introduce an unreachable terminator

Use it instead of a `panic` for inexhaustive matches and correct the
comment. I think we trust our match-generation algorithm enough to
generate these blocks, and not generating an `unreachable` means that
LLVM won't optimize `match void() {}` to an `unreachable`.

src/librustc/mir/repr.rs

@@ -374,6 +374,9 @@ pub enum TerminatorKind<'tcx> {
     /// have been filled in by now. This should occur at most once.
     Return,
 
+    /// Indicates a terminator that can never be reached.
+    Unreachable,
+
     /// Drop the Lvalue
     Drop {
         location: Lvalue<'tcx>,
@@ -432,6 +435,7 @@ impl<'tcx> TerminatorKind<'tcx> {
             SwitchInt { targets: ref b, .. } => b[..].into_cow(),
             Resume => (&[]).into_cow(),
             Return => (&[]).into_cow(),
+            Unreachable => (&[]).into_cow(),
             Call { destination: Some((_, t)), cleanup: Some(c), .. } => vec![t, c].into_cow(),
             Call { destination: Some((_, ref t)), cleanup: None, .. } =>
                 slice::ref_slice(t).into_cow(),
@@ -461,6 +465,7 @@ impl<'tcx> TerminatorKind<'tcx> {
             SwitchInt { targets: ref mut b, .. } => b.iter_mut().collect(),
             Resume => Vec::new(),
             Return => Vec::new(),
+            Unreachable => Vec::new(),
             Call { destination: Some((_, ref mut t)), cleanup: Some(ref mut c), .. } => vec![t, c],
             Call { destination: Some((_, ref mut t)), cleanup: None, .. } => vec![t],
             Call { destination: None, cleanup: Some(ref mut c), .. } => vec![c],
@@ -539,6 +544,7 @@ impl<'tcx> TerminatorKind<'tcx> {
             SwitchInt { discr: ref lv, .. } => write!(fmt, "switchInt({:?})", lv),
             Return => write!(fmt, "return"),
             Resume => write!(fmt, "resume"),
+            Unreachable => write!(fmt, "unreachable"),
             Drop { ref location, .. } => write!(fmt, "drop({:?})", location),
             DropAndReplace { ref location, ref value, .. } =>
                 write!(fmt, "replace({:?} <- {:?})", location, value),
@@ -582,7 +588,7 @@ impl<'tcx> TerminatorKind<'tcx> {
     pub fn fmt_successor_labels(&self) -> Vec<Cow<'static, str>> {
         use self::TerminatorKind::*;
         match *self {
-            Return | Resume => vec![],
+            Return | Resume | Unreachable => vec![],
             Goto { .. } => vec!["".into()],
             If { .. } => vec!["true".into(), "false".into()],
             Switch { ref adt_def, .. } => {

src/librustc/mir/visit.rs

@@ -386,7 +386,8 @@ macro_rules! make_mir_visitor {
                     }
 
                     TerminatorKind::Resume |
-                    TerminatorKind::Return => {
+                    TerminatorKind::Return |
+                    TerminatorKind::Unreachable => {
                     }
 
                     TerminatorKind::Drop { ref $($mutability)* location,