Rustfmt can generate invalid code when using `space_after_colon=false`

formatting the following code with space_after_colon=false will generate invalid code

struct SomeStruct {
    some_field: ::some_crate::Thing,
}

since it removes the space between the field colon and the :: prefix on some_crate
i cant see this as very high priority because i dont know anyone who prefixes things with :: asides from maybe alloc or std, and space_after_colon=false isnt widely used, but this should probably be looked into

Start by reproducing the Rust snippet with space_after_colon=false and checking whether the formatted output still parses. Then trace rustfmt's handling of spaces after struct-field colons and add a regression test for the :: path prefix; done means formatting preserves valid Rust code.