Rustfmt: Leading :: removed but necessary for disambiguation in 2018 edition

[yoshuawuyts]

Heya, I think I ran into a a bug with the uniform_paths feature. I've included the expected output, and error case below. Run on rustfmt 0.99.2-nightly (2018-08-07). I hope this comes in useful. Thanks! :sparkles:

Expected

This is the code I'm trying to write. We need the ::log prefix because the compiler is unsure which instace of log we're trying to refer to.

#![feature(rust_2018_preview, uniform_paths)]

use futures::prelude::*;
use http_03::cli::Cli;
use hyper::{service::service_fn_ok, Body, Response, Server};
use ::log::{error, info, log};
use structopt::StructOpt;

Current

rustfmt changes ::log to log which prevents the code from compiling (error included below).

#![feature(rust_2018_preview, uniform_paths)]

use futures::prelude::*;
use http_03::cli::Cli;
use hyper::{service::service_fn_ok, Body, Response, Server};
use log::{error, info, log};   // The `::` prefix is removed here
use structopt::StructOpt;

Error Message

[orium]

It doesn't need to be macro imports. In this code:

#![feature(rust_2018_preview, uniform_paths)]
extern crate criterion;
use ::criterion::Criterion;

rustfmt will also rewrite use ::criterion::Criterion; to use criterion::Criterion;

[crw5996]

I'm confused about this issue. I have a patch written but it is failing ./tests/import.rs at line 62 because the tests expect the formatter to remove the leading ModSpace (::) operator.

How should I go around fixing this issue? Check to see if the user is using the uniform_paths feature?

[nrc]

How should I go around fixing this issue?

There is an option to set for which edition Rustfmt is working under. We should only preserve leading :: in the 2018 edition. For the test for this issue we should use the special comments for setting options to set the edition to 2018.

[scampi]

@nrc Should this issue be closed, given the PR got merged ?

[orium]

@nrc what is that option?

[crw5996]

@orium you can add edition = "2018" to rustfmt.toml to fix the problem

[orium]

That lead me to discover #2989 :)

Edit: ...which was just fixed a few hours ago by #2984.

[orium]

This bug is still present even if you set the edition to 2018:

orium@laptop-orion tmp % cat .rustfmt.toml
edition = "Edition2018"
orium@laptop-orion tmp % cargo +nightly fmt -- --check
Diff in /tmp/orium/tmp/src/lib.rs at line 1:
 #![feature(rust_2018_preview, uniform_paths)]
 extern crate criterion;
-use ::criterion::Criterion;
+use criterion::Criterion;

[jonhoo]

The title of this issue should probably be updated. I filed #3026 before finding this by digging through the commit logs :p I guess this has now been fixed, but a new rustfmt release for nightly hasn't been cut yet?

[yoshuawuyts]

@jonhoo you're right -- changed it to use the title of the issue you opened, because that seems like a better description of what's going on.

[nrc]

Yeah, this is fixed on master. I'll update nightly rustfmt later in the week.

[orium]

@nrc Will this be released as part of the next beta? I'm mentioning this because it would the current beta is the 2018 edition RC.

[nrc]

Probably. I am planning a beta uplift, not sure exactly when it'll happen.