Fix matching of rvalues with destructors

Fixes #4542.

Author: nikomatsakis

src/librustc/middle/trans/_match.rs

@@ -1701,7 +1701,7 @@ pub fn trans_match_inner(scope_cx: block,
             None
         }
     };
-    let lldiscr = discr_datum.to_ref_llval(bcx);
+    let lldiscr = discr_datum.to_zeroable_ref_llval(bcx);
     compile_submatch(bcx, matches, [lldiscr], chk);
 
     let mut arm_cxs = ~[];

src/librustc/middle/trans/datum.rs

@@ -471,6 +471,31 @@ pub impl Datum {
         }
     }
 
+    fn to_zeroable_ref_llval(&self, bcx: block) -> ValueRef {
+        /*!
+         * Returns a by-ref llvalue that can be zeroed in order to
+         * cancel cleanup. This is a kind of hokey bridge used
+         * to adapt to the match code. Please don't use it for new code.
+         */
+
+        match self.mode {
+            // All by-ref datums are zeroable, even if we *could* just
+            // cancel the cleanup.
+            ByRef(_) => self.val,
+
+            // By value datums can't be zeroed (where would you store
+            // the zero?) so we have to spill them. Add a temp cleanup
+            // for this spilled value and cancel the cleanup on this
+            // current value.
+            ByValue => {
+                let slot = self.to_ref_llval(bcx);
+                self.cancel_clean(bcx);
+                add_clean_temp_mem(bcx, slot, self.ty);
+                slot
+            }
+        }
+    }
+
     fn appropriate_mode(&self) -> DatumMode {
         /*! See the `appropriate_mode()` function */
 

src/test/run-pass/match-vec-rvalue.rs

@@ -0,0 +1,12 @@
+// Tests that matching rvalues with drops does not crash.
+
+fn main() {
+    match ~[1, 2, 3] {
+        x => {
+            assert_eq!(x.len(), 3);
+            assert_eq!(x[0], 1);
+            assert_eq!(x[1], 2);
+            assert_eq!(x[2], 3);
+        }
+    }
+}