src/main/starlark/builtins_bzl/common/rule_util.bzl

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"""Defines create_rule and create_dep macros"""

def create_rule(impl, attrs = {}, deps = [], fragments = [], remove_attrs = [], **kwargs):
    """Creates a rule composed from dependencies.

    Args:
        impl: The implementation function of the rule, taking as parameters the
            rule ctx followed by the executable function of each dependency
        attrs: Dict of attributes required by the rule. These will override any
            conflicting attributes specified by dependencies
        deps: Dict of name->dependency mappings, with each dependency struct
            created using 'create_dep'. The keys of this dict are the parameter
            names received by 'impl'
        fragments: List of configuration fragments required by the rule
        remove_attrs: List of attributes to remove from the implementation.
        **kwargs: extra args to be passed for rule creation

    Returns:
        The composed rule
    """
    merged_attrs = dict()
    fragments = list(fragments)
    merged_mandatory_attrs = []

    for dep in deps:
        merged_attrs.update(dep.attrs)
        fragments.extend(dep.fragments)
        merged_mandatory_attrs.extend(dep.mandatory_attrs)
    merged_attrs.update(attrs)

    for attr in remove_attrs:
        if attr in merged_mandatory_attrs:
            fail("Cannot remove mandatory attribute %s" % attr)
        merged_attrs.pop(attr)

    return rule(
        implementation = impl,
        attrs = merged_attrs,
        fragments = fragments,
        **kwargs
    )

def create_dep(call, attrs = {}, fragments = [], mandatory_attrs = None):
    """Combines a dependency's executable function, attributes, and fragments.

    Args:
        call: the executable function
        attrs: dict of required rule attrs
        fragments: list of required configuration fragments
        mandatory_attrs: list of attributes that can't be removed later
          (when not set, all attributes are mandatory)
    Returns:
        The struct
    """
    return _create_dep(call, attrs, fragments, mandatory_attrs if mandatory_attrs else attrs.keys())

def _create_dep(call, attrs = {}, fragments = [], mandatory_attrs = []):
    return struct(
        call = call,
        attrs = attrs,
        fragments = fragments,
        mandatory_attrs = mandatory_attrs,
    )

def create_composite_dep(merge_func, *deps):
    """Creates a dependency struct from multiple dependencies

    Args:
        merge_func: The executable function to evaluate the dependencies.
        *deps: The dependencies to compose provided as keyword args

    Returns:
        A dependency struct
    """
    merged_attrs = dict()
    merged_frags = []
    merged_mandatory_attrs = []
    for dep in deps:
        merged_attrs.update(dep.attrs)
        merged_frags.extend(dep.fragments)
        merged_mandatory_attrs.extend(dep.mandatory_attrs)

    return _create_dep(
        call = merge_func,
        attrs = merged_attrs,
        fragments = merged_frags,
        mandatory_attrs = merged_mandatory_attrs,
    )