Apply suggestions from code review

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: folkertdev

library/core/src/ffi/va_list.rs

@@ -190,12 +190,9 @@ impl Clone for VaList<'_> {
     #[inline]
     fn clone(&self) -> Self {
         // We only implement Clone and not Copy because some future target might not be able to
-        // implement Copy (e.g. because it allocates).
-
-        // We still use a `va_copy` intrinsic to provide a hook for const evaluation. The hook is
-        // used to report UB when a variable argument list is duplicated with a manual `memcpy`.
-        // While that works in practice for all current targets, we want to be able to support
-        // targets in the future where that is not the case.
+        // implement Copy (e.g. because it allocates). For the same reason we use an intrinsic
+        // to do the copying: the fact that on all current targets, this is just `memcpy`, is an implementation
+        // detail. The intrinsic lets Miri catch UB from code incorrectly relying on that implementation detail.
         va_copy(self)
     }
 }

library/core/src/intrinsics/mod.rs

@@ -3473,7 +3473,7 @@ pub unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaList<'_>) -> T;
 #[rustc_intrinsic]
 #[rustc_nounwind]
 pub fn va_copy<'f>(src: &VaList<'f>) -> VaList<'f> {
-    // NOTE: this intrinsic exists only as a hook for constant evaluation, and is used to detect UB
+    // NOTE: this intrinsic exists only as a hook for Miri and constant evaluation, and is used to detect UB
     // when `VaList` is used incorrectly. Codegen backends should not have custom behavior for this
     // intrinsic, they should always use this fallback implementation.
     src.duplicate()