[workerd-cxx] Add test_expand build target to expand proc_macros

Just adds a test_expand build target which expands the //kj-rs/tests:awaitables-rust_test target macros, as well as a justfile target for doing so

Author: LordGoatius

MODULE.bazel

@@ -16,7 +16,7 @@ cc_configure = use_extension("@rules_cc//cc:extensions.bzl", "cc_configure_exten
 use_repo(cc_configure, "local_config_cc")
 
 rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
-rust.toolchain(versions = ["1.86.0"])
+rust.toolchain(versions = ["1.86.0", "nightly/2025-06-06"])
 use_repo(rust, "rust_toolchains")
 
 register_toolchains("@rust_toolchains//:all")

justfile

@@ -14,6 +14,9 @@ test:
 clippy:
     bazel build --config=clippy  //...
 
+expand:
+    bazel build //kj-rs/tests:expand-rust_test
+
 cargo-update:
     bazel run //third-party:vendor
 

kj-rs/tests/BUILD.bazel

@@ -1,5 +1,5 @@
 load("@rules_cc//cc:defs.bzl", "cc_library")
-load("@rules_rust//rust:defs.bzl", "rust_library", "rust_test")
+load("@rules_rust//rust:defs.bzl", "rust_library", "rust_test", "rust_unpretty")
 load("//tools/bazel:rust_cxx_bridge.bzl", "rust_cxx_bridge")
 
 # https://bazel.build/configure/windows#clang
@@ -24,6 +24,14 @@ rust_library(
     ],
 )
 
+rust_unpretty(
+    name = "expand-rust_test",
+    testonly = True,
+    deps = [
+        ":awaitables-rust_test"
+    ]
+)
+
 rust_test(
     name = "awaitables-rust_test",
     crate = "awaitables-rust",