Visibility

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This page covers Bazel's two visibility systems: target visibility and load visibility.

Both types of visibility help other developers distinguish between your library's public API and its implementation details, and help enforce structure as your workspace grows. You can also use visibility when deprecating a public API to allow current users while denying new ones.

Target visibility

Target visibility controls who may depend on your target — that is, who may use your target's label inside an attribute such as deps.

A target A is visible to a target B if they are in the same package, or if A grants visibility to B's package. Thus, packages are the unit of granularity for deciding whether or not to allow access. If B depends on A but A is not visible to B, then any attempt to build B fails during analysis.

Note that granting visibility to a package does not by itself grant visibility to its subpackages. For more details on package and subpackages, see Concepts and terminology.

For prototyping, you can disable target visibility enforcement by setting the flag --check_visibility=false. This should not be done for production usage in submitted code.

The primary way to control visibility is with the visibility attribute on rule targets. This section describes the format of this attribute, and how to determine a target's visibility.

Visibility specifications

All rule targets have a visibility attribute that takes a list of labels. Each label has one of the following forms. With the exception of the last form, these are just syntactic placeholders that do not correspond to any actual target.

  • "//visibility:public": Grants access to all packages. (May not be combined with any other specification.)

  • "//visibility:private": Does not grant any additional access; only targets in this package can use this target. (May not be combined with any other specification.)

  • "//foo/bar:__pkg__": Grants access to //foo/bar (but not its subpackages).

  • "//foo/bar:__subpackages__": Grants access //foo/bar and all of its direct and indirect subpackages.

  • "//some_pkg:my_package_group": Grants access to all of the packages that are part of the given package_group.

    • Package groups use a different syntax for specifying packages. Within a package group, the forms "//foo/bar:__pkg__" and "//foo/bar:__subpackages__" are respectively replaced by "//foo/bar" and "//foo/bar/...". Likewise, "//visibility:public" and "//visibility:private" are just "public" and "private".

For example, if //some/package:mytarget has its visibility set to [":__subpackages__", "//tests:__pkg__"], then it could be used by any target that is part of the //some/package/... source tree, as well as targets defined in //tests/BUILD, but not by targets defined in //tests/integration/BUILD.

Best practice: To make several targets visible to the same set of packages, use a package_group instead of repeating the list in each target's visibility attribute. This increases readability and prevents the lists from getting out of sync.

Rule target visibility

A rule target's visibility is:

  1. The value of its visibility attribute, if set; or else

  2. The value of the default_visibility argument of the package statement in the target's BUILD file, if such a declaration exists; or else

  3. //visibility:private.

Best practice: Avoid setting default_visibility to public. It may be convenient for prototyping or in small codebases, but the risk of inadvertently creating public targets increases as the codebase grows. It's better to be explicit about which targets are part of a package's public interface.

Example

File //frobber/bin/BUILD:

# This target is visible to everyone
cc_binary(
    name = "executable",
    visibility = ["//visibility:public"],
    deps = [":library"],
)

# This target is visible only to targets declared in the same package
cc_library(
    name = "library",
    # No visibility -- defaults to private since no
    # package(default_visibility = ...) was used.
)

# This target is visible to targets in package //object and //noun
cc_library(
    name = "subject",
    visibility = [
        "//noun:__pkg__",
        "//object:__pkg__",
    ],
)

# See package group "//frobber:friends" (below) for who can
# access this target.
cc_library(
    name = "thingy",
    visibility = ["//frobber:friends"],
)

File //frobber/BUILD:

# This is the package group declaration to which target
# //frobber/bin:thingy refers.
#
# Our friends are packages //frobber, //fribber and any
# subpackage of //fribber.
package_group(
    name = "friends",
    packages = [
        "//fribber/...",
        "//frobber",
    ],
)

Generated file target visibility

A generated file target has the same visibility as the rule target that generates it.

Source file target visibility

You can explicitly set the visibility of a source file target by calling exports_files. When no visibility argument is passed to exports_files, it makes the visibility public. exports_files may not be used to override the visibility of a generated file.

For source file targets that do not appear in a call to exports_files, the visibility depends on the value of the flag --incompatible_no_implicit_file_export:

  • If the flag is set, the visibility is private.

  • Else, the legacy behavior applies: The visibility is the same as the BUILD file's default_visibility, or private if a default visibility is not specified.

Avoid relying on the legacy behavior. Always write an exports_files declaration whenever a source file target needs non-private visibility.

Best practice: When possible, prefer to expose a rule target rather than a source file. For example, instead of calling exports_files on a .java file, wrap the file in a non-private java_library target. Generally, rule targets should only directly reference source files that live in the same package.

Example

File //frobber/data/BUILD:

exports_files(["readme.txt"])

File //frobber/bin/BUILD:

cc_binary(
  name = "my-program",
  data = ["//frobber/data:readme.txt"],
)

Config setting visibility

Historically, Bazel has not enforced visibility for config_setting targets that are referenced in the keys of a select(). There are two flags to remove this legacy behavior:

  • --incompatible_enforce_config_setting_visibility enables visibility checking for these targets. To assist with migration, it also causes any config_setting that does not specify a visibility to be considered public (regardless of package-level default_visibility).

  • --incompatible_config_setting_private_default_visibility causes config_settings that do not specify a visibility to respect the package's default_visibility and to fallback on private visibility, just like any other rule target. It is a no-op if --incompatible_enforce_config_setting_visibility is not set.

Avoid relying on the legacy behavior. Any config_setting that is intended to be used outside the current package should have an explicit visibility, if the package does not already specify a suitable default_visibility.

Package group target visibility

package_group targets do not have a visibility attribute. They are always publicly visible.

