auto merge of #16970 : kmcallister/rust/llvm-unreachable, r=thestinger

I'm not sure how to add an automated test for this.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index a3d63bbe06c..7d86b65168f 100644
--- a/src/libcore/intrinsics.rs
+++ b/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();