Add Tuple marker trait

Author: compiler-errors

compiler/rustc_hir/src/lang_items.rs

@@ -290,6 +290,8 @@ language_item_table! {
 
     Try,                     sym::Try,                 try_trait,                  Target::Trait,          GenericRequirement::None;
 
+    Tuple,                   sym::tuple_trait,         tuple_trait,                Target::Trait,          GenericRequirement::Exact(0);
+
     SliceLen,                sym::slice_len_fn,        slice_len_fn,               Target::Method(MethodKind::Inherent), GenericRequirement::None;
 
     // Language items from AST lowering

compiler/rustc_span/src/symbol.rs

@@ -1481,6 +1481,7 @@ symbols! {
         tuple,
         tuple_from_req,
         tuple_indexing,
+        tuple_trait,
         two_phase,
         ty,
         type_alias_enum_variants,

library/core/src/marker.rs

@@ -800,6 +800,15 @@ impl<T: ?Sized> Unpin for *mut T {}
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
 pub trait Destruct {}
 
+/// A marker for tuple types.
+///
+/// The implementation of this trait is built-in and cannot be implemented
+/// for any user type.
+#[unstable(feature = "tuple_trait", issue = "none")]
+#[cfg_attr(not(bootstrap), lang = "tuple_trait")]
+#[rustc_on_unimplemented(message = "`{Self}` is not a tuple")]
+pub trait Tuple {}
+
 /// Implementations of `Copy` for primitive types.
 ///
 /// Implementations that cannot be described in Rust