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BXL and Anonymous Targets

Anonymous targets

Anonymous targets are supported in BXL. Anonymous targets are keyed by the attributes, and allow you to share/cache work more effectively.

You might want to use anonymous targets if there is some heavy Starlark evaluation which can be cached, or if you want to cache local actions.

Note: The context object within the anon target rule is not a BXL context, but a normal rule analysis context.


The actions object returned from ctx.bxl_actions().actions (equivalent of ctx.actions in normal rules) has the following functions for anonymous targets:

  • anon_target(rule: "rule", attrs: Dict[str, Any]) -> "promise": generates a single anonymous target. Return type is an unresolved promise.
  • anon_targets(rules: [("rule", Dict[str, Any])]) -> "promise": generates a list of anonymous targets. Return type is an unresolved promise representing the list of anonymous targets.
  • artifact_promise(promise: "promise") -> "promise_artifact": turns an unresolved promise into a kind of artifact. See Convert promise to artifact for more info on why you might want to use this.

The resulting promise also has map() and join() functions. map() applies a function to the promise's results, and join() turns multiple promises into a single promise.

To resolve promises in BXL, bxl_ctx has a resolve() function, which takes in the analysis actions instance (actions object returned from ctx.bxl_actions().actions) and a single promise and returns an optional promise value, if there is one. If you intend to create multiple promises, using join() to produce a single promise will allow you to resolve them concurently with a single resolve() call.

Small example:

def _my_impl(ctx):
bxl_actions = ctx.bxl_actions() # pass in relevant params to configure the execution platform resolution
actions = bxl_actions.actions

promise1 = actions.anon_target(my_anon_rule1, my_attrs1).promise
promise2 = actions.anon_target(my_anon_rule2, my_attrs2)

joined = promise1.join(promise2)

resolved = ctx.resolve(actions, joined)

# do some more stuff ...

Complete Example

## anon_bxl_rules.bzl ############

# Define an anonymous rule.

MirrorInfo = provider(fields = ["mirrored_attrs"])

# Anonymous rule which writes some silly output, and also mirrors all attributes received
def _mirror_impl(ctx: "context") -> ["provider"]:
out = ctx.actions.declare_output("my_output")
ctx.actions.write(out, "my_content")
return [DefaultInfo(default_outputs = [out]), MirrorInfo(mirrored_attrs = ctx.attrs)]

my_mirror_rule = rule(impl = _mirror_impl, attrs = {
"false": attrs.bool(),
"list_string": attrs.list(attrs.string()),
"string": attrs.string(),
"true": attrs.bool(),

# Will be used in a map function in my_script.bxl below
StringInfo = provider(fields = ["my_string"])

## my_script.bxl ############

load(":anon_bxl_rules.bzl", "MirrorInfo", "StringInfo", "my_mirror_rule")

def _anon_target_example(ctx):
bxl_actions = ctx.bxl_actions()
actions = bxl_actions.actions

# Attrs to pass into the anonymous target. An anonymous target is defined by the hash of its attributes
my_attrs = {
"false": False,
"int": 42,
"list_string": ["a", "b", "c"],
"string": "foo-bar-string",
"true": True,

# A function to be applied to the promise (result of anon target), producing a promise with the resulting value.
def my_function(providers):
# Do something with the attrs. In this example, we are validating that the attrs are what we expect.
mirrored_fields = providers[MirrorInfo].mirrored_attrs
assert_eq(mirrored_fields.true, True)
assert_eq(mirrored_fields.false, False)
assert_eq(, 42)
assert_eq(mirrored_fields.string, "foo-bar-string")
assert_eq(mirrored_fields.list_string, ["a", "b", "c"])

outputs = providers[DefaultInfo].default_outputs
# These are the providers this target returns
return [DefaultInfo(default_outputs = outputs), StringInfo(my_string = "map function succeeded!")]

# Create an anonymous target by passing in "my_attrs" into "my_mirror_rule", and returns providers.
# Specifically, it returns "DefaultInfo" and "MirrorInfo", as defined in "my_mirror_rule"
# Then, we map the result to "my_function", which does some validation
promise = actions.anon_target(my_mirror_rule, my_attrs)

# Resolving the promise returns a "provider_collection", which was defined by "my_function" above.
# `DefaultInfo` is at index 0, `StringInfo` is at index 1
promise_result = ctx.resolve(actions, promise)

ensured = ctx.output.ensure(promise_result[0].default_outputs[0])
# should print out location of the output, which contains the "my_content" string as defined in anon_bxl_rules.bzl above

# should print out "map function succeeded!"

def assert_eq(a, b):
if a != b:
fail("Expected {} == {}".format(a, b))

anon_target_example = bxl_main(
impl = _anon_target_example,
cli_args = {