auto merge of #19448 : japaric/rust/binops-by-value, r=nikomatsakis

- The following operator traits now take their arguments by value: `Add`, `Sub`, `Mul`, `Div`, `Rem`, `BitAnd`, `BitOr`, `BitXor`, `Shl`, `Shr`. This breaks all existing implementations of these traits.

- The binary operation `a OP b` now "desugars" to `OpTrait::op_method(a, b)` and consumes both arguments.

- `String` and `Vec` addition have been changed to reuse the LHS owned value, and to avoid internal cloning. Only the following asymmetric operations are available: `String + &str` and `Vec<T> + &[T]`, which are now a short-hand for the "append" operation.

[breaking-change]

---

This passes `make check` locally. I haven't touch the unary operators in this PR, but converting them to by value should be very similar to this PR. I can work on them after this gets the thumbs up.

@nikomatsakis r? the compiler changes
@aturon r? the library changes. I think the only controversial bit is the semantic change of the `Vec`/`String` `Add` implementation.
cc #19148 

1 files changed, 24 insertions, 0 deletions

diff --git a/src/libsyntax/ext/tt/transcribe.rs b/src/libsyntax/ext/tt/transcribe.rs
index 99799fecb78..e2439bad178 100644
--- a/src/libsyntax/ext/tt/transcribe.rs
+++ b/src/libsyntax/ext/tt/transcribe.rs
@@ -106,6 +106,8 @@ enum LockstepIterSize {
     LisContradiction(String),
 }
 
+// NOTE(stage0): Remove impl after a snapshot
+#[cfg(stage0)]
 impl Add<LockstepIterSize, LockstepIterSize> for LockstepIterSize {
     fn add(&self, other: &LockstepIterSize) -> LockstepIterSize {
         match *self {
@@ -127,6 +129,28 @@ impl Add<LockstepIterSize, LockstepIterSize> for LockstepIterSize {
     }
 }
 
+#[cfg(not(stage0))]  // NOTE(stage0): Remove cfg after a snapshot
+impl Add<LockstepIterSize, LockstepIterSize> for LockstepIterSize {
+    fn add(self, other: LockstepIterSize) -> LockstepIterSize {
+        match self {
+            LisUnconstrained => other,
+            LisContradiction(_) => self,
+            LisConstraint(l_len, ref l_id) => match other {
+                LisUnconstrained => self.clone(),
+                LisContradiction(_) => other,
+                LisConstraint(r_len, _) if l_len == r_len => self.clone(),
+                LisConstraint(r_len, r_id) => {
+                    let l_n = token::get_ident(l_id.clone());
+                    let r_n = token::get_ident(r_id);
+                    LisContradiction(format!("inconsistent lockstep iteration: \
+                                              '{}' has {} items, but '{}' has {}",
+                                              l_n, l_len, r_n, r_len).to_string())
+                }
+            },
+        }
+    }
+}
+
 fn lockstep_iter_size(t: &TokenTree, r: &TtReader) -> LockstepIterSize {
     match *t {
         TtDelimited(_, ref delimed) => {