Support bzlmod (#1528)

This makes rules_rust load dependencies via bzlmod. Currently only basic functionality is completed, such as registering rustc toolchains and compiling crates. Note that it cannot interact with cargo, wasm or load any other rust toolchains such as rustfmt.
    
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
bazel run //:hello_world
```

To register toolchains in an ergonomic way, it defines a new "hub" repository that contains all the toolchain proxies, so they can all be registered like so:

```
register_toolchains("@rust_toolchains//:all")
```

closes #1493

Author: cameron-martin

.bazelci/presubmit.yml

@@ -342,6 +342,12 @@ tasks:
       - "//..."
     build_flags: *aspects_flags
     soft_fail: yes
+  ubuntu2004_examples_bzlmod:
+    name: Bzlmod Examples
+    platform: ubuntu2004
+    working_directory: examples/bzlmod/hello_world
+    build_targets:
+      - "//..."
   rbe_ubuntu1604_examples:
     name: Examples
     platform: rbe_ubuntu1604

MODULE.bazel

@@ -0,0 +1,17 @@
+module(
+    name = "rules_rust",
+    version = "0.20.0",
+)
+
+print("WARNING: The rules_rust Bazel module is still highly experimental and subject to change at any time. Only use it to try out bzlmod for now.")  # buildifier: disable=print
+
+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 = "1.3.1")
+
+internal_deps = use_extension("//rust/private:extensions.bzl", "internal_deps")
+use_repo(
+    internal_deps,
+    "rules_rust_tinyjson",
+)

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 +1,19 @@
+module(
+    name = "hello_world",
+    version = "1.0",
+)
+
+bazel_dep(name = "rules_rust", version = "0.9.0")
+local_path_override(
+    module_name = "rules_rust",
+    path = "../../..",
+)
+
+rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
+rust.toolchain(edition = "2021")
+use_repo(
+    rust,
+    "rust_toolchains",
+)
+
+register_toolchains("@rust_toolchains//:all")

examples/bzlmod/hello_world/WORKSPACE

@@ -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!");
+}

examples/bzlmod/hello_world/src/main.rs

@@ -0,0 +1,42 @@
+"Module extensions for using rules_rust with bzlmod"
+
+load(
+    "//rust/private:repository_utils.bzl",
+    "DEFAULT_EXTRA_TARGET_TRIPLES",
+    "DEFAULT_NIGHTLY_VERSION",
+    "DEFAULT_STATIC_RUST_URL_TEMPLATES",
+)
+load(":repositories.bzl", "rust_register_toolchains")
+
+def _rust_impl(ctx):
+    mod = ctx.modules[0]
+    for toolchain in mod.tags.toolchain:
+        rust_register_toolchains(
+            dev_components = toolchain.dev_components,
+            edition = toolchain.edition,
+            allocator_library = toolchain.allocator_library,
+            rustfmt_version = toolchain.rustfmt_version,
+            rust_analyzer_version = toolchain.rust_analyzer_version,
+            sha256s = toolchain.sha256s,
+            extra_target_triples = toolchain.extra_target_triples,
+            urls = toolchain.urls,
+            versions = toolchain.versions,
+            register_toolchains = False,
+        )
+
+rust_toolchain = tag_class(attrs = {
+    "allocator_library": attr.string(),
+    "dev_components": attr.bool(default = False),
+    "edition": attr.string(),
+    "extra_target_triples": attr.string_list(default = DEFAULT_EXTRA_TARGET_TRIPLES),
+    "rust_analyzer_version": attr.string(),
+    "rustfmt_version": attr.string(default = DEFAULT_NIGHTLY_VERSION),
+    "sha256s": attr.string_dict(),
+    "urls": attr.string_list(default = DEFAULT_STATIC_RUST_URL_TEMPLATES),
+    "versions": attr.string_list(default = []),
+})
+
+rust = module_extension(
+    implementation = _rust_impl,
+    tag_classes = {"toolchain": rust_toolchain},
+)

rust/extensions.bzl

@@ -0,0 +1,12 @@
+"""Bzlmod module extensions that are only used internally"""
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+load("//rust/private:repository_utils.bzl", "TINYJSON_KWARGS")
+
+def _internal_deps_impl(_module_ctx):
+    http_archive(**TINYJSON_KWARGS)
+
+internal_deps = module_extension(
+    doc = "Dependencies for rules_rust",
+    implementation = _internal_deps_impl,
+)