Simplify the return lvalue

src/eval_context.rs

@@ -66,8 +66,7 @@ pub struct Frame<'tcx> {
     pub return_to_block: StackPopCleanup,
 
     /// The location where the result of the current stack frame should be written to.
-    /// None if the function is a diverging function
-    pub return_lvalue: Option<Lvalue<'tcx>>,
+    pub return_lvalue: Lvalue<'tcx>,
 
     /// The list of locals for this stack frame, stored in order as
     /// `[arguments..., variables..., temporaries...]`. The locals are stored as `Option<Value>`s.
@@ -272,7 +271,7 @@ pub fn push_stack_frame(
         instance: ty::Instance<'tcx>,
         span: codemap::Span,
         mir: &'tcx mir::Mir<'tcx>,
-        return_lvalue: Option<Lvalue<'tcx>>,
+        return_lvalue: Lvalue<'tcx>,
         return_to_block: StackPopCleanup,
     ) -> EvalResult<'tcx> {
         ::log_settings::settings().indentation += 1;
@@ -329,7 +328,7 @@ pub(super) fn pop_stack_frame(&mut self) -> EvalResult<'tcx> {
         ::log_settings::settings().indentation -= 1;
         let frame = self.stack.pop().expect("tried to pop a stack frame, but there were none");
         match frame.return_to_block {
-            StackPopCleanup::MarkStatic(mutable) => if let Lvalue::Global(id) = frame.return_lvalue.expect("diverging static") {
+            StackPopCleanup::MarkStatic(mutable) => if let Lvalue::Global(id) = frame.return_lvalue {
                 let global_value = self.globals.get_mut(&id)
                     .expect("global should have been cached (static)");
                 match global_value.value {

src/lvalue.rs

@@ -64,6 +64,14 @@ pub struct Global<'tcx> {
 }
 
 impl<'tcx> Lvalue<'tcx> {
+    /// Produces an Lvalue that will error if attempted to be read from
+    pub fn undef() -> Self {
+        Lvalue::Ptr {
+            ptr: PrimVal::Undef,
+            extra: LvalueExtra::None,
+        }
+    }
+
     pub fn zst() -> Self {
         Self::from_ptr(Pointer::zst_ptr())
     }
@@ -148,7 +156,7 @@ pub(super) fn eval_and_read_lvalue(&mut self, lvalue: &mir::Lvalue<'tcx>) -> Eva
     pub(super) fn eval_lvalue(&mut self, mir_lvalue: &mir::Lvalue<'tcx>) -> EvalResult<'tcx, Lvalue<'tcx>> {
         use rustc::mir::Lvalue::*;
         let lvalue = match *mir_lvalue {
-            Local(mir::RETURN_POINTER) => self.frame().return_lvalue.expect("diverging function returned"),
+            Local(mir::RETURN_POINTER) => self.frame().return_lvalue,
             Local(local) => Lvalue::Local { frame: self.stack.len() - 1, local, field: None },
 
             Static(ref static_) => {

src/step.rs

@@ -192,7 +192,7 @@ fn global_item(
                 instance,
                 span,
                 mir,
-                Some(Lvalue::Global(cid)),
+                Lvalue::Global(cid),
                 cleanup,
             )
         });
@@ -235,7 +235,7 @@ fn visit_constant(&mut self, constant: &mir::Constant<'tcx>, location: mir::Loca
                     this.ecx.push_stack_frame(this.instance,
                                               constant.span,
                                               mir,
-                                              Some(Lvalue::Global(cid)),
+                                              Lvalue::Global(cid),
                                               StackPopCleanup::MarkStatic(false),
                     )
                 });

src/terminator/drop.rs

@@ -49,7 +49,7 @@ pub(crate) fn drop(&mut self, arg: Value, mut instance: ty::Instance<'tcx>, ty:
             instance,
             span,
             mir,
-            Some(Lvalue::zst()),
+            Lvalue::zst(),
             StackPopCleanup::None,
         )?;
 

src/terminator/mod.rs

@@ -30,7 +30,7 @@ pub(super) fn eval_terminator(
         use rustc::mir::TerminatorKind::*;
         match terminator.kind {
             Return => {
-                self.dump_local(self.frame().return_lvalue.expect("diverging function returned"));
+                self.dump_local(self.frame().return_lvalue);
                 self.pop_stack_frame()?
             }
 
@@ -430,8 +430,8 @@ fn eval_fn_call_inner(
             Err(other) => return Err(other),
         };
         let (return_lvalue, return_to_block) = match destination {
-            Some((lvalue, block)) => (Some(lvalue), StackPopCleanup::Goto(block)),
-            None => (None, StackPopCleanup::None),
+            Some((lvalue, block)) => (lvalue, StackPopCleanup::Goto(block)),
+            None => (Lvalue::undef(), StackPopCleanup::None),
         };
 
         self.push_stack_frame(
@@ -606,7 +606,7 @@ fn call_c_abi(
                     f_instance,
                     mir.span,
                     mir,
-                    Some(Lvalue::zst()),
+                    Lvalue::zst(),
                     StackPopCleanup::Goto(dest_block),
                 )?;