Build plan should more clearly distinguish custom build steps

[luser]

Currently each build script in a build produces two invocations in the build plan--one to compile the build script, and one to run it. However, they both have "target_kind": ["custom-build"] so it's difficult to distinguish them when consuming a build plan.

In my cargo-build-plan-graph tool I wound up using a heuristic that I do not love. It would be better if they had unique target_kind keys or some other straightforward way to distinguish them.

Here's a sample build plan from sccache.

[Mark-Simulacrum]

For now a better heuristic might be endswith("build-script-build")

[ehuss]

I think that would be starts_with("build-script-") since the user can change the name with the build key.

[Mark-Simulacrum]

Huh, I never thought about the build being user dependent; is that documented anywhere? It seems needlessly complicated...

[ehuss]

The key is documented in build scripts and the manifest reference. It doesn't really talk about the binary/crate name of the build script, presumably because that's not something that's really relevant to most users.

[dwijnand]

Is changing the target_kind something we're happy to change in Cargo? (post soft freeze)

[ehuss]

It might get complicated if something like #5855 is implemented. Something like Doctest would appear with target_kind=["lib"] with no other distinction. target_kind could be sort of a mix between TargetKind and CompileMode. I'm trying to think if it would be better to keep them separate, but I can't think of a good one.

[ehuss]

cc #5508 which also discusses the problems with target_kind in the build-plan.