Export RustDefaultVersion to Bazel

The config constant is consumed by rust toolchain implemented in aosp/2709668.

Bug: 295918553
Test: go test
Change-Id: I0a28a802ad28f6ee24dfb1b5bdab72094534ae04
diff --git a/bp2build/conversion.go b/bp2build/conversion.go
index f280924..7791419 100644
--- a/bp2build/conversion.go
+++ b/bp2build/conversion.go
@@ -12,6 +12,7 @@
 	"android/soong/cc"
 	cc_config "android/soong/cc/config"
 	java_config "android/soong/java/config"
+	rust_config "android/soong/rust/config"
 	"android/soong/starlark_fmt"
 
 	"github.com/google/blueprint/proptools"
@@ -37,6 +38,9 @@
 	files = append(files, newFile("java_toolchain", GeneratedBuildFileName, "")) // Creates a //java_toolchain package.
 	files = append(files, newFile("java_toolchain", "constants.bzl", java_config.BazelJavaToolchainVars(cfg)))
 
+	files = append(files, newFile("rust_toolchain", GeneratedBuildFileName, "")) // Creates a //rust_toolchain package.
+	files = append(files, newFile("rust_toolchain", "constants.bzl", rust_config.BazelRustToolchainVars(cfg)))
+
 	files = append(files, newFile("apex_toolchain", GeneratedBuildFileName, "")) // Creates a //apex_toolchain package.
 	apexToolchainVars, err := apex.BazelApexToolchainVars()
 	if err != nil {