1# Go with Bzlmod
2
3This document describes how to use rules_go and Gazelle with Bazel's new external dependency subsystem [Bzlmod](https://bazel.build/external/overview#bzlmod), which is meant to replace `WORKSPACE` files eventually.
4Usages of rules_go and Gazelle in `BUILD` files are not affected by this; refer to the existing documentation on rules and configuration options for them.
5
6## Setup
7
8Add the following lines to your `MODULE.bazel` file:
9
10```starlark
11bazel_dep(name = "rules_go", version = "0.39.1")
12bazel_dep(name = "gazelle", version = "0.31.0")
13```
14
15The latest versions are always listed on https://registry.bazel.build/.
16
17If you have WORKSPACE dependencies that reference rules_go and/or Gazelle, you can still use the legacy repository names for the two repositories:
18
19```starlark
20bazel_dep(name = "rules_go", version = "0.39.1", repo_name = "io_bazel_rules_go")
21bazel_dep(name = "gazelle", version = "0.31.0", repo_name = "bazel_gazelle")
22```
23
24## Go SDKs
25
26rules_go automatically downloads and registers a recent Go SDK, so unless a particular version is required, no manual steps are required.
27
28To register a particular version of the Go SDK, use the `go_sdk` module extension:
29
30```starlark
31go_sdk = use_extension("@rules_go//go:extensions.bzl", "go_sdk")
32
33# Download an SDK for the host OS & architecture as well as common remote execution platforms.
34go_sdk.download(version = "1.20.3")
35
36# Alternately, download an SDK for a fixed OS/architecture.
37go_sdk.download(
38 version = "1.20.3",
39 goarch = "amd64",
40 goos = "linux",
41)
42
43# Register the Go SDK installed on the host.
44go_sdk.host()
45```
46
47You can register multiple Go SDKs and select which one to use on a per-target basis using [`go_cross_binary`](rules.md#go_cross_binary).
48As long as you specify the `version` of an SDK, it will be downloaded lazily, that is, only when it is actually needed during a particular build.
49The usual rules of [toolchain resolution](https://bazel.build/extending/toolchains#toolchain-resolution) apply, with SDKs registered in the root module taking precedence over those registered in dependencies.
50
51### Using a Go SDK
52
53By default, Go SDK repositories are created with mangled names and are not expected to be referenced directly.
54
55For build actions, toolchain resolution is used to select the appropriate SDK for a given target.
56[`go_cross_binary`](rules.md#go_cross_binary) can be used to influence the outcome of the resolution.
57
58The `go` tool of the SDK registered for the host is available via the `@rules_go//go` target.
59Prefer running it via this target over running `go` directly to ensure that all developers use the same version.
60The `@rules_go//go` target can be used in scripts executed via `bazel run`, but cannot be used in build actions.
61Note that `go` command arguments starting with `-` require the use of the double dash separator with `bazel run`:
62
63```sh
64bazel run @rules_go//go -- mod tidy -v
65```
66
67If you really do need direct access to a Go SDK, you can provide the `name` attribute on the `go_sdk.download` or `go_sdk.host` tag and then bring the repository with that name into scope via `use_repo`.
68Note that modules using this attribute cannot be added to registries such as the Bazel Central Registry (BCR).
69If you have a use case that would require this, please explain it in an issue.
70
71### Configuring `nogo`
72
73The `nogo` tool is a static analyzer for Go code that is run as part of compilation.
74It is configured via an instance of the [`nogo`](/go/nogo.rst) rule, which can then be registered with the `go_sdk` extension:
75
76```starlark
77go_sdk = use_extension("@rules_go//go:extensions.bzl", "go_sdk")
78go_sdk.nogo(nogo = "//:my_nogo")
79```
80
81By default, the `nogo` tool is executed for all Go targets in the main repository, but not any external repositories.
82Each module can only provide at most one `go_sdk.nogo` tag and only the tag of the root module is honored.
83
84It is also possible to include only or exclude particular packages from `nogo` analysis, using syntax that matches the `visibility` attribute on rules:
85
86```starlark
87go_sdk = use_extension("@rules_go//go:extensions.bzl", "go_sdk")
88go_sdk.nogo(
89 nogo = "//:my_nogo",
90 includes = [
91 "//:__subpackages__",
92 "@my_own_go_dep//logic:__pkg__",
93 ],
94 excludes = [
95 "//third_party:__subpackages__",
96 ],
97)
98```
99
100### Not yet supported
101
102* `go_local_sdk`
103* `go_wrap_sdk`
104
105## Generating BUILD files
106
107Add the following to your top-level BUILD file:
108
109```starlark
110load("@gazelle//:def.bzl", "gazelle")
111
112gazelle(name = "gazelle")
113```
114
115If there is no `go.mod` file in the same directory as your top-level BUILD file, also add the following [Gazelle directive](https://github.com/bazelbuild/bazel-gazelle#directives) to that BUILD file to supply Gazelle with your Go module's path:
116
117```starlark
118# gazelle:prefix github.com/example/project
119```
120
121Then, use `bazel run //:gazelle` to (re-)generate BUILD files.
122
123## External dependencies
124
125External Go dependencies are managed by the `go_deps` module extension provided by Gazelle.
126`go_deps` performs [Minimal Version Selection](https://go.dev/ref/mod#minimal-version-selection) on all transitive Go dependencies of all Bazel modules, so compared to the old WORKSPACE setup, every Bazel module only needs to declare its own Go dependencies.
127For every major version of a Go module, there will only ever be a single version in the entire build, just as in regular Go module builds.
128
129### Specifying external dependencies
130
131Even though this is not a strict requirement, for interoperability with Go tooling that isn't Bazel-aware, it is recommended to manage Go dependencies via `go.mod`.
