921,7 @@ impl Item_ {
             ItemStruct(..) => "struct",
             ItemUnion(..) => "union",
             ItemTrait(..) => "trait",
+            ItemTraitAlias(..) => "trait alias",
             ItemImpl(..) |
             ItemAutoImpl(..) => "item",
         }

src/librustc/hir/print.rs

@@ -747,6 +747,27 @@ impl<'a> State<'a> {
                 }
                 self.bclose(item.span)?;
             }
+            hir::ItemTraitAlias(ref generics, ref bounds) => {
+                self.head("")?;
+                self.print_visibility(&item.vis)?;
+                self.word_nbsp("trait")?;
+                self.print_name(item.name)?;
+                self.print_generics(generics)?;
+                let mut real_bounds = Vec::with_capacity(bounds.len());
+                // FIXME(durka) this seems to be some quite outdated syntax
+                for b in bounds.iter() {
+                    if let TraitTyParamBound(ref ptr, hir::TraitBoundModifier::Maybe) = *b {
+                        self.s.space()?;
+                        self.word_space("for ?")?;
+                        self.print_trait_ref(&ptr.trait_ref)?;
+                    } else {
+                        real_bounds.push(b.clone());
+                    }
+                }
+                self.print_bounds(" = ", &real_bounds[..])?;
+                self.print_where_clause(&generics.where_clause)?;
+                self.s.word(";")?;
+            }
         }
         self.ann.post(self, NodeItem(item))
     }

src/librustc/ich/impls_hir.rs

@@ -848,6 +848,7 @@ impl_stable_hash_for!(enum hir::Item_ {
     ItemStruct(variant_data, generics),
     ItemUnion(variant_data, generics),
     ItemTrait(is_auto, unsafety, generics, bounds, item_refs),
+    ItemTraitAlias(generics, bounds),
     ItemAutoImpl(unsafety, trait_ref),
     ItemImpl(unsafety, impl_polarity, impl_defaultness, generics, trait_ref, ty, impl_item_refs)
 });
@@ -1004,6 +1005,7 @@ impl_stable_hash_for!(enum hir::def::Def {
     Variant(def_id),
     Trait(def_id),
     TyAlias(def_id),
+    TraitAlias(def_id),
     AssociatedTy(def_id),
     PrimTy(prim_ty),
     TyParam(def_id),

src/librustc/middle/reachable.rs

@@ -268,7 +268,7 @@ impl<'a, 'tcx> ReachableContext<'a, 'tcx> {
                     hir::ItemExternCrate(_) | hir::ItemUse(..) |
                     hir::ItemTy(..) | hir::ItemStatic(..) |
                     hir::ItemMod(..) | hir::ItemForeignMod(..) |
-                    hir::ItemImpl(..) | hir::ItemTrait(..) |
+                    hir::ItemImpl(..) | hir::ItemTrait(..) | hir::ItemTraitAlias(..) |
                     hir::ItemStruct(..) | hir::ItemEnum(..) |
                     hir::ItemUnion(..) | hir::ItemAutoImpl(..) |
                     hir::ItemGlobalAsm(..) => {}

src/librustc/middle/resolve_lifetime.rs

@@ -469,6 +469,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
             | hir::ItemStruct(_, ref generics)
             | hir::ItemUnion(_, ref generics)
             | hir::ItemTrait(_, _, ref generics, ..)
+            | hir::ItemTraitAlias(ref generics, ..)
             | hir::ItemImpl(_, _, _, ref generics, ..) => {
                 // These kinds of items have only early bound lifetime parameters.
                 let mut index = if let hir::ItemTrait(..) = item.node {