Only install runtime/cgo with pure = False (bazelbuild#3492)

We don't know why installing `runtime/cgo` even if cgo is disabled
doesn't seem to break anything, so we shouldn't do it.

go/private/actions/stdlib.bzl

@@ -82,6 +82,9 @@ def _build_stdlib(go):
     if go.mode.race:
         args.add("-race")
     args.add_all(go.sdk.experiments, before_each = "-experiment")
+    args.add("-package", "std")
+    if not go.mode.pure:
+        args.add("-package", "runtime/cgo")
     args.add_all(link_mode_args(go.mode))
     env = go.env
     if go.mode.pure:

go/tools/builders/stdlib.go

@@ -33,6 +33,8 @@ func stdlib(args []string) error {
 	race := flags.Bool("race", false, "Build in race mode")
 	shared := flags.Bool("shared", false, "Build in shared mode")
 	dynlink := flags.Bool("dynlink", false, "Build in dynlink mode")
+	var packages multiFlag
+	flags.Var(&packages, "package", "Packages to build")
 	var experiments multiFlag
 	flags.Var(&experiments, "experiment", "Go experiments to enable via GOEXPERIMENT")
 	if err := flags.Parse(args); err != nil {
@@ -164,8 +166,7 @@ You may need to use the flags --cpu=x64_windows --compiler=mingw-gcc.`)
 		return fmt.Errorf("error modifying cgo environment to absolute path: %v", err)
 	}
 
-	// TODO(#1885): don't install runtime/cgo in pure mode.
-	installArgs = append(installArgs, "std", "runtime/cgo")
+	installArgs = append(installArgs, packages...)
 	if err := goenv.runCommand(installArgs); err != nil {
 		return err
 	}

tests/core/stdlib/BUILD.bazel

@@ -5,7 +5,7 @@ go_test(
     name = "buildid_test",
     srcs = ["buildid_test.go"],
     data = [":stdlib_files"],
-    rundir = ".",
+    deps = ["//go/runfiles"],
 )
 
 stdlib_files(name = "stdlib_files")

tests/core/stdlib/buildid_test.go

@@ -1,3 +1,4 @@
+//go:build go1.10
 // +build go1.10
 
 /* Copyright 2018 The Bazel Authors. All rights reserved.
@@ -24,6 +25,8 @@ import (
 	"os/exec"
 	"path/filepath"
 	"testing"
+
+	"github.com/bazelbuild/rules_go/go/runfiles"
 )
 
 func TestEmptyBuildID(t *testing.T) {
@@ -39,20 +42,17 @@ func TestEmptyBuildID(t *testing.T) {
 		"aes.a": "",
 		"cgo.a": "",
 	}
+	stdlibPkgDir, err := runfiles.Rlocation("io_bazel_rules_go/stdlib_/pkg")
+	if err != nil {
+		t.Fatal(err)
+	}
 	n := len(pkgPaths)
 	done := errors.New("done")
 	var visit filepath.WalkFunc
 	visit = func(path string, info os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
-		if (info.Mode() & os.ModeType) == os.ModeSymlink {
-			path, err = filepath.EvalSymlinks(path)
-			if err != nil {
-				return err
-			}
-			return filepath.Walk(path, visit)
-		}
 		if filepath.Base(path) == "buildid" && (info.Mode()&0111) != 0 {
 			buildidPath = path
 		}
@@ -67,9 +67,7 @@ func TestEmptyBuildID(t *testing.T) {
 		}
 		return nil
 	}
-	if err := filepath.Walk(".", visit); err == nil {
-		t.Fatal("could not locate stdlib ROOT file")
-	} else if err != done {
+	if err = filepath.Walk(stdlibPkgDir, visit); err != nil && err != done {
 		t.Fatal(err)
 	}
 	if buildidPath == "" {

tests/core/stdlib/stdlib_files.bzl

@@ -14,18 +14,18 @@
 
 load("//go/private:providers.bzl", "GoStdLib")
 
-def _pure_transition_impl(settings, attr):
-    return {"//go/config:pure": True}
+def _force_rebuild_transition_impl(settings, attr):
+    return {"//go/config:race": True}
 
-pure_transition = transition(
-    implementation = _pure_transition_impl,
-    inputs = ["//go/config:pure"],
-    outputs = ["//go/config:pure"],
+force_rebuild_transition = transition(
+    implementation = _force_rebuild_transition_impl,
+    inputs = ["//go/config:race"],
+    outputs = ["//go/config:race"],
 )
 
 def _stdlib_files_impl(ctx):
-    # When a transition is used, ctx.attr._stdlib is a list of Target instead
-    # of a Target. Possibly a bug?
+    # When an outgoing transition (aka split transition) is used,
+    # ctx.attr._stdlib is a list of Target.
     stdlib = ctx.attr._stdlib[0][GoStdLib]
     libs = stdlib.libs
     runfiles = ctx.runfiles(files = libs)
@@ -40,7 +40,7 @@ stdlib_files = rule(
         "_stdlib": attr.label(
             default = "@io_bazel_rules_go//:stdlib",
             providers = [GoStdLib],
-            cfg = pure_transition,  # force recompilation
+            cfg = force_rebuild_transition,
         ),
         "_allowlist_function_transition": attr.label(
             default = "@bazel_tools//tools/allowlists/function_transition_allowlist",