Auto merge of #1260 - RalfJung:unreachable, r=RalfJung

Implement unreachable intrinsic

Fixes https://github.com/rust-lang/miri/issues/1254

src/machine.rs

@@ -106,8 +106,7 @@ pub fn new(rng: StdRng, stacked_borrows: bool, tracked_pointer_tag: Option<PtrId
     pub fn init_extern_statics<'tcx, 'mir>(
         this: &mut MiriEvalContext<'mir, 'tcx>,
     ) -> InterpResult<'tcx> {
-        let target_os = this.tcx.sess.target.target.target_os.as_str();
-        match target_os {
+        match this.tcx.sess.target.target.target_os.as_str() {
             "linux" => {
                 // "__cxa_thread_atexit_impl"
                 // This should be all-zero, pointer-sized.

src/shims/fs.rs

@@ -802,7 +802,7 @@ fn mkdir(
 
         this.check_no_isolation("mkdir")?;
 
-        let _mode = if this.tcx.sess.target.target.target_os.as_str() == "macos" {
+        let _mode = if this.tcx.sess.target.target.target_os == "macos" {
             u32::from(this.read_scalar(mode_op)?.not_undef()?.to_u16()?)
         } else {
             this.read_scalar(mode_op)?.to_u32()?

src/shims/intrinsics.rs

@@ -36,6 +36,7 @@ fn call_intrinsic(
         let (dest, ret) = match ret {
             None => match intrinsic_name {
                 "miri_start_panic" => return this.handle_miri_start_panic(args, unwind),
+                "unreachable" => throw_ub!(Unreachable),
                 _ => throw_unsup_format!("unimplemented (diverging) intrinsic: {}", intrinsic_name),
             },
             Some(p) => p,

tests/compile-fail/unreachable.rs

@@ -0,0 +1,4 @@
+// error-pattern: entering unreachable code
+fn main() {
+    unsafe { std::hint::unreachable_unchecked() }
+}