Support querying repo mappings

[cameron-martin]

Description of the feature request:

Add a way of querying for repo mappings, for usage in external tools (such as language servers). The repo mapping is necessary for these tools to resolve labels to files on disk, since the files live under the canonical repo name.

Which category does this issue belong to?

External Dependency

What underlying problem are you trying to solve with this feature?

Facebook have a starlark parser & lsp which works pretty great with Bazel. However, it doesn't support bzlmod because it assumes the repo name of a label is the canonical name, i.e. it looks up @foo in $(bazel info output_base)/external/foo.

According to @fmeum, this is needed for intellij too.

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

No response

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse master; git rev-parse HEAD ?

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

Slack discussion: https://bazelbuild.slack.com/archives/C014RARENH0/p1703107424515999

[fmeum]

I started working on this in the past in #18990, but noticed a problems with my implementation: All repositories generated by a module extension share the same repository mapping containing all these repos. If stored naively, this results in memory usage that is quadratic in the number of extension repos, which can easily be in the 1,000s. Thus, I think it's necessary to "intern" the repo mappings by having one array containing the deduplicated mappings and a map that references them by ID, e.g.:

{
  "mappings": [{"foo": "foo~1.2.3, ...}, {"bar": "bar~4.5.6", ...}, ...],
  "repoToMappingIndex": {"foo~1.2.3": 0, ...}
}

Happy to support anyone who wants to pick this up!

[fmeum]

CC @mai93 as this format will also be needed to make IntelliJ's "Go to Definition" features work for labels with Bzlmod.

[meteorcloudy]

Looks like this one is done! \o/

[iancha1992]

A fix for this issue has been included in Bazel 7.1.0 RC1. Please test out the release candidate and report any issues as soon as possible. Thanks!