Rollup merge of rust-lang#126927 - workingjubilee:vaargsafe-is-unsafe…

…, r=joboet

core: VaArgSafe is an unsafe trait

`T: VaArgSafe` is relied on for soundness. Safe impls promise nothing. Therefore this must be an unsafe trait. Slightly pedantic, as only core can impl this, but we *could* choose to unseal the trait. That would allow soundly (but unsafely) implementing this for e.g. a `#[repr(C)] struct` that should be passable by varargs.

Relates to rust-lang#44930

library/core/src/ffi/mod.rs

@@ -484,7 +484,7 @@ mod sealed_trait {
                   all supported platforms",
         issue = "44930"
     )]
-    pub trait VaArgSafe {}
+    pub unsafe trait VaArgSafe {}
 }
 
 macro_rules! impl_va_arg_safe {
@@ -494,7 +494,7 @@ macro_rules! impl_va_arg_safe {
                        reason = "the `c_variadic` feature has not been properly tested on \
                                  all supported platforms",
                        issue = "44930")]
-            impl sealed_trait::VaArgSafe for $t {}
+            unsafe impl sealed_trait::VaArgSafe for $t {}
         )+
     }
 }
@@ -509,14 +509,15 @@ impl_va_arg_safe! {f64}
               all supported platforms",
     issue = "44930"
 )]
-impl<T> sealed_trait::VaArgSafe for *mut T {}
+unsafe impl<T> sealed_trait::VaArgSafe for *mut T {}
+
 #[unstable(
     feature = "c_variadic",
     reason = "the `c_variadic` feature has not been properly tested on \
               all supported platforms",
     issue = "44930"
 )]
-impl<T> sealed_trait::VaArgSafe for *const T {}
+unsafe impl<T> sealed_trait::VaArgSafe for *const T {}
 
 #[unstable(
     feature = "c_variadic",