Gazelle build file generator

Gazelle is a build file generator for Go projects. It can create new BUILD.bazel files for a project that follows "go build" conventions, and it can update existing build files to include new files and options. Gazelle can be invoked directly in a project workspace, or it can be run on an external repository during the build as part of the go_repository rule.

Gazelle is under active development. Its interface and the rules it generates may change. Gazelle is not an official Google product.

Contents

Setup

Running Gazelle with Bazel

To use Gazelle in a new project, add the code below to the WORKSPACE file in the root directory of your repository. This should come after io_bazel_rules_go and its dependencies are loaded.

http_archive(
    name = "bazel_gazelle",
    url = "https://github.com/bazelbuild/bazel-gazelle/releases/download/0.8/bazel-gazelle-0.8.tar.gz",
    sha256 = "e3dadf036c769d1f40603b86ae1f0f90d11837116022d9b06e4cd88cae786676",
)
load("@bazel_gazelle//:deps.bzl", "gazelle_dependencies")
gazelle_dependencies()

Add the code below to the BUILD or BUILD.bazel file in the root directory of your repository. Replace the string in prefix with the portion of your import path that corresponds to your repository.

load("@bazel_gazelle//:def.bzl", "gazelle")

gazelle(
    name = "gazelle",
    prefix = "github.com/example/project",
)

After adding this code, you can run Gazelle with Bazel.

bazel run //:gazelle

This will generate new BUILD.bazel files for your project. You can run the same command in the future to update existing BUILD.bazel files to include new source files or options.

Running Gazelle with Go

If you have a Go SDK installed, you can install Gazelle in your GOPATH with the command below:

go get -u github.com/bazelbuild/bazel-gazelle/cmd/gazelle

Make sure to re-run this command to upgrade Gazelle whenever you upgrade rules_go in your repository.

To generate BUILD.bazel files in a new project, run the command below, replacing the prefix with the portion of your import path that corresponds to your repository.

gazelle -go_prefix github.com/my/project

The prefix only needs to be specified the first time you run Gazelle. To update existing BUILD.bazel files, you can just run gazelle without arguments.

Compatibility

Gazelle generates build files that require a minimum version of rules_go to build. Check the table below to ensure that you're using compatible versions.

Gazelle version	Minimum rules_go version	Maximum rules_go version
0.8.0	0.8.0	n/a

Usage

Command line

gazelle <command> [flags...] [package-dirs...]

The first argument to Gazelle may be one of the commands below. If no command is specified, update is assumed. The remaining arguments are specific to each command and are documented below.

update

Scans sources files, then generates and updates build files.

fix

Same as the update command, but it also fixes deprecated usage of rules.

update-repos

Updates repository rules in the WORKSPACE file.

Bazel rule

Gazelle may be run via a rule. See Running Gazelle with Bazel for setup instructions. This rule builds Gazelle and generates a wrapper script that executes Gazelle with baked-in set of arguments. You can run this script with bazel run, or you can copy it into your workspace and run it directly.

The following attributes are available on the gazelle rule.

Name	Type	Default value
gazelle	label	@bazel_gazelle//cmd/gazelle
The go_binary rule version of Gazelle wit	that builds Gazelle. h this.	You can substitute a modified
external	string	external
The method for resolvi external or :	ng unknown imports to value:vendored.	Bazel dependencies. May be
build_tags	string_list	[]
The last of Go build t	ags that Gazelle shoul	d consider to always be true.
prefix	string	mandatory value
The import path that c TODO(#26): this should	orresponds to the repo be optional.	sitory root directory.
extra_args	string_list	[]
A list of extra comman	d line arguments passe	d to Gazelle.
command	string	update
The Gazelle command to a different command, e invoke the generated w	use. May be :value:`f .g., :value:`update-re rapper script directly	ix` or update. To run pos`, you'll need to copy the with explicit arguments.

fix and update

The update command is the most common way of running Gazelle. Gazelle will scan sources in directories throughout the repository, then create and update build files.

The fix command does everything update does, but it also fixes deprecated usage of rules, analogous to go fix. For example, cgo_library will be consolidated with go_library. This command may delete or rename rules, so it's not on by default. See Fix command transformations for details.

Both commands accept a list of directories to process as positional arguments. If no directories are specified, Gazelle will process the current directory. Subdirectories will be processed recursively.

The following flags are accepted:

Name	Default value
-build_file_name file1,file2,...	BUILD.bazel,BUILD
Comma-separated list of file names. Gazell build files. New files will use the first your project contains non-Bazel files name case-insensitive file systems).	e recognizes these files as Bazel name in this list. Use this if d BUILD (or build on
-build_tags tag1,tag2
List of Go build tags Gazelle will conside constraints when generating Go rules. It a certain platforms (for example, amd64,li tags are true (for example,go1.8). It (for example,ignore``). This flag overr	r to be true. Gazelle applies ssumes certain tags are true on nux``). It assumes all Go release considers other tags to be false ides that behavior.
-external external|vendored	external
Determines how Gazelle resolves import pat vendored. Gazelle translates Go i resolving library dependencies. Import pat go_prefix are resolved to local labels are resolved based on this mode. In :value resolved using an external dependency in t not create or maintain these dependencies paths are resolved to a library in the ven	hs. May be external or mport paths to Bazel labels when hs that start with the , but other imports :external mode, paths are he WORKSPACE file (Gazelle does yet). In vendored mode, dor directory.
-go_prefix example.com/repo
A prefix of import paths for libraries in the repository root. Gazelle infers this f root BUILD.bazel file, if it exists. If no This prefix is used to determine whether a in the current repository or an external d	the repository that corresponds to rom the go_prefix rule in the t, this option is mandatory. n import path refers to a library ependency.
-known_import example.com
Skips import path resolution for a known d When Gazelle resolves an import path to an to discover the remote repository root ove discovery step for a few well-known domain golang.org and github.com. This flag speci which is useful in situations where the lo	omain. May be repeated. external dependency, it attempts r HTTP. Gazelle skips this s with predictable structure, like fies additional domains to skip, okup would fail for some reason.
-mode fix|print|diff	fix
Method for emitting merged build files. In fix mode, Gazelle writes generated print mode, it prints them to stdout. unified diff.	and merged files to disk. In In diff mode, it prints a
-proto default|legacy|disable	default
Determines how Gazelle should generate rul in Directives below.	es for .proto files. See details
-repo_root dir
The root directory of the repository. Gaze directory containing the WORKSPACE file. Gazelle will not process packages outside	lle normally infers this to be the this directory.

