c-variadic: unseal core::ffi::VaArgSafe

And update the safety documentation

Author: folkertdev

library/core/src/ffi/va_list.rs

@@ -185,36 +185,19 @@ impl<'a, 'f: 'a> DerefMut for VaList<'a, 'f> {
     }
 }
 
-mod sealed {
-    pub trait Sealed {}
-
-    impl Sealed for i32 {}
-    impl Sealed for i64 {}
-    impl Sealed for isize {}
-
-    impl Sealed for u32 {}
-    impl Sealed for u64 {}
-    impl Sealed for usize {}
-
-    impl Sealed for f64 {}
-
-    impl<T> Sealed for *mut T {}
-    impl<T> Sealed for *const T {}
-}
-
 /// Trait which permits the allowed types to be used with [`VaListImpl::arg`].
 ///
 /// # Safety
 ///
-/// This trait must only be implemented for types that C passes as varargs without implicit promotion.
+/// Implementing this trait asserts that the type can be soundly passed via varargs.
 ///
 /// In C varargs, integers smaller than [`c_int`] and floats smaller than [`c_double`]
 /// are implicitly promoted to [`c_int`] and [`c_double`] respectively. Implementing this trait for
 /// types that are subject to this promotion rule is invalid.
 ///
 /// [`c_int`]: core::ffi::c_int
 /// [`c_double`]: core::ffi::c_double
-pub unsafe trait VaArgSafe: sealed::Sealed {}
+pub unsafe trait VaArgSafe {}
 
 // i8 and i16 are implicitly promoted to c_int in C, and cannot implement `VaArgSafe`.
 unsafe impl VaArgSafe for i32 {}