custom MIR: add support for tail calls

core/src/intrinsics/mir.rs

@@ -247,6 +247,8 @@
 //!       otherwise branch.
 //!  - [`Call`] has an associated function as well, with special syntax:
 //!    `Call(ret_val = function(arg1, arg2, ...), ReturnTo(next_block), UnwindContinue())`.
+//!  - [`TailCall`] does not have a return destination or next block, so its syntax is just
+//!    `TailCall(function(arg1, arg2, ...))`.
 
 #![unstable(
     feature = "custom_mir",
@@ -350,6 +352,12 @@ define!("mir_call",
     /// - [`UnwindCleanup`]
     fn Call(call: (), goto: ReturnToArg, unwind_action: UnwindActionArg)
 );
+define!("mir_tail_call",
+    /// Call a function.
+    ///
+    /// The argument must be of the form `fun(arg1, arg2, ...)`.
+    fn TailCall<T>(call: T)
+);
 define!("mir_unwind_resume",
     /// A terminator that resumes the unwinding.
     fn UnwindResume()