An anonymous target is defined by the hash of its attributes, rather than its name. During analysis, rules can define and access the providers of anonymous targets before producing their own providers. Two distinct rules might ask for the same anonymous target, sharing the work it performs.

This solves two distinct problems:

The sharing problem - if you have two processes that want to share some work, you can create an anon target that does that work once, which is then reused by the two processes. Without such a mechanism, all sharing must be present in the target graph: you can't create any new sharing.
The overlay problem - this is the idea that you want to have a shadow-graph, similar in structure to the normal graph, but with additional information attached. Bazel accomplishes this with Aspects. With Anonymous (anon) targets, you can create a shadow-graph by convention, just by using the target name you wish to shadow as the attribute.

Dynamic dependencies, in their full generality, enable users to do a thing, look at the result, then ask for fresh things. However, this full generality is not provided as it breaks processes, like query, that power the Target Determinator.

In Buck2, dynamic dependencies are implemented using dynamic_output, which provides users with the ability to create new actions, after running actions, then look at the result. dynamic_output is restricted in its power when compared to fully generic dynamic dependencies, as detailed in the Dynamic Dependencies page.

Anon targets enable users to create a new analysis (that is, call an anon target that may not have existed before) after looking at the result of a previous analysis (which is passed in, or after looking at an anon target). In many ways, anon target is the version of dynamic_output at analysis time, rather than action time.

The execution platform for an anon target is that of the inherited from the calling target, which is part of the hash. If that is too restrictive, you could use execution groups, where an anon target gets told which execution group to use.

Creating anon targets

Anon rule

An anonymous rule is defined using rule or anon_rule.

Example:

my_anon_rule = rule(
    impl = _anon_impl,
    attrs = {},
)

# Or:

my_anon_rule = anon_rule(
    impl = _anon_impl,
    attrs = {},
    artifact_promise_mappings = {} # only available for anon_rule
)

For rule, these are normal rules, with the difference that they are not in a configuration, so ctx.actions.label won't show configuration information, but just unspecified.

For anon_rule, the configuration restrictions also apply, and there is an artifact_promise_mappings field which you can specify a dict of artifact promise names to the map function, which would be applied to the anon target's promise during rule resolution.

Anon target

An anonymous rule is used via ctx.actions.anon_target or ctx.actions.anon_targets, passing in the rule and the attributes for the rule.

The return values of those functions are a AnonTarget and AnonTargets type, respectively.

Example:

my_anon_rule1 = anon_rule(
    impl = _anon_impl,
    attrs = {},
    artifact_promise_mappings = {}
)

my_anon_rule2 = anon_rule(
    impl = _anon_impl,
    attrs = {},
    artifact_promise_mappings = {}
)

# <elsewhere>
anon_target = ctx.actions.anon_target(my_anon_rule1, {})

anon_targets = ctx.actions.anon_targets([(my_anon_rule1, {}), (my_anon_rule2, {})])

`AnonTarget` and `AnonTargets`

AnonTarget has a promise attribute, and artifact() and artifacts() functions. AnonTargets has a promise attribute and anon_targets attribute.

The promise attribute for both types returns the anon target's promise (type is promise), which when evaluated returns the providers of the anonymous target. The promise type has a few special behaviors.

It has a map function, which takes a function and applies it to the future, returning a new future
All promises will eventually resolve to a list of providers

For AnonTarget, the artifact() and artifacts() functions only return something if using anon_rule. artifact() takes in an artifact name, which should be found in the artifact_promise_mappings dict, and returns the artifact promise. artifacts() returns the dict of all promise artifact names to the artifact promise itself, as defined in artifact_promise_mappings. See Convert promise to artifact below for more information about artifact promises.

