Add intrinsics::unreachable

src/libcore/intrinsics.rs

@@ -250,6 +250,13 @@ extern "rust-intrinsic" {
     /// Abort the execution of the process.
     pub fn abort() -> !;
 
+    /// Tell LLVM that this point in the code is not reachable,
+    /// enabling further optimizations.
+    ///
+    /// NB: This is very different from the `unreachable!()` macro!
+    #[cfg(not(stage0))]
+    pub fn unreachable() -> !;
+
     /// Execute a breakpoint trap, for inspection by a debugger.
     pub fn breakpoint();
 

src/librustc/middle/trans/intrinsic.rs

@@ -228,6 +228,10 @@ pub fn trans_intrinsic_call<'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>, node: ast::N
             Unreachable(bcx);
             v
         }
+        (_, "unreachable") => {
+            Unreachable(bcx);
+            C_nil(ccx)
+        }
         (_, "breakpoint") => {
             let llfn = ccx.get_intrinsic(&("llvm.debugtrap"));
             Call(bcx, llfn, [], None)

src/librustc/middle/typeck/check/mod.rs

@@ -5589,6 +5589,7 @@ pub fn check_intrinsic_type(ccx: &CrateCtxt, it: &ast::ForeignItem) {
     } else {
         match name.get() {
             "abort" => (0, Vec::new(), ty::mk_bot()),
+            "unreachable" => (0, Vec::new(), ty::mk_bot()),
             "breakpoint" => (0, Vec::new(), ty::mk_nil()),
             "size_of" |
             "pref_align_of" | "min_align_of" => (1u, Vec::new(), ty::mk_uint()),