Error code E0320

Recursion limit reached while creating drop-check rules.

Example of erroneous code:

fn main() {
enum A<T> {
    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 Dropped, 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, 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.

It is not possible to define a type with recursive drop-check rules. All such recursion must be removed.