Investigate preserving comments

[dtolnay]

This seems highly desirable for cargo crev's use case.

From each syn::ItemMod that gets parsed, you can find the Span of the semicolon and use Span::start from proc-macro2's "span-locations" feature to figure out the input file line and column of the semicolon. Then you can replace that semicolon character in the original source string with a set of braces containing an indented content of the child module. The final output would be a string of source code rather than syn::File, which the caller can parse to syn::File if they want.

[dtolnay]

FYI @dpc. This seems pretty necessary for crev-dev/cargo-crev#89 so you may be interested in giving it a shot.