Merge pull request #1839 from folkertdev/c-variadic-unsafe-at-any-speed

add a warning about using `safe` on extern c-variadic functions

Author: traviscross

src/items/external-blocks.md

@@ -179,9 +179,15 @@ unsafe extern "C" {
     unsafe fn foo(...);
     unsafe fn bar(x: i32, ...);
     unsafe fn with_name(format: *const u8, args: ...);
+    // SAFETY: This function guarantees it will not access
+    // variadic arguments.
+    safe fn ignores_variadic_arguments(x: i32, ...);
 }
 ```
 
+> [!WARNING]
+> The `safe` qualifier should not be used on a function in an `extern` block unless that function guarantees that it will not access the variadic arguments at all. Passing an unexpected number of arguments or arguments of unexpected type to a variadic function may lead to [undefined behavior][undefined].
+
 r[items.extern.attributes]
 ## Attributes on extern blocks