NRVO pass for acyclic CFGs

Author: jonas-schievink

src/librustc_middle/mir/interpret/error.rs

@@ -368,8 +368,6 @@ pub enum UndefinedBehaviorInfo {
     InvalidUndefBytes(Option<Pointer>),
     /// Working with a local that is not currently live.
     DeadLocal,
-    /// Trying to read from the return place of a function.
-    ReadFromReturnPlace,
 }
 
 impl fmt::Debug for UndefinedBehaviorInfo {
@@ -431,7 +429,6 @@ impl fmt::Debug for UndefinedBehaviorInfo {
                 "using uninitialized data, but this operation requires initialized memory"
             ),
             DeadLocal => write!(f, "accessing a dead local variable"),
-            ReadFromReturnPlace => write!(f, "tried to read from the return place"),
         }
     }
 }

src/librustc_mir/interpret/operand.rs

@@ -445,7 +445,14 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
         let base_op = match place.local {
-            mir::RETURN_PLACE => throw_ub!(ReadFromReturnPlace),
+            mir::RETURN_PLACE => {
+                if let Some(place) = self.frame().return_place {
+                    self.place_to_op(place)?
+                } else {
+                    // Tried to access return place, but the function cannot return
+                    throw_ub!(Unreachable)
+                }
+            }
             local => {
                 // Do not use the layout passed in as argument if the base we are looking at
                 // here is not the entire place.

src/librustc_mir/lib.rs

@@ -26,6 +26,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(trait_alias)]
 #![feature(option_expect_none)]
 #![feature(or_patterns)]
+#![feature(bindings_after_at)]
 #![recursion_limit = "256"]
 
 #[macro_use]