What version of gazelle are you using?

0.37.0

What version of rules_go are you using?

0.49.0

What version of Bazel are you using?

7.2.1

Does this issue reproduce with the latest releases of all the above?

Yes

What operating system and processor architecture are you using?

macOS 15.1 aarch64

What did you do?

I generated some empty rules to remove existing rules in BUILD files:

func cleanUndefinedRules(res *language.GenerateResult, f *rule.File) {
	for _, fr := range f.Rules {
		if ... {
				continue
		}
		r := rule.NewRule(fr.Kind(), fr.Name())
		res.Empty = append(res.Empty, r)
	}
}

The kind definition has:

		MergeableAttrs: map[string]bool{
			"srcs": true,
		}

and the current build file has:

some_rule(
	name = "test",
    srcs = glob([...]),
)

What did you expect to see?

the "test" target gets removed.

What did you see instead?

gazelle: BUILD.bazel:124.12-124.29: could not merge expression

and the target is untouched after the run.

This is because when merging the attrs, if the dst is not a scalar, it can't be removed.