Enable TrapUnreachable in LLVM.

src/rustllvm/PassWrapper.cpp

@@ -398,6 +398,12 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine(
   Options.DataSections = DataSections;
   Options.FunctionSections = FunctionSections;
 
+  // Tell LLVM to translate `unreachable` into an explicit trap instruction.
+  // This limits the extent of possible undefined behavior in some cases, as it
+  // prevents control flow from "falling through" into whatever code happens to
+  // be laid out next in memory.
+  Options.TrapUnreachable = true;
+
   TargetMachine *TM = TheTarget->createTargetMachine(
       Trip.getTriple(), RealCPU, Feature, Options, RM, CM, OptLevel);
   return wrap(TM);

src/test/run-make/intrinsic-unreachable/exit-ret.rs

@@ -11,10 +11,12 @@
 #![feature(asm)]
 #![crate_type="lib"]
 
-pub fn exit(n: usize) {
+#[deny(unreachable_code)]
+pub fn exit(n: usize) -> i32 {
     unsafe {
         // Pretend this asm is an exit() syscall.
         asm!("" :: "r"(n) :: "volatile");
         // Can't actually reach this point, but rustc doesn't know that.
     }
+    42
 }

src/test/run-make/intrinsic-unreachable/exit-unreachable.rs

@@ -13,10 +13,12 @@
 
 use std::intrinsics;
 
-pub fn exit(n: usize) -> ! {
+#[allow(unreachable_code)]
+pub fn exit(n: usize) -> i32 {
     unsafe {
         // Pretend this asm is an exit() syscall.
         asm!("" :: "r"(n) :: "volatile");
         intrinsics::unreachable()
     }
+    42
 }