template_rule.bzl

# Copyright 2022 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Rule for simple expansion of template files."""

def _template_rule_impl(ctx):
    """Modifies ctx.file.src keys in ctx.attr.substitutions with corresponding values."""
    ctx.actions.expand_template(
        template = ctx.file.src,
        output = ctx.outputs.out,
        substitutions = ctx.attr.substitutions,
    )

# Rule for simple expansion of template files. Modifies the templates in src substituting the keys
# in substitutions with the corresponding values; the result is output to out.
#
# Borrowed from TensorFlow (https://github.com/tensorflow/tensorflow)
#
# Typical usage:
#   template_rule(
#       name = "ExpandMyTemplate",
#       src = "my.template",
#       out = "my.txt",
#       substitutions = {
#         "$VAR1": "foo",
#         "$VAR2": "bar",
#       }
#   )
#
# Args:
#   name: The name of the rule.
#   src: The template file to expand
#   out: The destination of the expanded file
#   substitutions: A dictionary mapping strings to their substitutions
template_rule = rule(
    attrs = {
        "src": attr.label(
            mandatory = True,
            allow_single_file = True,
        ),
        "substitutions": attr.string_dict(mandatory = True),
        "out": attr.output(mandatory = True),
    },
    # output_to_genfiles is required for header files.
    output_to_genfiles = True,
    implementation = _template_rule_impl,
)