Visibility of implicit dependencies

Some rules have implicit dependencies — dependencies that are not spelled out in a BUILD file but are inherent to every instance of that rule. For example, a cc_library rule might create an implicit dependency from each of its rule targets to an executable target representing a C++ compiler.

Currently, for visibility purposes these implicit dependencies are treated like any other dependency. This means that the target being depended on (such as our C++ compiler) must be visible to every instance of the rule. In practice this usually means the target must have public visibility.

You can change this behavior by setting --incompatible_visibility_private_attributes_at_definition. When enabled, the target in question need only be visible to the rule declaring it an implicit dependency. That is, it must be visible to the package containing the .bzl file in which the rule is defined. In our example, the C++ compiler could be private so long as it lives in the same package as the definition of the cc_library rule.

Load visibility

Load visibility controls whether a .bzl file may be loaded from other BUILD or .bzl files.

In the same way that target visibility protects source code that is encapsulated by targets, load visibility protects build logic that is encapsulated by .bzl files. For instance, a BUILD file author might wish to factor some repetitive target definitions into a macro in a .bzl file. Without the protection of load visibility, they might find their macro reused by other collaborators in the same workspace, so that modifying the macro breaks other teams' builds.

Note that a .bzl file may or may not have a corresponding source file target. If it does, there is no guarantee that the load visibility and the target visibility coincide. That is, the same BUILD file might be able to load the .bzl file but not list it in the srcs of a filegroup, or vice versa. This can sometimes cause problems for rules that wish to consume .bzl files as source code, such as for documentation generation or testing.

For prototyping, you may disable load visibility enforcement by setting --check_bzl_visibility=false. As with --check_visibility=false, this should not be done for submitted code.

Load visibility is available as of Bazel 6.0.

Declaring load visibility

To set the load visibility of a .bzl file, call the visibility() function from within the file. The argument to visibility() is a list of package specifications, just like the packages attribute of package_group. However, visibility() does not accept negative package specifications.

The call to visibility() must only occur once per file, at the top level (not inside a function), and ideally immediately following the load() statements.

Unlike target visibility, the default load visibility is always public. Files that do not call visibility() are always loadable from anywhere in the workspace. It is a good idea to add visibility("private") to the top of any new .bzl file that is not specifically intended for use outside the package.

Example

# //mylib/internal_defs.bzl

# Available to subpackages and to mylib's tests.
visibility(["//mylib/...", "//tests/mylib/..."])

def helper(...):
    ...
# //mylib/rules.bzl

load(":internal_defs.bzl", "helper")
# Set visibility explicitly, even though public is the default.
# Note the [] can be omitted when there's only one entry.
visibility("public")

myrule = rule(
    ...
)
# //someclient/BUILD

load("//mylib:rules.bzl", "myrule")          # ok
load("//mylib:internal_defs.bzl", "helper")  # error

...

Load visibility practices

This section describes tips for managing load visibility declarations.

Factoring visibilities

When multiple .bzl files should have the same visibility, it can be helpful to factor their package specifications into a common list. For example:

# //mylib/internal_defs.bzl

visibility("private")

clients = [
    "//foo",
    "//bar/baz/...",
    ...
]
# //mylib/feature_A.bzl

load(":internal_defs.bzl", "clients")
visibility(clients)

...
# //mylib/feature_B.bzl

load(":internal_defs.bzl", "clients")
visibility(clients)

...

This helps prevent accidental skew between the various .bzl files' visibilities. It also is more readable when the clients list is large.

Composing visibilities

Sometimes a .bzl file might need to be visible to an allowlist that is composed of multiple smaller allowlists. This is analogous to how a package_group can incorporate other package_groups via its includes attribute.

Suppose you are deprecating a widely used macro. You want it to be visible only to existing users and to the packages owned by your own team. You might write:

# //mylib/macros.bzl

load(":internal_defs.bzl", "our_packages")
load("//some_big_client:defs.bzl", "their_remaining_uses)

# List concatenation. Duplicates are fine.
visibility(our_packages + their_remaining_uses)

Deduplicating with package groups

Unlike target visibility, you cannot define a load visibility in terms of a package_group. If you want to reuse the same allowlist for both target visibility and load visibility, it's best to move the list of package specifications into a .bzl file, where both kinds of declarations may refer to it. Building off the example in Factoring visibilities above, you might write:

# //mylib/BUILD

load(":internal_defs", "clients")

package_group(
    name = "my_pkg_grp",
    packages = clients,
)

This only works if the list does not contain any negative package specifications.

Protecting individual symbols

Any Starlark symbol whose name begins with an underscore cannot be loaded from another file. This makes it easy to create private symbols, but does not allow you to share these symbols with a limited set of trusted files. On the other hand, load visibility gives you control over what other packages may see your .bzl file, but does not allow you to prevent any non-underscored symbol from being loaded.

Luckily, you can combine these two features to get fine-grained control.

# //mylib/internal_defs.bzl

# Can't be public, because internal_helper shouldn't be exposed to the world.
visibility("private")

# Can't be underscore-prefixed, because this is
# needed by other .bzl files in mylib.
def internal_helper(...):
    ...

def public_util(...):
    ...
# //mylib/defs.bzl

load(":internal_defs", "internal_helper", _public_util="public_util")
visibility("public")

# internal_helper, as a loaded symbol, is available for use in this file but
# can't be imported by clients who load this file.
...

# Re-export public_util from this file by assigning it to a global variable.
# We needed to import it under a different name ("_public_util") in order for
# this assignment to be legal.
public_util = _public_util

bzl-visibility Buildifier lint

There is a Buildifier lint that provides a warning if users load a file from a directory named internal or private, when the user's file is not itself underneath the parent of that directory. This lint predates the load visibility feature and is unnecessary in workspaces where .bzl files declare visibilities.