Make libclang attr as optional in the rust_bindgen_toolchain (#3788)

The `libclang` attr is currently used to set env variables for the
underlying `clang-sys` crate to dynamically link libclang. In build
systems that use static linking, these are redundant. Making libclang
attr optional simplifies the toolchain config for static linking.

---------

Co-authored-by: Krasimir Georgiev <krasimir@google.com>

Author: vaibhav-1207

extensions/bindgen/private/bindgen.bzl

@@ -216,22 +216,24 @@ def _rust_bindgen_impl(ctx):
     toolchain = ctx.toolchains[Label("//:toolchain_type")]
     bindgen_bin = toolchain.bindgen
     clang_bin = toolchain.clang
-    libclang = toolchain.libclang
-    libstdcxx = toolchain.libstdcxx
+    libclang = getattr(toolchain, "libclang", None)
+    libstdcxx = getattr(toolchain, "libstdcxx", None)
 
     output = ctx.outputs.out
 
     cc_toolchain, feature_configuration = find_cc_toolchain(ctx = ctx)
 
     tools = depset(([clang_bin] if clang_bin else []), transitive = [cc_toolchain.all_files])
 
-    # libclang should only have 1 output file
-    libclang_dir = _get_libs_for_static_executable(libclang).to_list()[0].dirname
-
     env = {
-        "LIBCLANG_PATH": libclang_dir,
         "RUST_BACKTRACE": "1",
     }
+
+    if libclang:
+        # libclang should only have 1 output file
+        libclang_dir = _get_libs_for_static_executable(libclang).to_list()[0].dirname
+        env["LIBCLANG_PATH"] = libclang_dir
+
     if clang_bin:
         env["CLANG_PATH"] = clang_bin.path
 
@@ -381,8 +383,9 @@ def _rust_bindgen_impl(ctx):
             [header],
             transitive = [
                 cc_lib[CcInfo].compilation_context.headers,
-                _get_libs_for_static_executable(libclang),
             ] + ([
+                _get_libs_for_static_executable(libclang),
+            ] if libclang else []) + ([
                 _get_libs_for_static_executable(libstdcxx),
             ] if libstdcxx else []),
         ),
@@ -528,10 +531,11 @@ For additional information, see the [Bazel toolchains documentation](https://doc
             default = True,
         ),
         "libclang": attr.label(
-            doc = "A cc_library that provides bindgen's runtime dependency on libclang.",
+            doc = "A cc_library providing bindgen's runtime dependency on libclang. This attribute is required for hermeticity when bindgen is dynamically linked. If None, bindgen must be statically linked; else, system libraries will be used instead.",
             cfg = "exec",
             providers = [CcInfo],
             allow_files = True,
+            mandatory = False,
         ),
         "libstdcxx": attr.label(
             doc = "A cc_library that satisfies libclang's libstdc++ dependency. This is used to make the execution of clang hermetic. If None, system libraries will be used instead.",