go_sdk extension: create go_host_compatible_sdk repository

[ylecornec]

What type of PR is this?
Feature

What does this PR do? Why is it needed?

As discussed in this issue, this PR makes the go_sdk module extension define a new repository called go_host_compatible_sdk.

This repository exposes the label of the first host compatible sdk declared by a module via the download or host tag (according to bazel's iteration order over modules).

The gazelle module could then rely on this toolchain for bootstrapping instead of always using go_default_sdk.

Which issues(s) does this PR fix?

This PR is a first step toward closing bazelbuild/bazel-gazelle#1469

Other notes for review

• As discussed in the issue, this PR does not yet provide a way do use toolchains declared outside of rules_go. This is left for another PR.

[ylecornec]

Thanks, I applied your suggestions.

Indeed, since we only need the first element of the host_compatible_toolchains list for now, I replaced this list by a first_host_compatible_toolchain variable.

[fmeum]

@tyler-french Could you take a look as well?

[ylecornec]

As a heads-up, I will be out of office for 3 months starting from July. So I will not be able to help with this (and #3546) during this period.

[fmeum]

Sorry for the delay, this looks good and can be merged after these remaining changes.

@tyler-french is working on porting go_wrap_sdk to Bzlmod and once that's done, we probably have an easier way to realize your follow-up PR.

[ylecornec]

Alright, I applied the suggestions.

[fmeum]

Tested in Gazelle with this diff:

diff --git a/MODULE.bazel b/MODULE.bazel
index f8f3b26..5aa210d 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -14,7 +14,7 @@ go_sdk = use_extension("@io_bazel_rules_go//go:extensions.bzl", "go_sdk")
 # Known to exist since it is instantiated by rules_go itself.
 use_repo(
     go_sdk,
-    go_sdk = "go_default_sdk",
+    "go_host_compatible_sdk_label",
 )
 
 non_module_deps = use_extension("//internal/bzlmod:non_module_deps.bzl", "non_module_deps")
diff --git a/internal/bzlmod/non_module_deps.bzl b/internal/bzlmod/non_module_deps.bzl
index 8885452..b49e466 100644
--- a/internal/bzlmod/non_module_deps.bzl
+++ b/internal/bzlmod/non_module_deps.bzl
@@ -6,14 +6,18 @@ load(
     "//internal:go_repository_tools.bzl",
     "go_repository_tools",
 )
+load(
+    "@go_host_compatible_sdk_label//:defs.bzl",
+    "host_compatible_sdk",
+)
 
 visibility("//")
 
 def _non_module_deps_impl(_):
     go_repository_cache(
         name = "bazel_gazelle_go_repository_cache",
-        # Always provided by rules_go.
-        go_sdk_name = "go_sdk",
+        # Label.workspace_name is always a canonical name, so use a canonical label.
+        go_sdk_name = "@" + host_compatible_sdk.workspace_name,
         go_env = {},
     )
     go_repository_tools(
diff --git a/tests/bcr/MODULE.bazel b/tests/bcr/MODULE.bazel
index b4e3ff9..b61cb26 100644
--- a/tests/bcr/MODULE.bazel
+++ b/tests/bcr/MODULE.bazel
@@ -9,7 +9,10 @@ local_path_override(
 )
 
 bazel_dep(name = "rules_go", version = "0.39.1")
-
+local_path_override(
+    module_name = "rules_go",
+    path = "../../../rules_go",
+)
 go_deps = use_extension("@gazelle//:extensions.bzl", "go_deps")
 
 # Validate a go.mod replace directive works.

[tyler-french]

Tested at Uber