Initial support for auto traits with default bounds

compiler/rustc_session/src/options.rs

@@ -2187,6 +2187,8 @@ options! {
         "Use WebAssembly error handling for wasm32-unknown-emscripten"),
     enforce_type_length_limit: bool = (false, parse_bool, [TRACKED],
         "enforce the type length limit when monomorphizing instances in codegen"),
+    experimental_default_bounds: bool = (false, parse_bool, [TRACKED],
+        "enable default bounds for experimental group of auto traits"),
     export_executable_symbols: bool = (false, parse_bool, [TRACKED],
         "export symbols from executables, as if they were dynamic libraries"),
     external_clangrt: bool = (false, parse_bool, [UNTRACKED],