Restore transitions.

Author: johnedmonds

rust/private/rust.bzl

@@ -320,6 +320,9 @@ _rust_library_attrs = {
         """),
         default = "rlib",
     ),
+    "_whitelist_function_transition": attr.label(
+        default = "//tools/whitelists/function_transition_whitelist",
+    ),
 }
 
 _rust_test_attrs = {
@@ -340,6 +343,7 @@ rust_library = rule(
                  _rust_library_attrs.items()),
     fragments = ["cpp"],
     host_fragments = ["cpp"],
+    cfg = wasm_transition,
     toolchains = [
         "@io_bazel_rules_rust//rust:toolchain",
         "@bazel_tools//tools/cpp:toolchain_type"