librustc: Always parse macro!()/macro![] as expressions if not

followed by a semicolon.

This allows code like `vec![1i, 2, 3].len();` to work.

This breaks code that uses macros as statements without putting
semicolons after them, such as:

    fn main() {
        ...
        assert!(a == b)
        assert!(c == d)
        println(...);
    }

It also breaks code that uses macros as items without semicolons:

    local_data_key!(foo)

    fn main() {
        println("hello world")
    }

Add semicolons to fix this code. Those two examples can be fixed as
follows:

    fn main() {
        ...
        assert!(a == b);
        assert!(c == d);
        println(...);
    }

    local_data_key!(foo);

    fn main() {
        println("hello world")
    }

RFC #378.

Closes #18635.

[breaking-change]

src/libsyntax/fold.rs

@@ -1485,7 +1485,7 @@ mod test {
     }
 
     // maybe add to expand.rs...
-    macro_rules! assert_pred (
+    macro_rules! assert_pred {
         ($pred:expr, $predname:expr, $a:expr , $b:expr) => (
             {
                 let pred_val = $pred;
@@ -1497,7 +1497,7 @@ mod test {
                 }
             }
         )
-    )
+    }
 
     // make sure idents get transformed everywhere
     #[test] fn ident_transformation () {
@@ -1523,6 +1523,6 @@ mod test {
             matches_codepattern,
             "matches_codepattern",
             pprust::to_string(|s| fake_print_crate(s, &folded_crate)),
-            "zz!zz((zz$zz:zz$(zz $zz:zz)zz+=>(zz$(zz$zz$zz)+)))".to_string());
+            "zz!zz((zz$zz:zz$(zz $zz:zz)zz+=>(zz$(zz$zz$zz)+)));".to_string());
     }
 }