Add Identity trait

Author: GuillaumeGomez

compiler/rustc_hir/src/lang_items.rs

@@ -165,6 +165,7 @@ language_item_table! {
     DynMetadata,             sym::dyn_metadata,        dyn_metadata,               Target::Struct,         GenericRequirement::None;
 
     Freeze,                  sym::freeze,              freeze_trait,               Target::Trait,          GenericRequirement::Exact(0);
+    Identity,                sym::identity,            identity_trait,             Target::Trait,          GenericRequirement::Exact(0);
 
     Drop,                    sym::drop,                drop_trait,                 Target::Trait,          GenericRequirement::None;
     Destruct,                sym::destruct,            destruct_trait,             Target::Trait,          GenericRequirement::None;

compiler/rustc_span/src/symbol.rs

@@ -790,6 +790,7 @@ symbols! {
         i64,
         i8,
         ident,
+        identity,
         identity_future,
         if_let,
         if_let_guard,

library/core/src/marker.rs

@@ -779,6 +779,22 @@ unsafe impl<T: ?Sized> Freeze for *mut T {}
 unsafe impl<T: ?Sized> Freeze for &T {}
 unsafe impl<T: ?Sized> Freeze for &mut T {}
 
+/// Compiler-internal trait used to handle more easily trait aliases.
+/// They get generated like projections. For example:
+/// ```ignore (pseudo-code)
+/// type Foo = Bar;
+/// // becomes:
+/// type Foo = <Bar as Identity>::Identity;
+/// ```
+pub(crate) trait Identity: Sized {
+    #[cfg_attr(not(bootstrap), lang = "identity")]
+    type Identity;
+}
+
+impl<T> Identity for T {
+    type Identity = T;
+}
+
 /// Types that can be safely moved after being pinned.
 ///
 /// Rust itself has no notion of immovable types, and considers moves (e.g.,