`cargo metadata` does not take CARGO_BUILD_TARGET into account

[jyn514]

Problem

cargo metadata should print the correct target directory, but when CARGO_BUILD_TARGET is set, still prints the default instead of the directory that will actually be used.

Steps

$ rm -rf target
$ export CARGO_BUILD_TARGET=x86_64-unknown-linux-gnu
$ cargo doc
 Documenting test-doc v0.1.0 (/home/joshua/src/rust/test-doc)
    Finished dev [unoptimized + debuginfo] target(s) in 0.80s
$ ls target/doc
ls: cannot access 'target/doc': No such file or directory
$ ls target/x86_64-unknown-linux-gnu/doc
ayu.css         favicon.ico            LICENSE-MIT.txt  rustdoc.css      settings.js                  source-script.js                 src
brush.svg       FiraSans-LICENSE.txt   light.css        rust-logo.png    SourceCodePro-LICENSE.txt    SourceSerifPro-Bold.ttf.woff     storage.js
COPYRIGHT.txt   FiraSans-Medium.woff   main.js          search-index.js  SourceCodePro-Regular.woff   SourceSerifPro-It.ttf.woff       test_doc
dark.css        FiraSans-Regular.woff  normalize.css    settings.css     SourceCodePro-Semibold.woff  SourceSerifPro-LICENSE.md        theme.js
down-arrow.svg  LICENSE-APACHE.txt     noscript.css     settings.html    source-files.js              SourceSerifPro-Regular.ttf.woff  wheel.svg
$ cargo metadata | jq .target_directory
warning: please specify `--format-version` flag explicitly to avoid compatibility problems
"/home/joshua/src/rust/test-doc/target"

Possible Solution(s)

Add the target directory to target_directory.

Notes

Output of cargo version:

cargo 1.48.0-nightly (9d1a4863a 2020-10-05)

[ehuss]

CARGO_BUILD_TARGET doesn't change the target directory, though it does change the layout within the target directory. If specifying a target, then the output is in $target_dir/$target/.... This is explained a bit in https://doc.rust-lang.org/cargo/guide/build-cache.html.

[jyn514]

I see. Is there a way to get the doc directory from cargo metadata? I don't see target/doc anywhere in the metadata output, but I might have missed it.

I'd prefer to avoid having to mimic cargo's logic for determining the structure within the target/ directory, AFAIK that's not intended to be stable.

[ehuss]

I think the specific layout rules are stable and not something we will likely change. That is:

• $CARGO_TARGET_DIR/doc is the doc output if --target is not specified.
• $CARGO_TARGET_DIR/$TARGET/doc is the doc output if --target is specified.

It doesn't look like cargo metadata gives you $TARGET, so that is a little awkward if you are using config files. If possible, you can also use JSON messages from cargo which will tell you the output path from rustdoc.

[jyn514]

I'm not running cargo directly, so I have to be able to find the information after the fact. It sounds like there isn't currently a way to do that - would you take a PR adding $TARGET to cargo metadata?

[ehuss]

We are a little reluctant to add the default target to cargo metadata because Cargo is moving towards supporting multiple targets during the same build, so it is not really clear how that would be exposed. Additionally, cargo metadata is geared for pre-build information, and we don't want to go further down the road towards including things like artifact outputs.

Would it work to have something like cargo config build.target which would fetch and display the confg value if it is defined?

[jyn514]

I'm a little confused why this needs a new command ... cargo config build.target would work fine for me but I guess I'm not sure why this is so different from the rest of cargo metadata?

Cargo is moving towards supporting multiple targets during the same build

Where could I learn more about this?

[jyn514]

Also, FWIW cargo-deadlinks looks at all cargo targets, so if this information was per-target instead of global that would be perfectly fine.

[ehuss]

cargo metadata is mostly intended for querying the workspace and resolve configuration. It's actually a bug that it looks at the target at all (#8462).

Where could I learn more about this?

https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#multitarget and #7004 and #6784