Update rules_rust to use Bazel's toolchains feature. (#52)

This allows rust targets to only depend on (and thus download) the
toolchains that are used, dynamically chosen based on the current target
platform.

Fixes #14.

rust/BUILD

@@ -1,45 +1,7 @@
 package(default_visibility = ["//visibility:public"])
 
-exports_files(["rust.bzl"])
-
-config_setting(
-    name = "darwin",
-    values = {"host_cpu": "darwin"},
-)
-
-config_setting(
-    name = "k8",
-    values = {"host_cpu": "k8"},
-)
-
-filegroup(
-    name = "rustc",
-    srcs = select({
-        ":darwin": ["@rust_darwin_x86_64//:rustc"],
-        ":k8": ["@rust_linux_x86_64//:rustc"],
-    }),
-)
-
-filegroup(
-    name = "rustc_lib",
-    srcs = select({
-        ":darwin": ["@rust_darwin_x86_64//:rustc_lib"],
-        ":k8": ["@rust_linux_x86_64//:rustc_lib"],
-    }),
-)
-
-filegroup(
-    name = "rustdoc",
-    srcs = select({
-        ":darwin": ["@rust_darwin_x86_64//:rustdoc"],
-        ":k8": ["@rust_linux_x86_64//:rustdoc"],
-    }),
-)
-
-filegroup(
-    name = "rust_lib",
-    srcs = select({
-        ":darwin": ["@rust_darwin_x86_64//:rust_lib"],
-        ":k8": ["@rust_linux_x86_64//:rust_lib"],
-    }),
-)
+exports_files([
+    "rust.bzl",
+    "toolchain.bzl",
+    "repositories.bzl",
+])

rust/repositories.bzl

@@ -64,6 +64,58 @@ filegroup(
 )
 """
 
+# This defines the default toolchain separately from the actual repositories, so that the remote
+# repositories will only be downloaded if they are actually used.
+DEFAULT_TOOLCHAINS = """
+load("@io_bazel_rules_rust//rust:toolchain.bzl", "rust_toolchain")
+
+toolchain(
+    name = "rust-linux-x86_64",
+    exec_compatible_with = [
+        "@bazel_tools//platforms:linux",
+        "@bazel_tools//platforms:x86_64",
+    ],
+    target_compatible_with = [
+        "@bazel_tools//platforms:linux",
+        "@bazel_tools//platforms:x86_64",
+    ],
+    toolchain = ":rust-linux-x86_64_impl",
+    toolchain_type = "@io_bazel_rules_rust//rust:toolchain",
+)
+
+rust_toolchain(
+    name = "rust-linux-x86_64_impl",
+    rust_doc = "@rust_linux_x86_64//:rustdoc",
+    rust_lib = ["@rust_linux_x86_64//:rust_lib"],
+    rustc = "@rust_linux_x86_64//:rustc",
+    rustc_lib = ["@rust_linux_x86_64//:rustc_lib"],
+    visibility = ["//visibility:public"],
+)
+
+toolchain(
+    name = "rust-darwin-x86_64",
+    exec_compatible_with = [
+        "@bazel_tools//platforms:osx",
+        "@bazel_tools//platforms:x86_64",
+    ],
+    target_compatible_with = [
+        "@bazel_tools//platforms:osx",
+        "@bazel_tools//platforms:x86_64",
+    ],
+    toolchain = ":rust-darwin-x86_64_impl",
+    toolchain_type = "@io_bazel_rules_rust//rust:toolchain",
+)
+
+rust_toolchain(
+    name = "rust-darwin-x86_64_impl",
+    rust_doc = "@rust_darwin_x86_64//:rustdoc",
+    rust_lib = ["@rust_darwin_x86_64//:rust_lib"],
+    rustc = "@rust_darwin_x86_64//:rustc",
+    rustc_lib = ["@rust_darwin_x86_64//:rustc_lib"],
+    visibility = ["//visibility:public"],
+)
+"""
+
 # Eventually with better toolchain hosting options we could load only one of these, not both.
 def rust_repositories():
   native.new_http_archive(
@@ -81,3 +133,13 @@ def rust_repositories():
       sha256 = "38606e464b31a778ffa7d25d490a9ac53b472102bad8445b52e125f63726ac64",
       build_file_content = RUST_DARWIN_BUILD_FILE,
   )
+
+  native.new_local_repository(
+      name = "rust_default_toolchains",
+      path = ".",
+      build_file_content = DEFAULT_TOOLCHAINS)
+
+  # Register toolchains
+  native.register_toolchains(
+      "@rust_default_toolchains//:rust-linux-x86_64",
+      "@rust_default_toolchains//:rust-darwin-x86_64")