Auto merge of #14184 - lowr:feat/trait-alias-def, r=Veykril

Handle trait alias definitions

Part of #2773

This PR adds a bunch of structs and enum variants for trait aliases. Trait aliases should be handled as an independent item because they are semantically distinct from traits.

I basically started by adding `TraitAlias{Id, Loc}` to `hir_def::item_tree` and iterated adding necessary stuffs until compiler stopped complaining what's missing. Let me know if there's still anything I need to add.

I'm opening up this PR for early review and stuff. I'm planning to add tests for IDE functionalities in this PR, but not type-related support, for which I put FIXME notes.

crates/hir/src/lib.rs

@@ -53,7 +53,7 @@ use hir_def::{
     AdtId, AssocItemId, AssocItemLoc, AttrDefId, ConstId, ConstParamId, DefWithBodyId, EnumId,
     EnumVariantId, FunctionId, GenericDefId, HasModule, ImplId, ItemContainerId, LifetimeParamId,
     LocalEnumVariantId, LocalFieldId, Lookup, MacroExpander, MacroId, ModuleId, StaticId, StructId,
-    TraitId, TypeAliasId, TypeOrConstParamId, TypeParamId, UnionId,
+    TraitAliasId, TraitId, TypeAliasId, TypeOrConstParamId, TypeParamId, UnionId,
 };
 use hir_expand::{name::name, MacroCallKind};
 use hir_ty::{
@@ -272,6 +272,7 @@ pub enum ModuleDef {
     Const(Const),
     Static(Static),
     Trait(Trait),
+    TraitAlias(TraitAlias),
     TypeAlias(TypeAlias),
     BuiltinType(BuiltinType),
     Macro(Macro),
@@ -284,6 +285,7 @@ impl_from!(
     Const,
     Static,
     Trait,
+    TraitAlias,
     TypeAlias,
     BuiltinType,
     Macro
@@ -310,6 +312,7 @@ impl ModuleDef {
             ModuleDef::Const(it) => Some(it.module(db)),
             ModuleDef::Static(it) => Some(it.module(db)),
             ModuleDef::Trait(it) => Some(it.module(db)),
+            ModuleDef::TraitAlias(it) => Some(it.module(db)),
             ModuleDef::TypeAlias(it) => Some(it.module(db)),
             ModuleDef::Macro(it) => Some(it.module(db)),
             ModuleDef::BuiltinType(_) => None,
@@ -338,6 +341,7 @@ impl ModuleDef {
             ModuleDef::Const(it) => it.name(db)?,
             ModuleDef::Adt(it) => it.name(db),
             ModuleDef::Trait(it) => it.name(db),
+            ModuleDef::TraitAlias(it) => it.name(db),
             ModuleDef::Function(it) => it.name(db),
             ModuleDef::Variant(it) => it.name(db),
             ModuleDef::TypeAlias(it) => it.name(db),
@@ -356,6 +360,7 @@ impl ModuleDef {
                 Adt::Union(it) => it.id.into(),
             },
             ModuleDef::Trait(it) => it.id.into(),
+            ModuleDef::TraitAlias(it) => it.id.into(),
             ModuleDef::Function(it) => it.id.into(),
             ModuleDef::TypeAlias(it) => it.id.into(),
             ModuleDef::Module(it) => it.id.into(),
@@ -398,6 +403,7 @@ impl ModuleDef {
             ModuleDef::Module(_)
             | ModuleDef::Adt(_)
             | ModuleDef::Trait(_)
+            | ModuleDef::TraitAlias(_)
             | ModuleDef::TypeAlias(_)
             | ModuleDef::Macro(_)
             | ModuleDef::BuiltinType(_) => None,
@@ -413,6 +419,7 @@ impl ModuleDef {
             ModuleDef::Const(it) => it.attrs(db),
             ModuleDef::Static(it) => it.attrs(db),
             ModuleDef::Trait(it) => it.attrs(db),
+            ModuleDef::TraitAlias(it) => it.attrs(db),
             ModuleDef::TypeAlias(it) => it.attrs(db),
             ModuleDef::Macro(it) => it.attrs(db),
             ModuleDef::BuiltinType(_) => return None,
@@ -429,6 +436,7 @@ impl HasVisibility for ModuleDef {
             ModuleDef::Const(it) => it.visibility(db),
             ModuleDef::Static(it) => it.visibility(db),
             ModuleDef::Trait(it) => it.visibility(db),
+            ModuleDef::TraitAlias(it) => it.visibility(db),
             ModuleDef::TypeAlias(it) => it.visibility(db),
             ModuleDef::Variant(it) => it.visibility(db),
             ModuleDef::Macro(it) => it.visibility(db),
@@ -1942,6 +1950,27 @@ impl HasVisibility for Trait {
     }
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct TraitAlias {
+    pub(crate) id: TraitAliasId,
+}
+
+impl TraitAlias {
+    pub fn module(self, db: &dyn HirDatabase) -> Module {
+        Module { id: self.id.lookup(db.upcast()).container }
+    }
+
+    pub fn name(self, db: &dyn HirDatabase) -> Name {
+        db.trait_alias_data(self.id).name.clone()
+    }
+}
+
+impl HasVisibility for TraitAlias {
+    fn visibility(&self, db: &dyn HirDatabase) -> Visibility {
+        db.trait_alias_data(self.id).visibility.resolve(db.upcast(), &self.id.resolver(db.upcast()))
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct TypeAlias {
     pub(crate) id: TypeAliasId,
@@ -2314,6 +2343,7 @@ pub enum GenericDef {
     Function(Function),
     Adt(Adt),
     Trait(Trait),
+    TraitAlias(TraitAlias),
     TypeAlias(TypeAlias),
     Impl(Impl),
     // enum variants cannot have generics themselves, but their parent enums
@@ -2326,6 +2356,7 @@ impl_from!(
     Function,
     Adt(Struct, Enum, Union),
     Trait,
+    TraitAlias,
     TypeAlias,
     Impl,
     Variant,
@@ -4074,6 +4105,12 @@ impl HasCrate for Trait {
     }
 }
 
+impl HasCrate for TraitAlias {
+    fn krate(&self, db: &dyn HirDatabase) -> Crate {
+        self.module(db).krate()
+    }
+}
+
 impl HasCrate for Static {
     fn krate(&self, db: &dyn HirDatabase) -> Crate {
         self.module(db).krate()