Example:

HelloInfo = provider(fields = ["output"])

my_anon_rule = anon_rule(
    impl = _anon_impl,
    attrs = {},
    artifact_promise_mappings = {
        "hello": lambda x: x[HelloInfo].output,
    }
)

# <elsewhere>
anon_target = ctx.actions.anon_target(my_anon_rule, {})
artifact = anon_target.artifact("hello")
artifact_from_dict = anon_target.artifacts()["hello"]

For AnonTargets, the anon_targets attribute returns a list of the underlying AnonTargets.

Example:

HelloInfo = provider(fields = ["output"])
GoodbyeInfo = provider(fields = ["output"])

my_anon_rule1 = anon_rule(
    impl = _anon_impl,
    attrs = {},
    artifact_promise_mappings = {
        "hello": lambda x: x[HelloInfo].output,
    }
)

my_anon_rule2 = anon_rule(
    impl = _anon_impl,
    attrs = {},
    artifact_promise_mappings = {
        "goodbye": lambda x: x[GoodbyeInfo].output,
    }
)

# <elsewhere>
all_targets = ctx.actions.anon_targets([(my_anon_rule1, {}), (my_anon_rule2, {})])
hello = all_targets.anon_targets[0].artifact("hello")
goodbye = all_targets.anon_targets[1].artifact("goodbye")

Attributes

Anon targets only support a subset of attributes that normal rules support.

Supported attributes:

bool
int
str
enum
dep
- deps attributes do not take strings, but dependencies, already in a configuration
- exec_deps are available if the passed in dep's execution platform matches
- Default attr.deps (as used for toolchains) are not permitted, as the default can't express a dependency. They must be passed forward from the caller. that of the anon target's caller
source
- Accepts bound artifacts or promise artifacts
arg
- Can only be used if anon_target_compatible is True when declaring attrs.arg (ex: attrs.arg(anon_target_compatible = True))
label
list
tuple
dict
one_of
option

You can use these attributes like you would in normal rules:

my_anon_rule = anon_rule(
    impl = _my_anon_impl,
    attrs = {
        "my_int": attrs.int(),
        "my_string_with_default": attrs.string(default = "foo"),
        "my_optional_source": attrs.option(attrs.source()),
        "my_list_of_labels": attrs.list(attrs.label()),
    },
    artifact_promise_mappings = {}
)

def _my_anon_impl(ctx: AnalysisContext) -> list[Provider]:
    my_int = ctx.attrs.my_int
    my_string_with_default = ctx.attrs.my_string_with_default
    my_optional_source = ctx.attrs.my_optional_source
    my_list_of_labels = ctx.attrs.my_list_of_labels

    # do something with the attributes...

    return [DefaultInfo()]

Attribute resolution

Attribute resolution is handled differently from normal code:

Transitions and more complex forms of attributes are banned.
The name attribute is a reserved attribute. It is an implicit attribute when defining a rule for an anon target, but can be optionally set when creating an anon target. If present, it must be a syntactically valid target, but could refer to a cell/package that does not exist. If not present, buck2 will generate a name for the target automatically.

`name` attribute example

# Rule definition for anon target
my_rule = rule(
    impl = _my_impl,
    attrs = {
        # `name` is already implicitly defined as an attribute, and will error
        # out if you try to define it again during rule declaration
    },
)

# Anon target instantiation, elsewhere
 ctx.actions.anon_target(
    my_rule,
    {
        # you can optionally pass `name` into the attributes even though it's
        # not explicitly defined in the `attrs` field for `my_rule`
        "name": "foo//bar:baz"
    },
)

To access the name attribute from an analysis context, you can use ctx.label.name.

