build: add Bazel Rust workflow

bazel/rust.bzl

@@ -0,0 +1,51 @@
+load("//bazel:utils.bzl", "execute", "require_env")
+
+
+def _rust_repository_impl(repository_ctx):
+    root = repository_ctx.path(require_env(repository_ctx, "RUST_TOOLCHAIN_ROOT"))
+    rustc = root.get_child("bin/rustc")
+
+    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(root, "toolchain")
+    repository_ctx.symlink(sysroot, "rust")
+
+    repository_ctx.file("WORKSPACE.bazel", "")
+    repository_ctx.template(
+        "BUILD.bazel",
+        Label("//bazel:rust.BUILD.template"),
+        {
+            "{host_triple}": host_triple,
+            "{release}": release,
+        },
+    )
+
+
+rust_repository = repository_rule(
+    implementation = _rust_repository_impl,
+    local = True,
+    environ = ["RUST_TOOLCHAIN_ROOT"],
+)
+
+
+def _rust_impl(module_ctx):
+    rust_repository(name = "rust")
+
+
+rust = module_extension(
+    implementation = _rust_impl,
+)