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+If an impl has a generic parameter with the `#[may_dangle]` attribute, then
+that impl must be declared as an `unsafe impl`.
+
+Erroneous code example:
+
+```compile_fail,E0569
+#![feature(dropck_eyepatch)]
+
+struct Foo<X>(X);
+impl<#[may_dangle] X> Drop for Foo<X> {
+    fn drop(&mut self) { }
+}
+```
+
+In this example, we are asserting that the destructor for `Foo` will not
+access any data of type `X`, and require this assertion to be true for
+overall safety in our program. The compiler does not currently attempt to
+verify this assertion; therefore we must tag this `impl` as unsafe.