fmt-debug option

Allows disabling `fmt::Debug` derive and debug formatting.

compiler/rustc_feature/src/unstable.rs

@@ -470,6 +470,8 @@ declare_features! (
     (unstable, ffi_const, "1.45.0", Some(58328)),
     /// Allows the use of `#[ffi_pure]` on foreign functions.
     (unstable, ffi_pure, "1.45.0", Some(58329)),
+    /// Controlling the behavior of fmt::Debug
+    (unstable, fmt_debug, "CURRENT_RUSTC_VERSION", Some(129709)),
     /// Allows using `#[repr(align(...))]` on function items
     (unstable, fn_align, "1.53.0", Some(82232)),
     /// Support delegating implementation of functions to other already implemented functions.