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/librustdoc/html/render.rs

@@ -246,9 +246,9 @@ struct IndexItem {
 
 // TLS keys used to carry information around during rendering.
 
-thread_local!(static CACHE_KEY: RefCell<Arc<Cache>> = Default::default())
+thread_local!(static CACHE_KEY: RefCell<Arc<Cache>> = Default::default());
 thread_local!(pub static CURRENT_LOCATION_KEY: RefCell<Vec<String>> =
-                    RefCell::new(Vec::new()))
+                    RefCell::new(Vec::new()));
 
 /// Generates the documentation for `crate` into the directory `dst`
 pub fn run(mut krate: clean::Crate,