rules/expand_template.bzl

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#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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#
#    http://www.apache.org/licenses/LICENSE-2.0
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"""A rule that performs template expansion.
"""

def _expand_template_impl(ctx):
    ctx.actions.expand_template(
        template = ctx.file.template,
        output = ctx.outputs.out,
        substitutions = ctx.attr.substitutions,
    )

expand_template = rule(
    implementation = _expand_template_impl,
    doc = """Template expansion

This performs a simple search over the template file for the keys in
substitutions, and replaces them with the corresponding values.

There is no special syntax for the keys. To avoid conflicts, you would need to
explicitly add delimiters to the key strings, for example "{KEY}" or "@KEY@".""",
    attrs = {
        "template": attr.label(
            mandatory = True,
            allow_single_file = True,
            doc = "The template file to expand.",
        ),
        "substitutions": attr.string_dict(
            mandatory = True,
            doc = "A dictionary mapping strings to their substitutions.",
        ),
        "out": attr.output(
            mandatory = True,
            doc = "The destination of the expanded file.",
        ),
    },
    output_to_genfiles = True,
)