Examples

Simple Example

# Define an anonymous rule
UpperInfo = provider(fields = ["message"])

def _impl_upper(ctx):
    return [UpperInfo(message = ctx.attrs.message.upper()]

upper = rule(
    attrs = {"message", attrs.string()},
    impl = _impl_upper
)

# Use an anonymous target
def impl(ctx):
    def k(providers):
        print(providers[UpperInfo].message)
        # These are the providers this target returns
        return [DefaultInfo()]
    return ctx.actions.anon_target(upper, {
        name: "my//:greeting",
        message: "Hello World",
    })
    .promise
    .map(k)

Longer example

The following code represents a scenario for a compile-and-link language where, if two targets end up compiling the same file (for example, they are in the same package and both list it, or it gets export_file'd), then that file is compiled just once:

## BUCK ##############
@load(":silly.bzl", "silly_binary")

silly_binary(
    name = "hello",
    srcs = ["hello.sil", "world.sil"],
)

## silly.bzl ############

_SillyCompilation = provider(fields = ["compiled"])

def _silly_compilation_impl(ctx):
    out = ctx.actions.declare_output("output.o")
    ctx.actions.run(cmd_args(
        ctx.attrs.toolchain.compiler,
        ctx.attrs.src,
        "-o",
        out.as_output(),
    ))
    return [DefaultInfo(), _SillyCompilation(compiled = out)]

_silly_compilation = rule(
    impl = _silly_compilation_impl,
    attrs = {
        "src": attrs.src(),
        "toolchain": attrs.dep(),
    },
)

def _silly_binary_impl(ctx):
    def k(providers):
        # Step 2: now link them all together
        out = ctx.actions.declare_output("out.exe")
        objs = [p[_SillyCompilation].compiled for p in providers]
        ctx.actions.run(cmd_args(
            ctx.attrs._silly_toolchain.linker,
            objs,
            "-o",
            out.as_output(),
        ))
        return [
            DefaultInfo(default_output = out),
            RunInfo(args = out),
        ]

    # Step 1: compile all my individual files
    return ctx.actions.anon_targets(
        [(_silly_compilation, {
            "src": src,
            "toolchain": ctx.attrs._silly_toolchain
        }) for src in ctx.attrs.srcs]
    ).map(k)

silly_binary = rule(
    impl = _silly_binary_impl,
    attrs = {
        "srcs": attr.list(attr.src()),
        "_silly_toolchain": attr.dep(default = "toolchains//:silly"),
    },
)

Convert promise to artifact

It can be challenging to pass around the promises from anon_target and structure functions to support that. If you only need an artifact (or multiple artifacts) from an anon_target, you can use artifact() function on the anon target to convert a promise to an artifact. This artifact can be passed to most things that expect artifacts, but until it is resolved (at the end of the current analysis) it can't be inspected with artifact functions like .extension, etc. .short_path is supported if ctx.actions.assert_short_path() was called, which produces an artifact type. The promise must resolve to a build (not source) artifact with no associated artifacts.

Example:

HelloInfo = provider(fields = ["hello", "world"])

def _anon_impl(ctx: AnalysisContext) -> ["provider"]:
    hello = ctx.actions.write("hello.out", "hello")
    world = ctx.actions.write("world.out", "world")
    return [DefaultInfo(), HelloInfo(hello = hello, world = world)]

_anon = anon_rule(
    impl = _anon_impl,
    attrs = {},
    artifact_promise_mappings = {
        "hello": lambda x: x[HelloInfo].hello,
        "world": lambda x: x[HelloInfo].world,
    }
)

def _use_impl(ctx: AnalysisContext) -> ["provider"]:
    anon = ctx.actions.anon_target(_anon, {})
    hello_artifact = anon.artifact("hello")
    world_artifact = anon.artifact("world")

    out = ctx.actions.declare_output("output")
    ctx.actions.run([
            ctx.attrs.some_tool,
            hello_artifact,
            world_artifact,
            out.as_output()
        ], category = "process")
    return [DefaultInfo(default_output = out)]

use_promise_artifact = rule(impl = _use_impl, attrs = {
    "some_tool": attr.exec_dep(),
})

Anon rule

Anon target

AnonTarget and AnonTargets

Attributes

Attribute resolution

name attribute example

Examples

Simple Example

Longer example

Convert promise to artifact

`AnonTarget` and `AnonTargets`

`name` attribute example