Eternal lint on a recursive tree

[ejjonny]

Summary

This code causes clippy to hang & never complete linting.

use std::mem::ManuallyDrop;
use std::ops::Deref;
use std::ops::DerefMut;

trait Scopable: Sized {
    type SubType: Scopable;
}

struct Subtree<T: Scopable>(ManuallyDrop<Box<Tree<T::SubType>>>);

impl<T: Scopable> Drop for Subtree<T> {
    fn drop(&mut self) {
        // SAFETY: The field cannot be used after we drop
        unsafe { ManuallyDrop::drop(&mut self.0) }
    }
}

impl<T: Scopable> Deref for Subtree<T> {
    type Target = Tree<T::SubType>;
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<T: Scopable> DerefMut for Subtree<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}

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

impl<T: Scopable> Tree<T> {
    fn foo(self) -> Self {
        self
    }
}

fn main() {}

Reproducer

I tried this code:(See above)

I expected to see this happen: Clippy finish linting within a reasonable time period

Instead, this happened: Clippy doesn't finish linting & hangs, eating 90% cpu

Version

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

Additional Labels

No response

[y21]

Looks like a hang in the significant_drop_tightening lint. Reduced a bit further:

use std::marker::PhantomData;

trait Trait {
    type Assoc: Trait;
}
struct S<T: Trait>(*const S<T::Assoc>, PhantomData<T>);

fn f<T: Trait>(x: &mut S<T>) {
    &mut x.0;
}

fn main() {}

We keep recursing into the first field of the struct and always miss the type cache because each level of nesting adds another projection (S<T::Assoc> -> S<T::Assoc::Assoc> -> S<T::Assoc::Assoc::Assoc> ...).

[c410-f3r]

Sign... There are many errors related to significant_drop_tightening. I hope to get back to working on it in the coming weeks.