`-w json` breaks `cargo doc --message-format=json`

[jyn514]

Problem

here is cargo, run on a toy crate:

$ RUSTDOCFLAGS="-Zunstable-options -wjson" cargo doc --message-format json
 Documenting whatever v0.1.0 (/home/jyn/src/whatever)
{"reason":"compiler-artifact","package_id":"path+file:///home/jyn/src/whatever#0.1.0","manifest_path":"/home/jyn/src/whatever/Cargo.toml","target":{"kind":["bin"],"crate_types":["bin"],"name":"whatever","src_path":"/home/jyn/src/whatever/src/main.rs","edition":"2024","doc":true,"doctest":false,"test":true},"profile":{"opt_level":"0","debuginfo":2,"debug_assertions":true,"overflow_checks":true,"test":false},"features":[],"filenames":[],"executable":null,"fresh":false}
{"reason":"build-finished","success":true}

this says "filenames": []. that is not correct. rustdoc generated an output file in /home/jyn/.local/lib/cargo/target/doc/whatever.json, which should be reported here.

my understanding is that for this to work, cargo needs rustdoc to support --print=file-names, which it doesn't currently.

Proposed Solution

1. Add support for --print=file-names to rustdoc. I think this can be insta-stable; any other values should probably be unstable.
2. Change cargo to run rustdoc --print=file-names before running it on the crate, just like it already does for rustc.
3. Use that to emit filenames in cargo's own json output.

Notes

here is an strace of cargo running rustc --print=file-names:

[pid 20757] execve("/home/jyn/.local/lib/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc", ["/home/jyn/.local/lib/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc", "-", "--crate-name", "___", "--print=file-names", "--crate-type", "bin", "--crate-type", "rlib", "--crate-type", "dylib", "--crate-type", "cdylib", "--crate-type", "staticlib", "--crate-type", "proc-macro", "--print=sysroot", "--print=split-debuginfo", "--print=crate-name", "--print=cfg", "-Wwarnings"], 0x55bb1434c940 /* 73 vars */) = 0

[weihanglo]

We have JSON output support in cargo rustdoc #13283 BTW. The --output-format json needs special handling for Cargo to track files.

cargo needs rustdoc to support --print=file-names, which it doesn't currently

I don't think it is needed, unless it starts emitting more than the logic here and Cargo cannot catch up with. The current output of cargo rustdoc --message-format json --output-format json -Zunstable-options looks good.

I think the action item is adding --output-format support in cargo doc.

[weihanglo]

That said, if there are other reasons Cargo wants --print=file-names support in rustdoc, we could do that and benefit from it.

[jyn514]

The current output of cargo rustdoc --message-format json --output-format json -Zunstable-options looks good.

oh, huh, TIL. i agree this is fine and we just need to add support to cargo doc.