132The `go_deps` extension parses this file directly, so external tooling such as `gazelle update-repos` is no longer needed.
133
134Register the `go.mod` file with the `go_deps` extension as follows:
135
136```starlark
137go_deps = use_extension("@gazelle//:extensions.bzl", "go_deps")
138go_deps.from_file(go_mod = "//:go.mod")
139
140# All *direct* Go dependencies of the module have to be listed explicitly.
141use_repo(
142 go_deps,
143 "com_github_gogo_protobuf",
144 "com_github_golang_mock",
145 "com_github_golang_protobuf",
146 "org_golang_x_net",
147)
148```
149
150Bazel emits a warning if the `use_repo` statement is out of date or missing entirely (requires Bazel 6.2.0 or higher).
151The warning contains a `buildozer` command to automatically fix the `MODULE.bazel` file (requires buildozer 6.1.1 or higher).
152
153Alternatively, you can specify a module extension tag to add an individual dependency:
154
155```starlark
156go_deps.module(
157 path = "google.golang.org/grpc",
158 sum = "h1:fPVVDxY9w++VjTZsYvXWqEf9Rqar/e+9zYfxKK+W+YU=",
159 version = "v1.50.0",
160)
161```
162
163#### Depending on tools
164
165If you need to depend on Go modules that are only used as tools, you can use the [`tools.go` technique](https://github.com/golang/go/issues/25922#issuecomment-1038394599):
166
1671. In a new subdirectory of your repository, create a `tools.go` file that imports the tools' main packages:
168
169 ```go
170 //go:build tools
171 // +build tools
172
173 package my_tools
174
175 import (
176 _ "github.com/the/tool"
177 _ "golang.org/x/tools/cmd/stringer"
178 )
179 ```
180
1812. Run `bazel run @rules_go//go mod tidy` to populate the `go.mod` file with the dependencies of the tools.
182
183Instead, if you want the tools' dependencies to be resolved independently of the dependencies of your regular code ([experimental](https://github.com/bazelbuild/bazel/issues/20186)):
184
1852. Run `bazel run @rules_go//go mod init` in the directory containing the `tools.go` file to create a new `go.mod` file and then run `bazel run @rules_go//go mod tidy` in that directory.
1863. Add `common --experimental_isolated_extension_usages` to your `.bazelrc` file to enable isolated usages of extensions.
1874. Add an isolated usage of the `go_deps` extension to your module file:
188
189 ```starlark
190 go_tool_deps = use_extension("@gazelle//:extensions.bzl", "go_deps", isolated = True)
191 go_tool_deps.from_file(go_mod = "//tools:go.mod")
192 ```
193
194### Managing `go.mod`
195
196An initial `go.mod` file can be created via
197
198```sh
199bazel run @rules_go//go mod init github.com/example/project
200```
201
202A dependency can be added via
203
204```sh
205bazel run @rules_go//go get golang.org/x/text@v0.3.2
206```
207
208### Overrides
209
210The root module can override certain aspects of the dependency resolution performed by the `go_deps` extension.
211
212#### `replace`
213
214[`replace` directives](https://go.dev/ref/mod#go-mod-file-replace) in `go.mod` can be used to replace particular or all versions of dependencies with other versions or entirely different modules.
215
216```
217replace(
218 golang.org/x/net v1.2.3 => example.com/fork/net v1.4.5
219 golang.org/x/mod => example.com/my/mod v1.4.5
220)
221```
222
223#### Gazelle directives
224
225Some external Go modules may require tweaking how Gazelle generates BUILD files for them via [Gazelle directives](https://github.com/bazelbuild/bazel-gazelle#directives).
226The `go_deps` extension provides a dedicated `go_deps.gazelle_override` tag for this purpose:
227
228```starlark
229go_deps.gazelle_override(
230 directives = [
231 "gazelle:go_naming_convention go_default_library",
232 ],
233 path = "github.com/stretchr/testify",
234)
235```
236
237If you need to use a `gazelle_override` to get a public Go module to build with Bazel, consider contributing the directives to the [public registry for default Gazelle overrides](https://github.com/bazelbuild/bazel-gazelle/blob/master/internal/bzlmod/default_gazelle_overrides.bzl) via a PR.
238This will allow you to drop the `gazelle_override` tag and also makes the Go module usable in non-root Bazel modules.
239
240#### `go_deps.module_override`
241
242A `go_deps.module_override` can be used to apply patches to a Go module:
243
244```starlark
245go_deps.module_override(
246 patch_strip = 1,
247 patches = [
248 "//patches:testify.patch",
249 ],
250 path = "github.com/stretchr/testify",
251)
252```
253
254#### `go_deps.archive_override`
255
256A `go_deps.archive_override` can be used to replace a Go module with an archive fetched from a URL and is very similar to the `archive_override` for Bazel modules:
257
258```starlark
259go_deps.archive_override(
260 urls = [
261 "https://github.com/bazelbuild/buildtools/archive/ae8e3206e815d086269eb208b01f300639a4b194.tar.gz",
262 ],
263 patch_strip = 1,
264 patches = [
265 "//patches:buildtools.patch",
266 ],
267 strip_prefix = "buildtools-ae8e3206e815d086269eb208b01f300639a4b194",
268 path = "github.com/bazelbuild/buildtools",
269 sha256 = "05d7c3d2bd3cc0b02d15672fefa0d6be48c7aebe459c1c99dced7ac5e598508f",
270)
271```
272
273### Not yet supported
274
275* Fetching dependencies from Git repositories
276* `go.mod` `replace` directives referencing local files
277* `go.mod` `exclude` directices
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