Difficult to know when to stop parsing JSON messages.

[ehuss]

Describe the problem you are trying to solve
It is currently difficult when parsing JSON messages to know when Cargo is finished. With cargo test or cargo run, additional things will be printed to stdout. Tools should scan for lines that start with { to detect JSON, but that doesn't help if the subsequent process also emits {.

Describe the solution you'd like
I think Cargo should emit some JSON message to indicate that it is finished. Tools can use this to know when to stop parsing. Or perhaps the message should be structured to indicate that Cargo is running an external process (with the path to the process). This may be complicated with tests since multiple tests will run (and would be good to have a story for how JSON test output works).

Either way, tools need some way to know when Cargo stops, and something else begins.

[alexcrichton]

I think this makes sense to do for a single-binary case like cargo run, but for cargo test I'd be worried about the multi-binary case as you've described. That'd probably make me lean a bit more toward "print we're running something" rather than "print the build is done". That would be a bit more flexible to running multiple binaries, and it also wouldn't have an odd message at the end of cargo build, only required messages on cargo run or cargo test