Support bzlmod

This is a rough proof of concept of making rules_rust load dependencies via bzlmod. Currently it can only compile and run rust crates, and cannot load crates from Cargo.toml.

There is one new module, `examples/bzlmod/hello_world`, that depends on the root `rules_rust` module. This example can be built and run using:

```
cd examples/bzlmod/hello_world
USE_BAZEL_VERSION=last_green bazel run //:hello_world
```

Currently toolchain registration has quite bad user experience, because all the toolchains to register have to be listed by the user in the MODULE.bazel file. This is because module extensions cannot directly register toolchains and instead must only expose repositories with toolchains, that are then registered using `toolchains_to_register`. It would be nice if all the toolchains could be aliased to one label and them all registered together. However, I can't see how this would be possible.

MODULE.bazel

@@ -0,0 +1,23 @@
+module(
+    name = "rules_rust",
+    version = "0.9.0",
+)
+
+bazel_dep(name = "platforms", version = "0.0.5")
+bazel_dep(name = "rules_cc", version = "0.0.1")
+bazel_dep(name = "bazel_skylib", version = "1.2.0")
+bazel_dep(name = "apple_support", version = "0.13.0")
+
+# Load tinyjson without using the process wrapper
+raw_cargo = use_extension("//:extensions.bzl", "raw_cargo")
+
+raw_cargo.crate(
+    name = "tinyjson",
+    sha256 = "1a8304da9f9370f6a6f9020b7903b044aa9ce3470f300a1fba5bc77c78145a16",
+    version = "2.3.0",
+)
+
+use_repo(
+    raw_cargo,
+    rules_rust_tinyjson = "raw_cargo_tinyjson_2_3_0",
+)

examples/bzlmod/hello_world/.bazelrc

@@ -0,0 +1 @@
+build --experimental_enable_bzlmod

examples/bzlmod/hello_world/.gitignore

@@ -0,0 +1 @@
+/bazel-*

examples/bzlmod/hello_world/BUILD.bazel

@@ -0,0 +1,13 @@
+load("@rules_rust//rust:defs.bzl", "rust_binary", "rust_doc")
+
+package(default_visibility = ["//visibility:public"])
+
+rust_binary(
+    name = "hello_world",
+    srcs = ["src/main.rs"],
+)
+
+rust_doc(
+    name = "hello_world_doc",
+    crate = ":hello_world",
+)

examples/bzlmod/hello_world/MODULE.bazel

@@ -0,0 +1,18 @@
+module(
+    name = "hello_world",
+    toolchains_to_register = ["@rust_linux_x86_64__x86_64-unknown-linux-gnu//:toolchain"],
+    version = "1.0",
+)
+
+bazel_dep(name = "rules_rust", version = "0.9.0")
+
+local_path_override(
+    module_name = "rules_rust",
+    path = "../../..",
+)
+
+rust_toolchains = use_extension("@rules_rust//:extensions.bzl", "toolchains")
+
+rust_toolchains.toolchain(edition = "2021")
+
+use_repo(rust_toolchains, "rust_linux_x86_64__x86_64-unknown-linux-gnu")

examples/bzlmod/hello_world/src/main.rs

@@ -0,0 +1,17 @@
+// Copyright 2015 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+fn main() {
+    println!("Hello, world!");
+}

extensions.bzl

@@ -0,0 +1,43 @@
+load("//rust:repositories.bzl", "rust_register_toolchains")
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+def _toolchains_impl(ctx):
+    for mod in ctx.modules:
+        for toolchain in mod.tags.toolchain:
+            rust_register_toolchains(edition = toolchain.edition, register_toolchains = False)
+
+toolchains_toolchain = tag_class(attrs = {"edition": attr.string()})
+toolchains = module_extension(
+    implementation = _toolchains_impl,
+    tag_classes = {"toolchain": toolchains_toolchain},
+)
+
+def _create_build_file_content(name):
+    return """
+load("@rules_rust//rust/private:rust.bzl", "rust_library_without_process_wrapper")
+
+rust_library_without_process_wrapper(
+    name = "{}",
+    srcs = glob(["src/*.rs"]),
+    edition = "2018",
+    visibility = ["@rules_rust//util/process_wrapper:__pkg__"],
+)
+""".format(name)
+
+def _raw_cargo_impl(ctx):
+    for mod in ctx.modules:
+        for crate in mod.tags.crate:
+            http_archive(
+                name = "raw_cargo_{}_{}".format(crate.name, crate.version.replace(".", "_")),
+                sha256 = crate.sha256,
+                url = "https://crates.io/api/v1/crates/{}/{}/download".format(crate.name, crate.version),
+                strip_prefix = "{}-{}".format(crate.name, crate.version),
+                type = "tar.gz",
+                build_file_content = _create_build_file_content(crate.name),
+            )
+
+raw_cargo_crate = tag_class(attrs = {"name": attr.string(), "version": attr.string(), "sha256": attr.string()})
+raw_cargo = module_extension(
+    implementation = _raw_cargo_impl,
+    tag_classes = {"crate": raw_cargo_crate},
+)

