Comments on extern crates are inconsistently indented

[shepmaster]

0.99.6-nightly (2018-10-18 750b252)

Before

extern crate serde; // 1.0.78
extern crate serde_derive; // 1.0.78
extern crate serde_json; // 1.0.27

After

extern crate serde;        // 1.0.78
extern crate serde_derive; // 1.0.78
extern crate serde_json; // 1.0.27

Expected

Either of

extern crate serde;        // 1.0.78
extern crate serde_derive; // 1.0.78
extern crate serde_json;   // 1.0.27

extern crate serde; // 1.0.78
extern crate serde_derive; // 1.0.78
extern crate serde_json; // 1.0.27

[scampi]

I think the core problem of this issue is similar to #3127 : the span for the list of extern statements does not include the last comment, i.e., // 1.0.27 here. Therefore, that comment is not aligned with the first two. The last comment is added thanks to code in src/missed_spans.rs.

Some context:

The walk_mod_items passes the list of mod items to be reordered alphabetically:
https://github.com/rust-lang-nursery/rustfmt/blob/284583fade72a8824153b36cceabe42118b57138/src/visitor.rs#L696-L698

Since the span of that list does not include the last comment, the ListItems misses it as a post_comment of the serde_json item.

https://github.com/rust-lang-nursery/rustfmt/blob/284583fade72a8824153b36cceabe42118b57138/src/lists.rs#L702

As a consequence, when the list is written down in write_list, it is as if the comment // 1.0.27 was never there to begin with.

https://github.com/rust-lang-nursery/rustfmt/blob/284583fade72a8824153b36cceabe42118b57138/src/lists.rs#L265