docs: add long error explanation for error E0320

compiler/rustc_error_codes/src/error_codes/E0320.md

@@ -0,0 +1,27 @@
+Recursion limit reached while creating drop-check rules.
+
+Example of erroneous code:
+
+```compile_fail,E0320
+enum A<T> {
+    B,
+    C(T, Box<A<(T, T)>>)
+}
+
+fn foo<T>() {
+    A::<T>::B; // error: overflow while adding drop-check rules for A<T>
+}
+```
+
+The Rust compiler must be able to reason about how a type is [`Drop`]ped, and
+by extension the types of its fields, to be able to generate the glue to
+properly drop a value. The code example above shows a type where this inference
+is impossible because it is recursive. Note that this is *not* the same as
+[E0072](E0072.html), where a type has an infinite size; the type here has a
+finite size but any attempt to `Drop` it would recurse infinitely. For more
+information, read [the `Drop` docs](../std/ops/trait.Drop.html).
+
+It is not possible to define a type with recursive drop-check rules. All such
+recursion must be removed.
+
+[`Drop`]: ../std/ops/trait.Drop.html