Add comments and rename a local variable

Author: ogoffart

src/librustc_codegen_ssa/mir/operand.rs

@@ -243,22 +243,25 @@ impl<'a, 'tcx: 'a, V: CodegenObject> OperandRef<'tcx, V> {
             _ => bug!("OperandRef::extract_field({:?}): not applicable", self)
         };
 
-        let bitcast = |bx: &mut Bx, val, ty| {
+        // HACK(eddyb) have to bitcast pointers until LLVM removes pointee types.
+        // Bools in union fields needs to be truncated.
+        let to_immediate_or_cast = |bx: &mut Bx, val, ty| {
             if ty == bx.cx().type_i1() {
                 bx.trunc(val, ty)
             } else {
                 bx.bitcast(val, ty)
             }
         };
 
-        // HACK(eddyb) have to bitcast pointers until LLVM removes pointee types.
         match val {
             OperandValue::Immediate(ref mut llval) => {
-                *llval = bitcast(bx, *llval, bx.cx().immediate_backend_type(field));
+                *llval = to_immediate_or_cast(bx, *llval, bx.cx().immediate_backend_type(field));
             }
             OperandValue::Pair(ref mut a, ref mut b) => {
-                *a = bitcast(bx, *a, bx.cx().scalar_pair_element_backend_type(field, 0, true));
-                *b = bitcast(bx, *b, bx.cx().scalar_pair_element_backend_type(field, 1, true));
+                *a = to_immediate_or_cast(bx, *a, bx.cx()
+                    .scalar_pair_element_backend_type(field, 0, true));
+                *b = to_immediate_or_cast(bx, *b, bx.cx()
+                    .scalar_pair_element_backend_type(field, 1, true));
             }
             OperandValue::Ref(..) => bug!()
         }