Absolute use path is erroneously stripped

[DomWilliams0]

Rustfmt is removing the :: prefix of absolute imports, which breaks when that import must differentiate between the crate of that name and a clashing module. An example:

$ cat src/lib.rs
// an empty module that shares the name of another crate, no problem
mod std {}

// i can access the original crate through ::std
use ::std::string;

$ cargo build
...
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s // <----- successful build

$ rustfmt --version
rustfmt 1.4.9-nightly (33e3667 2019-10-07)

$ rustfmt src/lib.rs

$ cat src/lib.rs
// an empty module that shares the name of another crate, no problem
mod std {}

// i can access the original crate through ::std
use std::string;      // <------------ note the removed :: prefix

$ cargo build
   Compiling fmt v0.1.0 (/tmp/fmt)
error[E0432]: unresolved import `std::string`
 --> src/lib.rs:5:5
  |
5 | use std::string;
  |     ^^^^^^^^^^^ no `string` in `std`

error[E0659]: `std` is ambiguous (name vs any other name during import resolution)
 --> src/lib.rs:5:5
  |
5 | use std::string;
  |     ^^^ ambiguous name
  |
  = note: `std` could refer to a built-in crate
  = help: use `::std` to refer to this crate unambiguously
note: `std` could also refer to the module defined here
 --> src/lib.rs:2:1
  |
2 | mod std {}
  | ^^^^^^^^^^
  = help: use `crate::std` to refer to this module unambiguously

error: aborting due to 2 previous errors

[calebcartwright]

Sounds like a potential dupe of #3501.

Have you tried including edition either via --edition 2018 arg or in your rustfmt config file? rustfmt 2.0 will switch the default edition to 2018 but it currently defaults to 2015

[topecongiro]

Closing as a dup of #3501.