Please add a Skylark builtin for parsing JSON to a dict (useful for repo rules)

When writing repository rules that inspect the state of the local system, I commonly want to run some local command and inspect its machine-readable output. The rule would then make configuration decisions based on available features, versions, installed plugins, and so on.

Nearly all tools that offer machine-readable version data use JSON, or at least offer it as an option. Rather than try to hack my way through with .split and .index, it would be much nicer if I could take a JSON string and have Bazel parse it directly into a dict within Skylark. A builtin json_to_dict(str) -> dict function would do nicely.

Using cmake as an example, I want to get its version using repository_ctx.execute():

$ cmake -E capabilities
{"version":{"isDirty":false,"major":3,"minor":9,"patch":1,"string":"3.9.1","suffix":""}}

… and then use that (3,9,1) tuple to decide whether it’s recent enough, or should be ignored in favor of an external repository.

There was a previous issue open about this (https://github.com/bazelbuild/bazel/issues/1813), but it had unclear motivations and was closed without action taken.

[jin]

FWIW, rules_jvm_external has been using @erickj’s bazel-json parser in Starlark and we haven’t ran into any issues with it since the beginning of the project.

[jmillikin-stripe]

There’s now a couple implementations of a JSON API in non-Bazel tools built on Starlark. To see if we can settle on a reasonable shared JSON API, I sent https://github.com/bazelbuild/starlark/pull/83 as a proposed standard JSON module in the Starlark spec.