cargo fmt --all downloads dependencies

[jonhoo]

Describe the bug

When running cargo fmt with the --all argument, it will download all dependencies in the current workspace, including for targets not used on the current platform.

To Reproduce

1. Create a new empty crate and add a dependency on serde = "1".
2. Remove serde-*.crate from ~/.cargo/registry/cache/github.com-*/
3. Run cargo fmt
4. Check that no serde-*.crate files were downloaded into ~/.cargo/registry/cache/
5. Run cargo fmt --all
6. Observe that the call takes longer, and that serde-*.crate was downloaded into ~/.cargo/registry/cache/

Furthermore, do the above with a cfg-gated dependency on serde using

[target.'cfg(bogus)'.dependencies]
serde = "0.2"

Notice that serde is still downloaded with --all, even though it does not even meet the target platform.

Expected behavior

cargo fmt should never download dependencies, whether run with --all or not.

Meta

• rustfmt version: rustfmt 1.4.24-stable (eb894d53 2020-11-05)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: cargo-fmt

Diagnosis

The origin of this issue is that --all invokes cargo metadata without --no-deps

rustfmt/src/cargo-fmt/main.rs

Line 456 in 81ad114

let metadata_with_deps = get_cargo_metadata(manifest_path, true)?;

This behavior was introduced in #3664, and the reasoning is spelled out in #3664 (comment). I think this probably requires a fix to upstream, but I'm not sure (ideas welcome!). I figured that regardless, a tracking issue would be useful.

The target issue could conceivably be fixed by passing --filter-platform to cargo metadata (it considers all targets by default). But, that would still only work subject to rust-lang/cargo#8987, and would also mean that cfg-guarded workspace members would not be walked by rustfmt (I think?).

[jonhoo]

The requisite upstream feature we need is rust-lang/cargo#7483

[calebcartwright]

Thanks for the report, but I'm going to close as a duplicate as there's a few open issues where this has been discussed already (e.g. #4247) and summarized in #4247 (comment)

[jonhoo]

I'm terrible at searching, my apologies! I'll try to help pushing on upstream to figure out a fix :)

[calebcartwright]

No worries, and thanks for your work in this space! It's a tricky issue on both sides and has negative impacts on some portion of the user base so getting this resolved would be fantastic