Use bazel query to provide completion on target names

[blais]

This CL enhances the build commands by automatically inferring the name of the target from the current buffer. If multiple targets match, completion is provided with the list.

This also changes the commands to run from the root of the workspace with a cd before the command name. There's an option to turn that behavior on. I think this is the most natural way to work with bazel (cwd at root of workspace).

If the current buffer is

• A file, bazel query is used to figure out the name of the target that the file is a src of.
• A directory (dired mode), the list of targets in the package is provided with default :all
• A BUILD file, the targets in the package as well.

Note that I did not add unit tests. I did check locally on all three files that it works. This PR for your consideration. I'll be running this on my box for a while. Happy to move to a branch if you prefer to work that way (let me know0.

Some further improvements are possible for the future:

• Adding an option to disable this might be useful, as it can be a little slow.
• When computing the completion list over a file, it may be a good idea to actually fetch it for the enclosing package, so that if the user wants to edit it there's completion from packages nearby.
• If a single target matches the file, maybe the command should just run. Using a prefix argument to allow the user to edit the target before running, perhaps.

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[blais]

@googlebot I signed it!

[phst]

This CL enhances the build commands by automatically inferring the name of the target from the current buffer. If multiple targets match, completion is provided with the list.

This also changes the commands to run from the root of the workspace with a cd before the command name.

Would you mind splitting up these two changes into separate PRs? They seem pretty unrelated to each other.

[phst]

I think especially for large repositories or when the Bazel server needs to be restarted, this approach can be too slow for an interactive command. I think there should at least be a customization option to switch between the precise-but-slow bazel query method and a faster-but-imprecise method like the one used before.

[phst]

I'd suggest making this a customization option and instead of a (non-extensible) Boolean provide choices such as workspace-root and default-directory.

[phst]

I don't think the entire standard error should be part of the error message, since it can be arbitrarily long.
I'd also suggest singaling a specific error symbol, so that callers can catch and handle the error.

[blais]

What summarization would you like to see?

[phst]

So, looking at this "holistically", I'd probably just return nil here and handle that accordingly in the completion prompt. This isn't a critical condition in the context of completion, as the user can always enter the target manually, so we souldn't fail hard here or pop up a warning.

[phst]

This will fail if there are no rules in the current package