Practical Bazel: Prefix All Generated Files

When writing a custom rule that generates files, be sure to add prefixes to all filenames so that multiple instances of your rule can be instantiated within the same Bazel package.

I like to place all generated files in a subdirectory prefixed with the rule’s label and %, as in:

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def _impl(ctx):
    prefix = ctx.label.name + "%/"
    file1 = ctx.actions.declare_file(prefix + "generated_file1.txt")
    # generate file1
    file2 = ctx.actions.declare_file(prefix + "generated_file2.txt")
    # generate file2
    ...

my_rule = rule(
    _impl,
    attrs = {
        ...
    },
)