Build only the default run binary

[dzfranklin]

I'd like a way to build only the default run binary, i.e. what cargo run runs. (I'm writing a subcommand that lets the user run the various binaries cargo builds under a time-travelling debugger. By default, I want to build and run only the default run binary)

Describe the solution you'd like

Either cargo build --default-bin, or some way to get the name of the default binary.

(I think --no-run is being phased out. If not, cargo run --no-run could also be added, although it sounds a little silly.)

Notes

If this is a wontfix I'd like to confirm that the algorithm used by cargo run is as follows.

• main.rs
• bin/foo.rs if only one exists
• value of default-run key in manifest (requires feature: default-run included in cargo metadata #9497 to get from cargo metadata)

[joshtriplett]

We discussed this in today's @rust-lang/cargo meeting. We'd be fine with an option to build what cargo run would run, but it should definitely be a cargo build option (e.g. --default-bin); we felt it would be confusing to have cargo run --no-run when what you're actually asking for is to build and not run.

On the other hand, you also filed #9497 to provide this information in cargo metadata, and our general sentiment in the meeting was that if metadata would suffice we'd be much happier providing it that way and not adding a new command-line option to cargo build.

Would metadata suffice for your use case?

[dzfranklin]

Hi Josh, sorry I missed your reply. Metadata would be sufficient.