Auto merge of rust-lang#89403 - camsteffen:fmt-unsafe-private, r=Mark-Simulacrum

Add private arg to fmt::UnsafeArg

As discussed [here](rust-lang#89139 (comment))

r? `@Mark-Simulacrum`

Author: bors

library/core/src/fmt/mod.rs

@@ -270,9 +270,10 @@ pub struct ArgumentV1<'a> {
 /// of `format_args!(..)` and reduce the scope of the `unsafe` block.
 #[allow(missing_debug_implementations)]
 #[doc(hidden)]
-#[non_exhaustive]
 #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-pub struct UnsafeArg;
+pub struct UnsafeArg {
+    _private: (),
+}
 
 impl UnsafeArg {
     /// See documentation where `UnsafeArg` is required to know when it is safe to
@@ -281,7 +282,7 @@ impl UnsafeArg {
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
     #[inline(always)]
     pub unsafe fn new() -> Self {
-        Self
+        Self { _private: () }
     }
 }