bzlmod: Add dedicated documentation

Co-authored-by: Steven E. Harris <seh@panix.com>

README.rst

@@ -7,6 +7,8 @@ Go rules for Bazel_
 .. _Bazel labels: https://docs.bazel.build/versions/master/build-ref.html#labels
 .. _Bazel: https://bazel.build/
 .. _Build modes: go/modes.rst
+.. _Bzlmod: https://bazel.build/external/overview#bzlmod
+.. _Go with Bzlmod: docs/go/core/bzlmod.md
 .. _Core rules: docs/go/core/rules.md
 .. _Coverage: https://bazel.google.cn/docs/coverage
 .. _Dependencies: go/dependencies.rst
@@ -226,6 +228,9 @@ Several additional tools need to be installed and configured.
 Initial project setup
 ~~~~~~~~~~~~~~~~~~~~~
 
+If you are using Bazel's new external dependency management system `Bzlmod`_,
+refer to the dedicated `Go with Bzlmod`_ guide instead.
+
 Create a file at the top of your repository named ``WORKSPACE``, and add the
 snippet below (or add to your existing ``WORKSPACE``). This tells Bazel to
 fetch rules_go and its dependencies. Bazel will download a recent supported

docs/go/core/bzlmod.md

@@ -0,0 +1,180 @@
+# Go with Bzlmod
+
+This 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.
+Usages 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.
+
+## Setup
+
+Add the following lines to your `MODULE.bazel` file:
+
+```starlark
+bazel_dep(name = "rules_go", version = "0.39.1")
+bazel_dep(name = "gazelle", version = "0.29.0")
+```
+
+The latest versions are always listed on https://registry.bazel.build/.
+
+If you have WORKSPACE dependencies that reference rules_go and/or Gazelle, you can still use the legacy repository names for the two repositories:
+
+```starlark
+bazel_dep(name = "rules_go", version = "0.39.1", repo_name = "io_bazel_rules_go")
+bazel_dep(name = "gazelle", version = "0.29.0", repo_name = "bazel_gazelle")
+```
+
+## Registering Go SDKs
+
+rules_go automatically downloads and registers a recent Go SDK, so unless a particular version is required, no manual steps are required.
+
+To register a particular version of the Go SDK, use the `go_sdk` module extension:
+
+```starlark
+go_sdk = use_extension("@rules_go//go:extensions.bzl", "go_sdk")
+
+# Download an SDK for the host OS & architecture.
+go_sdk.download(version = "1.20.3")
+
+# Alternately, download an SDK for a fixed OS/architecture, e.g. for remote execution.
+go_sdk.download(
+    version = "1.20.3",
+    goarch = "amd64",
+    goos = "linux",
+)
+
+# Register the Go SDK installed on the host.
+go_sdk.host()
+```
+
+You 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).
+The 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.
+
+### Using a Go SDK
+
+By default, Go SDK repositories are created with mangled names and are not expected to be referenced directly.
+
+For build actions, toolchain resolution is used to select the appropriate SDK for a given target.
+[`go_cross_binary`](rules.md#go_cross_binary) can be used to influence the outcome of the resolution.
+
+The `go` tool of the SDK registered for the host is available via the `@rules_go//go` target.
+Prefer running it via this target over running `go` directly to ensure that all developers use the same version.
+The `@rules_go//go` target can be used in scripts executed via `bazel run`, but cannot be used in build actions.
+Note that `go` command arguments starting with `-` require the use of the double dash separator with `bazel run`:
+
+```sh
+bazel run @rules_go//go -- mod tidy -v
+```
+
+If 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`.
+Note that modules using this attribute cannot be added to registries such as the Bazel Central Registry (BCR).
+If you have a use case that would require this, please explain it in an issue.
+
+### Not yet supported
+
+* `go_local_sdk`
+* `go_wrap_sdk`
+* nogo ([#3529](https://github.com/bazelbuild/rules_go/issues/3529))
+
+## Generating BUILD files
+
+Add the following to your top-level BUILD file, including your Go module's path in a [Gazelle directive](https://github.com/bazelbuild/bazel-gazelle#directives):
+
+```starlark
+load("@gazelle//:defs.bzl", "gazelle")
+
+# gazelle:prefix github.com/example/project
+gazelle(name = "gazelle")
+```
+
+Then, use `bazel run //:gazelle` to (re-)generate BUILD files.
+
+## External dependencies
+
+External Go dependencies are managed by the `go_deps` module extension provided by Gazelle.
+`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.
+For 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.
+
+### Specifying external dependencies
+
+Even 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`.
+The `go_deps` extension parses this file directly, so external tooling such as `gazelle update-repos` is no longer needed.
+
+Register the `go.mod` file with the `go_deps` extension as follows:
+
+```starlark
+go_deps = use_extension("@gazelle//:extensions.bzl", "go_deps")
+go_deps.from_file(go_mod = "//:go.mod")
+
+# All *direct* Go dependencies of the module have to be listed explicitly.
+use_repo(
+    go_deps,
+    "com_github_gogo_protobuf",
+    "com_github_golang_mock",
+    "com_github_golang_protobuf",
+    "org_golang_x_net",
+)
+```
+
+There is [ongoing work](https://github.com/bazelbuild/bazel-gazelle/pull/1511) targeted for Bazel 6.2.0 to automate the generation of the `use_repo` statement.
+
+Alternatively, you can specify a module extension tag to add an individual dependency.
+This can be useful for dependencies of generated code that `go mod tidy` would remove. (There is [ongoing work](https://github.com/bazelbuild/bazel-gazelle/pull/1495) to provide a Bazel-aware version of `tidy`.)
+
+```starlark
+go_deps.module(
+    path = "google.golang.org/grpc",
+    sum = "h1:fPVVDxY9w++VjTZsYvXWqEf9Rqar/e+9zYfxKK+W+YU=",
+    version = "v1.50.0",
+)
+```
+
+### Managing `go.mod`
+
+An initial `go.mod` file can be created via
+
+```sh
+bazel run @rules_go//go mod init github.com/example/project
+```
+
+A dependency can be added via
+
+```sh
+bazel run @rules_go//go get golang.org/x/text@v0.3.2
+```
+
+### Overrides
+
+The root module can override certain aspects of the dependency resolution performed by the `go_deps` extension.
+
+#### `replace`
+
+[`replace` directives](https://go.dev/ref/mod#go-mod-file-replace) in `go.mod` can be used to replace particular versions of dependencies with other versions or entirely different modules.
+At the moment the only supported form is:
+
+```
+replace(
+    golang.org/x/net v1.2.3 => example.com/fork/net v1.4.5
+)
+```
+
+#### Gazelle directives
+
+Some external Go modules may require tweaking how Gazelle generates BUILD files for them via [Gazelle directives](https://github.com/bazelbuild/bazel-gazelle#directives).
+The `go_deps` extension provides a dedicated `go_deps.gazelle_override` tag for this purpose:
+
+```starlark
+go_deps.gazelle_override(
+    directives = [
+        "gazelle:go_naming_convention go_default_library",
+    ],
+    path = "github.com/stretchr/testify",
+)
+```
+
+If 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 Gazelle directives](https://github.com/bazelbuild/bazel-gazelle/blob/master/internal/bzlmod/directives.bzl) via a PR.
+This will allow you to drop the `gazelle_override` tag and also makes the Go module usable in non-root Bazel modules.
+
+### Not yet supported
+
+* Fetching dependencies from Git repositories or via HTTP
+* `go.mod` `replace` directives matching all versions of a module
+* `go.mod` `replace` directives referencing local files
+* `go.mod` `exclude` directices