Debugging "keep" pragmas

[phst]

I use Gazelle for various languages for one of my projects (https://github.com/phst/rules_elisp), and found that I need to use a lot of # keep pragmas to keep Gazelle happy (https://github.com/search?q=repo%3Aphst%2Frules_elisp+%2F%23%5C+keep%24%2F+path%3A%2F%5C%2FBUILD%24%2F&type=code). In many cases I need to apply the pragma to an entire rule target just to prevent Gazelle from removing attributes or list elements (end-of-line pragmas seem to be ignored most of the time). I'd like to file more specific bugs about these issues (assuming they are shortcomings of Gazelle and its language plugins), but I'd like to debug them further first. Is there some way to visualize/debug Gazelle's decision-making graph, e.g. to answer the question "why does Gazelle insist on removing the attribute FOO of the BAR target"?

[jayconrod]

Unfortunately there's no verbose mode or anything, and I think at this level of detail, the output might be overwhelming. You might be best served running Gazelle with a debugging, setting a breakpoint in rule.MergeRules with a condition matching the file name and rule name you're interested in.

Most of the merging behavior is governed by rule.KindInfo, returned by the extension that generated the rule. MergeableAttrs and ResolveAttrs are sets attributes that Gazelle is expected to manage. It will delete whatever's there (modulo # keep) and overwrite it with the attribute from the generated rule.

Gazelle doesn't know how to merge all types of expressions, so merging may not work as expected on anything that's not a simple string or list. Gazelle doesn't evaluate expressions, so it won't be able to figure out a literal list concatenated with a variable, for example.

# keep comments are attached to AST nodes, depending on where they're located. For a "before" comment, it's the top-most node in the tree after that point. For a "suffix" comment, it's the right-most node on that line. If that's not a kind of expression that Gazelle knows how to recognize, it may be ignored.