Formatting for output of locate-project

[dtolnay]

This PR brings cargo tree's existing --format flag to cargo locate-project.

$ cargo locate-project --format '{root}'
/git/cargo/Cargo.toml

$ PROJECT_ROOT=$(cargo locate-project -f{root})

This allows scripts to select a script-friendly output representation. All the logic for interpreting such format flags already exists and is used by cargo tree --format. This solution is straightforwardly forward-compatible with introducing additional entries into what locate-project can produce, unlike such alternatives as cargo locate-project --output-format plain.

I like jq (am the third biggest contributor to it) but in 2020 we can't count on it being installed on target systems.

Closes #8009.

[rust-highfive]

r? @Eh2406

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[ehuss]

Thanks for the PR! This definitely seems useful. Can you say more why you went with a free-form "format" option? We were thinking it might be better to use a fixed-format flag like --message-format which would be more consistent with other subcommands. It could default to json (which is a little inconsistent, but is what it is), and then support another option like maybe script?

[dtolnay]

I was hoping to propose a way to locate the workspace's Cargo.toml, something I needed in https://github.com/dtolnay/cargo-rm and realized the only way to get it from a script is jq or a heroic sed.

For just the current crate root a new --message-format with a plain path output would be good, possibly adding flags for other project locations that get supported by locate-project later.

[phil-opp]

This PR (or some --message-format root-only option) would be also very useful for my locate_cargo_manifest crate, which currently parses the single line of JSON returned by cargo locate-project using the json crate. If there was some way to get only the path as output, we could make the crate completely dependency-free to further improve compile times.

(@dtolnay As an unrelated side note: The popular cargo-edit project also adds a cargo rm subcommand, so maybe you want to choose a different name for your project to avoid conflicts.)

[ehuss]

For just the current crate root a new --message-format with a plain path output would be good, possibly adding flags for other project locations that get supported by locate-project later.

Yea, I think I would probably prefer that. Would you be ok with making that change?

[dtolnay]

Closing in favor of #8707.