auto merge of #20190 : cmr/rust/gate-macro-args, r=alexcrichton

Uses the same approach as rust-lang/rust#17286 (and
subsequent changes making it more correct), where the visitor will skip any
pieces of the AST that are from "foreign code", where the spans don't line up,
indicating that that piece of code is due to a macro expansion.

If this breaks your code, read the error message to determine which feature
gate you should add to your crate.

Closes #18102

[breaking-change]

Author: bors

lib.rs

@@ -271,7 +271,8 @@ pub trait Rng {
     /// let choices = [1i, 2, 4, 8, 16, 32];
     /// let mut rng = thread_rng();
     /// println!("{}", rng.choose(&choices));
-    /// assert_eq!(rng.choose(choices[..0]), None);
+    /// # // replace with slicing syntax when it's stable!
+    /// assert_eq!(rng.choose(choices.slice_to(0)), None);
     /// ```
     fn choose<'a, T>(&mut self, values: &'a [T]) -> Option<&'a T> {
         if values.is_empty() {