Add comments to clarify function argument ownership

src/librustc/mir/mod.rs

@@ -650,7 +650,9 @@ pub enum TerminatorKind<'tcx> {
     Call {
         /// The function that’s being called
         func: Operand<'tcx>,
-        /// Arguments the function is called with
+        /// Arguments the function is called with. These are owned by the callee, which is free to
+        /// modify them. This is important as "by-value" arguments might be passed by-reference at
+        /// the ABI level.
         args: Vec<Operand<'tcx>>,
         /// Destination for the return value. If some, the call is converging.
         destination: Option<(Lvalue<'tcx>, BasicBlock)>,

src/librustc_mir/build/expr/into.rs

@@ -249,6 +249,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                         args.into_iter()
                             .map(|arg| {
                                 let scope = this.local_scope();
+                                // Function arguments are owned by the callee, so we need as_temp()
+                                // instead of as_operand() to enforce copies
                                 let operand = unpack!(block = this.as_temp(block, scope, arg));
                                 Operand::Consume(Lvalue::Local(operand))
                             })