Expand locations in x_defs values (bazelbuild#3473)

Allows embedding rlocationpaths into binaries to allow them to find
dependencies at runtime even when run from other tools, meaning that
`args` and `env` can't be used for this purpose.

docs/go/core/defines_and_stamping.md

@@ -21,6 +21,9 @@ value. You can also override stamp values from libraries using `x_defs`
 on the `go_binary` rule if needed. The `--[no]stamp` option controls whether
 stamping of workspace variables is enabled.
 
+The values of the `x_defs` dictionary are subject to
+[location expansion](https://bazel.build/reference/be/make-variables#predefined_label_variables).
+
 **Example**
 
 Suppose we have a small library that contains the current version.

go/private/context.bzl

@@ -268,6 +268,7 @@ def _library_to_source(go, attr, library, coverage_instrumented):
     source["deps"] = _dedup_deps(source["deps"])
     x_defs = source["x_defs"]
     for k, v in getattr(attr, "x_defs", {}).items():
+        v = _expand_location(go, attr, v)
         if "." not in k:
             k = "{}.{}".format(library.importmap, k)
         x_defs[k] = v
@@ -879,6 +880,9 @@ go_config = rule(
 def _expand_opts(go, attribute_name, opts):
     return [go._ctx.expand_make_variables(attribute_name, opt, {}) for opt in opts]
 
+def _expand_location(go, attr, s):
+    return go._ctx.expand_location(s, getattr(attr, "data", []))
+
 _LIST_TYPE = type([])
 
 # Used to get attribute values which may have been transitioned.

tests/core/go_test/x_defs/BUILD.bazel

@@ -48,3 +48,26 @@ go_library(
     visibility = ["//visibility:public"],
     x_defs = {"Qux": "Qux"},
 )
+
+go_library(
+    name = "x_defs_lib",
+    srcs = ["x_defs_lib.go"],
+    data = ["x_defs_lib.go"],
+    importpath = "github.com/bazelbuild/rules_go/tests/core/go_test/x_defs/x_defs_lib",
+    x_defs = {
+        "LibGo": "$(rlocationpath x_defs_lib.go)",
+    },
+)
+
+go_test(
+    name = "x_defs_test",
+    srcs = ["x_defs_test.go"],
+    data = ["x_defs_test.go"],
+    x_defs = {
+        "BinGo": "$(rlocationpath x_defs_test.go)",
+    },
+    deps = [
+        ":x_defs_lib",
+        "//go/runfiles",
+    ],
+)

tests/core/go_test/x_defs/x_defs_lib.go

@@ -0,0 +1,3 @@
+package x_defs_lib
+
+var LibGo = "not set"

tests/core/go_test/x_defs/x_defs_test.go

@@ -0,0 +1,34 @@
+package x_defs_lib_test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/bazelbuild/rules_go/go/runfiles"
+
+	"github.com/bazelbuild/rules_go/tests/core/go_test/x_defs/x_defs_lib"
+)
+
+var BinGo = "not set"
+
+func TestLibGoPath(t *testing.T) {
+	libGoPath, err := runfiles.Rlocation(x_defs_lib.LibGo)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = os.Stat(libGoPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestBinGoPath(t *testing.T) {
+	binGoPath, err := runfiles.Rlocation(BinGo)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = os.Stat(binGoPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+}