Rustfmt changes meaning of 2018 import starting with colons

[dtolnay]

Repro against current master branch (8ec0750):

echo 'use ::serde::Serialize; mod serde {}' | cargo run --bin rustfmt

Output:

use serde::Serialize;
mod serde {}

The original code imports ::serde::Serialize which refers to the external crate serde, not the local module. The formatted code imports serde::Serialize which refers to the local module and does not exist.

Interestingly, cargo fmt against the same code does not break the code, presumably because cargo fmt somehow understands that the edition of this code is 2018 and so the leading :: is meaningful. In the absence of knowing the edition for sure, would it be possible for rustfmt to treat this case more conservatively and keep the leading ::? We can continue to remove the colons if the edition is known to be 2015.

[scampi]

If you put edition = "2018" in the config file that should be fine

[scampi]

cargo fmt picks up the edition from Cargo.toml

[topecongiro]

If you are running rustfmt directly you need to pass --edition flag to rustfmt: cargo run --bin rustfmt -- --edition 2018. As the default edition of rustc and cargo are both 2018, I think it's reasonable to change the default edition of rustfmt to 2018 in 2.0.

[dtolnay]

Thanks, I think this is user error and I should be passing --edition 2018.

I don't know if this is helpful but I filed #3517 to track bumping the default edition that is assumed when no --edition is passed.

[topecongiro]

@dtolnay That is indeed helpful, thank you!