Add mapping from bazel labels to files in the runfiles manifest

[redsun82]

Recently I wanted to move the body of a very complex genrule to a bash script wrapped in an sh_binary used by that genrule, so that I could have a simple BUILD. What I clashed with is how the runfiles library actually works with respect to genrules.

In genrule, you use $(execpath @repo//a/bazel/label). The equivalent within the bash script using runfiles is $(rlocation some/obscure/path/to/what/the/label/stands/for). This poses several challenges:

• as a rule writer, I have no idea what exact file a label refers to. There's aliases and rule outputs to unwrap, and I need to stop and research that
• the label may (quite often) refer to different files on different platforms: in that case writing a cross-platform script is a real pain

Wouldn't it be cool if the mapping from bazel labels to data files was added to the runfiles manifest, so that one could write in the bash script (or whatever else using a runfiles library) $(rlocation @repo//a/bazel/label) just like in a genrule? There would be the case of labels unwrapping to multiple or no files, but that should be treated as an error just like in genrules. Providing rlocations in bash might be delicate because of separation of items in case spaces are in the files, but I'm sure there would be solutions to that.

[fmeum]

Have you seen $(rlocationpath(s) ...) (https://bazel.build/reference/be/make-variables#predefined_label_variables)? While not quite as convenient, it allows you to avoid hard-coding the file paths into your code.