build: add bazel rust workflow

bazel/BUILD

@@ -0,0 +1 @@
+exports_files(["rust_toolchain.bzl"])

bazel/rust_toolchain.bzl

@@ -0,0 +1,191 @@
+def _required_path(repository_ctx, command):
+    path = repository_ctx.which(command)
+    if path == None:
+        fail("{} must be available in PATH. Enter the rust dev shell first.".format(command))
+    return path
+
+
+def _execute(repository_ctx, arguments):
+    result = repository_ctx.execute(arguments)
+    if result.return_code != 0:
+        fail("command failed: {}\n{}".format(" ".join(arguments), result.stderr))
+    return result.stdout.strip()
+
+
+def _local_rust_repository_impl(repository_ctx):
+    rustc = _required_path(repository_ctx, "rustc")
+    rustdoc = _required_path(repository_ctx, "rustdoc")
+    cargo = _required_path(repository_ctx, "cargo")
+    cargo_clippy = _required_path(repository_ctx, "cargo-clippy")
+    clippy_driver = _required_path(repository_ctx, "clippy-driver")
+    rustfmt = _required_path(repository_ctx, "rustfmt")
+    cc = _required_path(repository_ctx, "cc")
+
+    sysroot = _execute(repository_ctx, [str(rustc), "--print", "sysroot"])
+    verbose_version = _execute(repository_ctx, [str(rustc), "-vV"])
+
+    host_triple = None
+    release = None
+    for line in verbose_version.splitlines():
+        if line.startswith("host: "):
+            host_triple = line.removeprefix("host: ")
+        if line.startswith("release: "):
+            release = line.removeprefix("release: ")
+
+    if host_triple == None:
+        fail("rustc -vV did not report a host triple")
+    if release == None:
+        fail("rustc -vV did not report a release")
+
+    repository_ctx.symlink(rustc, "bin/rustc")
+    repository_ctx.symlink(rustdoc, "bin/rustdoc")
+    repository_ctx.symlink(cargo, "bin/cargo")
+    repository_ctx.symlink(cargo_clippy, "bin/cargo-clippy")
+    repository_ctx.symlink(clippy_driver, "bin/clippy-driver")
+    repository_ctx.symlink(rustfmt, "bin/rustfmt")
+    repository_ctx.symlink(cc, "bin/cc")
+    repository_ctx.symlink(sysroot, "rust")
+
+    repository_ctx.file("WORKSPACE.bazel", "")
+    repository_ctx.file("BUILD.bazel", _build_file(host_triple, release))
+
+
+def _build_file(host_triple, release):
+    return """
+filegroup(
+    name = "rustc",
+    srcs = ["bin/rustc"],
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "rustdoc",
+    srcs = ["bin/rustdoc"],
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "cargo",
+    srcs = ["bin/cargo"],
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "cargo_clippy",
+    srcs = ["bin/cargo-clippy"],
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "clippy_driver",
+    srcs = ["bin/clippy-driver"],
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "rustfmt",
+    srcs = ["bin/rustfmt"],
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "cc",
+    srcs = ["bin/cc"],
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "rustc_lib",
+    srcs = glob(
+        [
+            "rust/lib/*.so*",
+            "rust/lib/rustlib/{host_triple}/codegen-backends/*.so",
+            "rust/lib/rustlib/{host_triple}/lib/*.so*",
+            "rust/lib/rustlib/{host_triple}/lib/*.rmeta",
+        ],
+        allow_empty = True,
+    ),
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "rust_objcopy",
+    srcs = glob(
+        ["rust/lib/rustlib/{host_triple}/bin/rust-objcopy"],
+        allow_empty = True,
+    ),
+    visibility = ["//visibility:public"],
+)
+
+load("@rules_rust//rust:toolchain.bzl", "rust_stdlib_filegroup")
+
+rust_stdlib_filegroup(
+    name = "rust_std",
+    srcs = glob(
+        [
+            "rust/lib/rustlib/{host_triple}/lib/*.rlib",
+            "rust/lib/rustlib/{host_triple}/lib/*.rmeta",
+            "rust/lib/rustlib/{host_triple}/lib/*.so*",
+            "rust/lib/rustlib/{host_triple}/lib/*.a",
+            "rust/lib/rustlib/{host_triple}/lib/self-contained/**",
+        ],
+        allow_empty = True,
+    ),
+    visibility = ["//visibility:public"],
+)
+
+load("@rules_rust//rust:toolchain.bzl", "rust_toolchain")
+
+rust_toolchain(
+    name = "rust_toolchain_impl",
+    rust_doc = ":rustdoc",
+    rust_std = ":rust_std",
+    rustc = ":rustc",
+    linker = ":cc",
+    linker_type = "indirect",
+    rust_objcopy = ":rust_objcopy",
+    rustfmt = ":rustfmt",
+    cargo = ":cargo",
+    clippy_driver = ":clippy_driver",
+    cargo_clippy = ":cargo_clippy",
+    rustc_lib = ":rustc_lib",
+    allocator_library = None,
+    global_allocator_library = None,
+    binary_ext = "",
+    staticlib_ext = ".a",
+    dylib_ext = ".so",
+    stdlib_linkflags = ["-ldl", "-lpthread"],
+    default_edition = "2024",
+    exec_triple = "{host_triple}",
+    target_triple = "{host_triple}",
+    version = "{release}",
+    channel = "stable",
+    visibility = ["//visibility:public"],
+)
+
+toolchain(
+    name = "local_rust_toolchain",
+    toolchain = ":rust_toolchain_impl",
+    toolchain_type = "@rules_rust//rust:toolchain_type",
+    visibility = ["//visibility:public"],
+)
+""".format(
+        host_triple = host_triple,
+        release = release,
+    )
+
+
+local_rust_repository = repository_rule(
+    implementation = _local_rust_repository_impl,
+    local = True,
+    environ = ["PATH"],
+)
+
+
+def _local_rust_impl(module_ctx):
+    local_rust_repository(name = "local_rust_toolchain")
+
+
+local_rust = module_extension(
+    implementation = _local_rust_impl,
+)