Support cargo build

[dmikusa]

Right now, cargo install is used to build and install the application binaries. Running cargo install and cargo build are not quite the same though. There are some subtle differences like build will use the lock file by default, build also handles workspaces differently, and build allows you to build tests.

The main trick will be running cargo build and then finding the right binaries to "install". You get that for free with cargo install. You can find the binaries by reading the cargo metadata, which we're already doing for some things.

Open questions:

1. Do we need to support both cargo build and cargo install? or should we replace cargo install with cargo build?
2. If we support both, how do you signal to the buildpack which one should be used? Possibly an env variable like BP_CARGO_COMMAND.
3. What should we default to? build or install?

[dmikusa]

@edude03 ^^ FYI.

[edude03]

Do we need to support both cargo build and cargo install? or should we replace cargo install with cargo build?

I'd like it if we still supported cargo install - just for building quick containers with tools - for example we use rover, so being able to do cargo install github.com/apollograph/rover is great.

If we support both, how do you signal to the buildpack which one should be used? Possibly an env variable like BP_CARGO_COMMAND.

Seems fine to me.

What should we default to? build or install?

Build has the "more correct" behaviour around using the lock file so I'd say build.

[dmikusa]

Thanks for the feedback!

Do we need to support both cargo build and cargo install? or should we replace cargo install with cargo build?
I'd like it if we still supported cargo install - just for building quick containers with tools - for example we use rover, so being able to do cargo install github.com/apollograph/rover is great.

To clarify, we wouldn't remove the support for installing third-party tools with cargo install like diesel cli. i.e. what you can presently install with BP_CARGO_INSTALL_TOOLS.

The change would be to how the primary application, i.e. your source code, gets built and installed.

Does that change your feedback at all?

[edude03]

(Sorry for the long delay) that makes sense and sounds like the right way to do it :)