Overflow while adding drop-check rules on a generic tree

[cyypherus]

I tried this code:

enum Tree<T: Scopable> {
    Group(Vec<Tree<T>>),
    Subtree(Box<Tree<T::SubType>>),
    Leaf(T),
}

trait Scopable: Sized {
    type SubType: Scopable;
}

impl<T: Scopable> Tree<T> {
    fn foo(self) -> Self { // error[E0320]: overflow while adding drop-check rules for Tree<T>
        self
    }
}

I expected this to compile as it doesn't seem to unconditionally introduce a type of infinite length, but no luck, drop checking does not succeed.

When the type is not passed in as an argument, or passed by reference of course, there are no issues & the code compiles.

fn bar() {
    let _ = Tree::Leaf(()); // no problem!
}

Meta

Reproduced on these versions,
rustc --version --verbose:

rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: aarch64-apple-darwin
release: 1.81.0
LLVM version: 18.1.7

rustc 1.83.0-nightly (1bc403daa 2024-10-11)
binary: rustc
commit-hash: 1bc403daadbebb553ccc211a0a8eebb73989665f
commit-date: 2024-10-11
host: aarch64-apple-darwin
release: 1.83.0-nightly
LLVM version: 19.1.1

Error

error[E0320]: overflow while adding drop-check rules for Tree<T>
  --> src/main.rs:68:21
   |
68 |     fn foo(self) -> Self {
   |                     ^^^^
   |
   = note: overflowed on Tree<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<T as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType as Scopable>::SubType>

[cyypherus]

Working around this drop-checking overflow also led to an issue with clippy
rust-lang/rust-clippy#13544

[theemathas]

Another variant of the issue:

trait Bar {
    type Element: Bar + Clone;
}

#[derive(Clone)]  // error occurs in the derive-generated code.
enum Foo<T: Bar> {
    A(Box<Foo<T::Element>>),
    B(T),
}