Add plain message format for locate-project

[dtolnay]

Supersedes #8683, as recommended in #8683 (comment). This PR adds a flag --message-format to cargo locate-project with possible values json (default) and plain.

$ cargo locate-project --message-format json
{"root":"/git/cargo/Cargo.toml"}

$ cargo locate-project --message-format plain
/git/cargo/Cargo.toml

Closes #8009.

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[dtolnay]

r? @ehuss

[ehuss]

Thanks!

@rfcbot fcp merge

@rust-lang/cargo This adds a new, stable command-line option to the cargo locate-project command to change the output format. --message-format=plain will print the project path without needing to parse JSON, which makes it much easier to use from a script (or really any tool). I think this is minor enough that I don't think it needs to go through the unstable process, but feel free to disagree.

I think --message-format is a good choice to maintain consistency with other commands. It is unfortunately inconsistent that the default is json, whereas other commands use different defaults. I think that's fine, and the docs are pretty straightforward. The only other concern is if the locate-project command is extended to print more information in the future, that will require adding additional command-line flags to control what is printed. I'm not too concerned about that, as I don't think there is much desire for additional information, and adding command-line flags is easy.

[ehuss]

@rfcbot fcp merge

[rfcbot]

Team member @ehuss has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Eh2406
• @alexcrichton
• @ehuss
• @joshtriplett

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[ehuss]

Thanks!
@bors r+

[bors]

📌 Commit 9ecf5e9 has been approved by ehuss

[bors]

⌛ Testing commit 9ecf5e9 with merge a9d6947...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing a9d6947 to master...