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,

src/librustc_borrowck/borrowck/mir/dataflow/mod.rs

@@ -449,7 +449,8 @@ impl<'a, 'tcx: 'a, D> DataflowAnalysis<'a, 'tcx, D>
     {
         match bb_data.terminator().kind {
             repr::TerminatorKind::Return |
-            repr::TerminatorKind::Resume => {}
+            repr::TerminatorKind::Resume |
+            repr::TerminatorKind::Unreachable => {}
             repr::TerminatorKind::Goto { ref target } |
             repr::TerminatorKind::Assert { ref target, cleanup: None, .. } |
             repr::TerminatorKind::Drop { ref target, location: _, unwind: None } |

src/librustc_borrowck/borrowck/mir/gather_moves.rs

@@ -621,7 +621,9 @@ fn gather_moves<'a, 'tcx>(mir: &Mir<'tcx>, tcx: TyCtxt<'a, 'tcx, 'tcx>) -> MoveD
 
         debug!("gather_moves({:?})", bb_data.terminator());
         match bb_data.terminator().kind {
-            TerminatorKind::Goto { target: _ } | TerminatorKind::Resume => { }
+            TerminatorKind::Goto { target: _ } |
+            TerminatorKind::Resume |
+            TerminatorKind::Unreachable => { }
 
             TerminatorKind::Return => {
                 let source = Location { block: bb,

src/librustc_mir/build/matches/mod.rs

@@ -78,12 +78,18 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         // branch to the appropriate arm block
         let otherwise = self.match_candidates(span, &mut arm_blocks, candidates, block);
 
-        // because all matches are exhaustive, in principle we expect
-        // an empty vector to be returned here, but the algorithm is
-        // not entirely precise
         if !otherwise.is_empty() {
-            let join_block = self.join_otherwise_blocks(span, otherwise);
-            self.panic(join_block, "something about matches algorithm not being precise", span);
+            // All matches are exhaustive. However, because some matches
+            // only have exponentially-large exhaustive decision trees, we
+            // sometimes generate an inexhaustive decision tree.
+            //
+            // In that case, the inexhaustive tips of the decision tree
+            // can't be reached - terminate them with an `unreachable`.
+            let source_info = self.source_info(span);
+
+            for block in otherwise {
+                self.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
+            }
         }
 
         // all the arm blocks will rejoin here

src/librustc_mir/build/scope.rs

@@ -90,12 +90,9 @@ use build::{BlockAnd, BlockAndExtension, Builder, CFG, ScopeAuxiliary, ScopeId};
 use rustc::middle::region::{CodeExtent, CodeExtentData};
 use rustc::middle::lang_items;
 use rustc::ty::subst::{Substs, Subst, VecPerParamSpace};
-use rustc::ty::{self, Ty, TyCtxt};
+use rustc::ty::{Ty, TyCtxt};
 use rustc::mir::repr::*;
-use syntax::codemap::{Span, DUMMY_SP};
-use syntax::parse::token::intern_and_get_ident;
-use rustc::middle::const_val::ConstVal;
-use rustc_const_math::ConstInt;
+use syntax::codemap::Span;
 use rustc_data_structures::indexed_vec::Idx;
 
 pub struct Scope<'tcx> {
@@ -556,50 +553,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         next_target.unit()
     }
 
-    /// Create diverge cleanup and branch to it from `block`.
-    // FIXME: Remove this (used only for unreachable cases in match).
-    pub fn panic(&mut self, block: BasicBlock, msg: &'static str, span: Span) {
-        // fn(&(msg: &'static str filename: &'static str, line: u32)) -> !
-        let region = ty::ReStatic; // FIXME(mir-borrowck): use a better region?
-        let func = self.lang_function(lang_items::PanicFnLangItem);
-        let args = self.hir.tcx().replace_late_bound_regions(&func.ty.fn_args(), |_| region).0;
-
-        let ref_ty = args[0];
-        let tup_ty = if let ty::TyRef(_, tyandmut) = ref_ty.sty {
-            tyandmut.ty
-        } else {
-            span_bug!(span, "unexpected panic type: {:?}", func.ty);
-        };
-
-        let (tuple, tuple_ref) = (self.temp(tup_ty), self.temp(ref_ty));
-        let (file, line) = self.span_to_fileline_args(span);
-        let message = Constant {
-            span: span,
-            ty: self.hir.tcx().mk_static_str(),
-            literal: self.hir.str_literal(intern_and_get_ident(msg))
-        };
-        let elems = vec![Operand::Constant(message),
-                         Operand::Constant(file),
-                         Operand::Constant(line)];
-        let source_info = self.source_info(span);
-        // FIXME: We should have this as a constant, rather than a stack variable (to not pollute
-        // icache with cold branch code), however to achieve that we either have to rely on rvalue
-        // promotion or have some way, in MIR, to create constants.
-        self.cfg.push_assign(block, source_info, &tuple, // [1]
-                             Rvalue::Aggregate(AggregateKind::Tuple, elems));
-        // [1] tuple = (message_arg, file_arg, line_arg);
-        // FIXME: is this region really correct here?
-        self.cfg.push_assign(block, source_info, &tuple_ref, // tuple_ref = &tuple;
-                             Rvalue::Ref(region, BorrowKind::Shared, tuple));
-        let cleanup = self.diverge_cleanup();
-        self.cfg.terminate(block, source_info, TerminatorKind::Call {
-            func: Operand::Constant(func),
-            args: vec![Operand::Consume(tuple_ref)],
-            cleanup: cleanup,
-            destination: None,
-        });
-    }
-
     /// Create an Assert terminator and return the success block.
     /// If the boolean condition operand is not the expected value,
     /// a runtime panic will be caused with the given message.
@@ -625,39 +578,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
 
