Rollup merge of #48106 - QuietMisdreavus:teleporting-crates, r=GuillaumeGomez

rustdoc: move manual "extern crate" statements outside automatic "fn main"s in doctests Gated on #48095 - I based the branch atop that so i could show off the change in one of its tests, the actual change in this PR is just the last commit There are a handful of unfortunate assumptions in the way rustdoc processes `extern crate` statements in doctests: 1. In the absence of an `extern crate` statement in the test, if the test also uses the local crate name, it will automatically insert an `extern crate cratename;` statement into the test. 2. If the doctest *does* include an `extern crate` statement, rustdoc will not automatically insert one, on the assumption that doing so would introduce a duplicate import. 3. If a doctest does not have the substring `fn main` outside a comment, rustdoc will wrap the whole doctest in a generated `fn main` so it can be compiled. In short, whenever you write a doctest like this... ```rust //! extern crate my_crate; //! my_crate::some_cool_thing(); ``` ...rustdoc will turn it into (something like) this: ```rust fn main() { extern crate my_crate; my_crate::some_cool_thing(); } ``` This creates issues when compiled, because now `my_crate` isn't even properly in scope! This forces people who want to have multiple crates in their doctests (or an explicit `extern crate` statement) to also manually include their own `fn main`, so rustdoc doesn't put their imports in the wrong place. This PR just taps into another processing step rustdoc does to doctests: Whenever you add an `#![inner_attribute]` to the beginning of a doctest, rustdoc will actually splice those out and put it before the generated `fn main`. Now, we can just do the same with `extern crate`s at the beginning, too, and get a much nicer experience. Now, the above example will be converted into this: ```rust extern crate my_crate; fn main() { my_crate::some_cool_thing(); } ```

Author: GuillaumeGomez

src/librustc_resolve/diagnostics.rs

@@ -325,6 +325,8 @@ Erroneous code example:
 extern crate core;
 
 struct core;
+
+fn main() {}
 ```
 
 There are two possible solutions:

src/librustdoc/test.rs

@@ -414,7 +414,8 @@ fn partition_source(s: &str) -> (String, String) {
  for line in s.lines() {
  let trimline = line.trim();
  let header = trimline.is_whitespace() ||
- trimline.starts_with("#![");
+ trimline.starts_with("#![") ||
+ trimline.starts_with("extern crate");
  if !header || after_header {
  after_header = true;
  after.push_str(line);
@@ -814,8 +815,8 @@ use asdf::qwop;
 assert_eq!(2+2, 4);";
  let expected =
 "#![allow(unused)]
-fn main() {
 extern crate asdf;
+fn main() {
 use asdf::qwop;
 assert_eq!(2+2, 4);
 }".to_string();