update-repos

The update-repos command updates repository rules in the WORKSPACE file. Currently, this can only be used to import repositories from a vendoring tool's lock file. More functionality will be added in the future.

The following flags are accepted:

Name	Default value
-from_file lock-file
Import repositories from a ven rules. These rules will be add existing rules. The lock file format is inferr Gopkg.lock is supported.	doring tool's lock file as go_repository ed to the bottom of WORKSPACE or merged with ed from the file's base name. Currently, only
-repo_root dir
The root directory of the repo directory containing the WORKS Gazelle will not process packa	sitory. Gazelle normally infers this to be the PACE file. ges outside this directory.

Bazel rule

When Gazelle is run by Bazel, most of the flags above can be encoded in the gazelle rule. For example:

load("@bazel_gazelle//:def.bzl", "gazelle")

gazelle(
    name = "gazelle",
    command = "fix",
    prefix = "github.com/example/project",
    external = "vendored",
    build_tags = [
        "integration",
        "debug",
    ],
    extra_args = [
        "-build_file_name",
        "BUILD,BUILD.bazel",
    ],
)

Directives

Gazelle supports several directives, written as comments in build files.

• # gazelle:ignore: may be written at the top level of any build file. Gazelle will not update files with this comment.
• # gazelle:exclude path: may be written at the top level of any build file. If the path refers to a source file, Gazelle won't include it in any rules. If the path refers to a directory, Gazelle won't recurse into it. The path may refer to something in a subdirectory, for example, a testdata directory somewhere in a vendor tree. This directive may be repeated to exclude multiple paths, one per line.
• # gazelle:proto <mode>: Tells Gazelle how to generate rules for .proto files. Applies to the current directory and subdirectories. Valid values for mode are:
  • default: proto_library, go_proto_library, go_grpc_library, and go_library rules are generated using @io_bazel_rules_go//proto:def.bzl. This is the default mode.
  • legacy: filegroup rules are generated for use by @io_bazel_rules_go//proto:go_proto_library.bzl. go_proto_library rules must be written by hand. Gazelle will run in this mode automatically if go_proto_library.bzl is loaded to avoid disrupting existing projects, but this can be overridden with a directive.
  • disable: .proto files are ignored. Gazelle will run in this mode automatically if go_proto_library is loaded from any other source, but this can be overridden with a directive.
• # keep: may be written before a rule to prevent the rule from being updated or after a source file, dependency, or flag to prevent it from being removed.

Example

Suppose you have a library that includes a generated .go file. Gazelle won't know what imports to resolve, so you may need to add dependencies manually with # keep comments.

load("@io_bazel_rules_go//go:def.bzl", "go_library")
load("@com_github_example_gen//:gen.bzl", "gen_go_file")

gen_go_file(
    name = "magic",
    srcs = ["magic.go.in"],
    outs = ["magic.go"],
)

go_library(
    name = "go_default_library",
    srcs = ["magic.go"],
    visibility = ["//visibility:public"],
    deps = [
        "@com_github_example_gen//:go_default_library",  # keep
    ],
)

Fix command transformations

When Gazelle is invoked with the fix command, in addition to updating source files and dependencies of existing rules, Gazelle will remove deprecated usage of the Go rules, analogous to go fix. The following transformations are performed.

Squash cgo libraries: Gazelle will remove cgo_library rules named cgo_default_library and merge their attributes with a go_library rule in the same package named go_default_library. If no such go_library rule exists, a new one will be created. Other cgo_library rules will not be removed.

cgo_library(
    name = "cgo_default_library",
    srcs = ["cgo.go"],
)

# AFTER
go_library(
    name = "go_default_library",
    srcs = [
        "cgo.go",
        "pure.go",
    ],
    cgo = True,
)

Remove legacy protos: Gazelle will remove usage of go_proto_library rules loaded from @io_bazel_rules_go//proto:go_proto_library.bzl and filegroup rules named go_default_library_protos. Newly generated proto rules will take their place. Since filegroup isn't needed anymore and go_proto_library has different attributes and was always written by hand, Gazelle will not attempt to merge anything from these rules with the newly generated rules.

This transformation is only applied in the default proto mode. Since Gazelle will run in legacy proto mode if go_proto_library.bzl is loaded, this transformation is not usually applied. You can set the proto mode explicitly using the directive # gazelle:proto default.

go_proto_library(
    name = "go_default_library",
    srcs = [":go_default_library_protos"],
)

filegroup(
    name = "go_default_library_protos",
    srcs = ["foo.proto"],
)

# AFTER
# The above rules are deleted. New proto_library, go_proto_library, and
# go_library rules will be generated automatically.