feat(mode): add purego tag when pure

The community has agreed[1] that "pure" builds of Go code should
use the `purego` build tag.

This change adds that de-facto tag.

[1]: golang/go#23172

go/modes.rst

@@ -68,6 +68,8 @@ or using `Bazel configuration transitions`_.
 | Disables cgo, even when a C/C++ toolchain is configured (similar to setting  |
 | ``CGO_ENABLED=0``). Packages that contain cgo code may still be built, but   |
 | the cgo code will be filtered out, and the ``cgo`` build tag will be false.  |
+| Sets the de-facto ``purego`` build tag for conditional inclusion of source   |
+| files.                                                                       |
 +-------------------+---------------------+------------------------------------+
 | :param:`debug`    | :type:`bool`        | :value:`false`                     |
 +-------------------+---------------------+------------------------------------+

go/private/mode.bzl

@@ -122,6 +122,8 @@ def get_mode(ctx, go_toolchain, cgo_context_info, go_config_info):
         tags.append("race")
     if msan:
         tags.append("msan")
+    if pure:
+        tags.append("purego")
 
     return struct(
         static = static,

tests/core/cgo/BUILD.bazel

@@ -334,6 +334,7 @@ go_test(
     data = [
         ":tag_cgo_bin",
         ":tag_pure_bin",
+        ":tag_purego_bin",
     ],
     rundir = ".",
     deps = ["//go/tools/bazel:go_default_library"],
@@ -350,6 +351,17 @@ go_binary(
     pure = "on",
 )
 
+go_binary(
+    name = "tag_purego_bin",
+    srcs = [
+        "tag_purego.go",
+        "tag_purego_err.c",
+        "tag_purego_err.go",
+    ],
+    cgo = True,
+    pure = "on",
+)
+
 go_binary(
     name = "tag_cgo_bin",
     srcs = [

tests/core/cgo/tag_purego.go

@@ -0,0 +1,9 @@
+// +build purego
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("purego")
+}

tests/core/cgo/tag_purego_err.c

@@ -0,0 +1 @@
+#error should not be compiled

tests/core/cgo/tag_purego_err.go

@@ -0,0 +1,6 @@
+// +build !purego
+
+package main
+
+// this file should not be compiled
+!!!

tests/core/cgo/tag_test.go

@@ -15,6 +15,9 @@ func Test(t *testing.T) {
 		{
 			name: "tag_pure_bin",
 			want: "pure",
+		}, {
+			name: "tag_purego_bin",
+			want: "purego",
 		}, {
 			name: "tag_cgo_bin",
 			want: "cgo",