rust/private/repository_utils.bzl

@@ -205,15 +205,15 @@ load("@rules_rust//rust:toolchain.bzl", "rust_toolchain")
 
 rust_toolchain(
     name = "{toolchain_name}",
-    rust_doc = "@{workspace_name}//:rustdoc",
-    rust_std = "@{workspace_name}//:rust_std-{target_triple}",
-    rustc = "@{workspace_name}//:rustc",
+    rust_doc = "//:rustdoc",
+    rust_std = "//:rust_std-{target_triple}",
+    rustc = "//:rustc",
     rustfmt = {rustfmt_label},
-    cargo = "@{workspace_name}//:cargo",
-    clippy_driver = "@{workspace_name}//:clippy_driver_bin",
+    cargo = "//:cargo",
+    clippy_driver = "//:clippy_driver_bin",
     llvm_cov = {llvm_cov_label},
     llvm_profdata = {llvm_profdata_label},
-    rustc_lib = "@{workspace_name}//:rustc_lib",
+    rustc_lib = "//:rustc_lib",
     rustc_srcs = {rustc_srcs},
     allocator_library = {allocator_library},
     binary_ext = "{binary_ext}",
@@ -265,15 +265,15 @@ def BUILD_for_rust_toolchain(
 
     rustc_srcs = "None"
     if include_rustc_srcs:
-        rustc_srcs = "\"@{workspace_name}//lib/rustlib/src:rustc_srcs\"".format(workspace_name = workspace_name)
+        rustc_srcs = "\"//lib/rustlib/src:rustc_srcs\"".format(workspace_name = workspace_name)
     rustfmt_label = "None"
     if include_rustfmt:
-        rustfmt_label = "\"@{workspace_name}//:rustfmt_bin\"".format(workspace_name = workspace_name)
+        rustfmt_label = "\"//:rustfmt_bin\"".format(workspace_name = workspace_name)
     llvm_cov_label = "None"
     llvm_profdata_label = "None"
     if include_llvm_tools:
-        llvm_cov_label = "\"@{workspace_name}//:llvm_cov_bin\"".format(workspace_name = workspace_name)
-        llvm_profdata_label = "\"@{workspace_name}//:llvm_profdata_bin\"".format(workspace_name = workspace_name)
+        llvm_cov_label = "\"//:llvm_cov_bin\"".format(workspace_name = workspace_name)
+        llvm_profdata_label = "\"//:llvm_profdata_bin\"".format(workspace_name = workspace_name)
     allocator_library_label = "None"
     if allocator_library:
         allocator_library_label = "\"{allocator_library}\"".format(allocator_library = allocator_library)

rust/repositories.bzl

@@ -610,20 +610,20 @@ def rust_repository_set(
         native.register_toolchains(*all_toolchain_names)
         native.register_toolchains(str(Label("//rust/private/dummy_cc_toolchain:dummy_cc_wasm32_toolchain")))
 
-    # Inform users that they should be using the canonical name if it's not detected
-    if "rules_rust" not in native.existing_rules():
-        message = "\n" + ("=" * 79) + "\n"
-        message += (
-            "It appears that you are trying to import rules_rust without using its\n" +
-            "canonical name, \"@rules_rust\" Please change your WORKSPACE file to\n" +
-            "import this repo with `name = \"rules_rust\"` instead."
-        )
-
-        if "io_bazel_rules_rust" in native.existing_rules():
-            message += "\n\n" + (
-                "Note that the previous name of \"@io_bazel_rules_rust\" is deprecated.\n" +
-                "See https://github.com/bazelbuild/rules_rust/issues/499 for context."
-            )
-
-        message += "\n" + ("=" * 79)
-        fail(message)
+    # # Inform users that they should be using the canonical name if it's not detected
+    # if "rules_rust" not in native.existing_rules():
+    #     message = "\n" + ("=" * 79) + "\n"
+    #     message += (
+    #         "It appears that you are trying to import rules_rust without using its\n" +
+    #         "canonical name, \"@rules_rust\" Please change your WORKSPACE file to\n" +
+    #         "import this repo with `name = \"rules_rust\"` instead."
+    #     )
+
+    #     if "io_bazel_rules_rust" in native.existing_rules():
+    #         message += "\n\n" + (
+    #             "Note that the previous name of \"@io_bazel_rules_rust\" is deprecated.\n" +
+    #             "See https://github.com/bazelbuild/rules_rust/issues/499 for context."
+    #         )
+
+    #     message += "\n" + ("=" * 79)
+    #     fail(message)