Add a pass-by-copy parameter passing convention

This is intended to solve the problem of how to pass arguments to
constructor functions -- you want to move in rvalues, but not have to
explicitly copy stuff that is not an rvalue. The by-copy passing
convention will ensure the callee gets its own copy of the value. For
rvalues, it'll just pass off the value. For lvalues, it'll make a
copy.

Issue #1177

1 files changed, 4 insertions, 1 deletions

diff --git a/src/comp/syntax/parse/parser.rs b/src/comp/syntax/parse/parser.rs
index 3731a6af7d0..d33bd89d5d8 100644
--- a/src/comp/syntax/parse/parser.rs
+++ b/src/comp/syntax/parse/parser.rs
@@ -573,7 +573,10 @@ fn parse_arg_mode(p: parser) -> ast::mode {
     if eat(p, token::BINOP(token::AND)) { ast::by_mut_ref }
     else if eat(p, token::BINOP(token::MINUS)) { ast::by_move }
     else if eat(p, token::ANDAND) { ast::by_ref }
-    else if eat(p, token::BINOP(token::PLUS)) { ast::by_val }
+    else if eat(p, token::BINOP(token::PLUS)) {
+        if eat(p, token::BINOP(token::PLUS)) { ast::by_val }
+        else { ast::by_copy }
+    }
     else { ast::mode_infer }
 }