         success_block
     }
-
-    fn lang_function(&mut self, lang_item: lang_items::LangItem) -> Constant<'tcx> {
-        let funcdid = match self.hir.tcx().lang_items.require(lang_item) {
-            Ok(d) => d,
-            Err(m) => {
-                self.hir.tcx().sess.fatal(&m)
-            }
-        };
-        Constant {
-            span: DUMMY_SP,
-            ty: self.hir.tcx().lookup_item_type(funcdid).ty,
-            literal: Literal::Item {
-                def_id: funcdid,
-                substs: self.hir.tcx().mk_substs(Substs::empty())
-            }
-        }
-    }
-
-    fn span_to_fileline_args(&mut self, span: Span) -> (Constant<'tcx>, Constant<'tcx>) {
-        let span_lines = self.hir.tcx().sess.codemap().lookup_char_pos(span.lo);
-        (Constant {
-            span: span,
-            ty: self.hir.tcx().mk_static_str(),
-            literal: self.hir.str_literal(intern_and_get_ident(&span_lines.file.name))
-        }, Constant {
-            span: span,
-            ty: self.hir.tcx().types.u32,
-            literal: Literal::Value {
-                value: ConstVal::Integral(ConstInt::U32(span_lines.line as u32)),
-            },
-        })
-    }
-
 }
 
 /// Builds drops for pop_scope and exit_scope.

src/librustc_mir/transform/no_landing_pads.rs

@@ -24,6 +24,7 @@ impl<'tcx> MutVisitor<'tcx> for NoLandingPads {
             TerminatorKind::Goto { .. } |
             TerminatorKind::Resume |
             TerminatorKind::Return |
+            TerminatorKind::Unreachable |
             TerminatorKind::If { .. } |
             TerminatorKind::Switch { .. } |
             TerminatorKind::SwitchInt { .. } => {

src/librustc_mir/transform/qualify_consts.rs

@@ -362,7 +362,8 @@ impl<'a, 'tcx> Qualifier<'a, 'tcx, 'tcx> {
                 TerminatorKind::Switch {..} |
                 TerminatorKind::SwitchInt {..} |
                 TerminatorKind::DropAndReplace { .. } |
-                TerminatorKind::Resume => None,
+                TerminatorKind::Resume |
+                TerminatorKind::Unreachable => None,
 
                 TerminatorKind::Return => {
                     // Check for unused values. This usually means

src/librustc_mir/transform/type_check.rs

@@ -379,6 +379,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
             TerminatorKind::Goto { .. } |
             TerminatorKind::Resume |
             TerminatorKind::Return |
+            TerminatorKind::Unreachable |
             TerminatorKind::Drop { .. } => {
                 // no checks needed for these
             }
@@ -595,6 +596,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                     span_mirbug!(self, block, "return on cleanup block")
                 }
             }
+            TerminatorKind::Unreachable => {}
             TerminatorKind::Drop { target, unwind, .. } |
             TerminatorKind::DropAndReplace { target, unwind, .. } |
             TerminatorKind::Assert { target, cleanup: unwind, .. } => {

src/librustc_trans/mir/analyze.rs

@@ -173,6 +173,7 @@ pub fn cleanup_kinds<'bcx,'tcx>(_bcx: Block<'bcx,'tcx>,
                 TerminatorKind::Goto { .. } |
                 TerminatorKind::Resume |
                 TerminatorKind::Return |
+                TerminatorKind::Unreachable |
                 TerminatorKind::If { .. } |
                 TerminatorKind::Switch { .. } |
                 TerminatorKind::SwitchInt { .. } => {

src/librustc_trans/mir/block.rs

@@ -191,6 +191,10 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                 })
             }
 
+            mir::TerminatorKind::Unreachable => {
+                bcx.unreachable();
+            }
+
             mir::TerminatorKind::Drop { ref location, target, unwind } => {
                 let lvalue = self.trans_lvalue(&bcx, location);
                 let ty = lvalue.ty.to_ty(bcx.tcx());