Confusing error message when returning impl Trait where the concrete type contains itself.

[chanks]

Code

#[derive(Debug)]
enum MyTreeNode {
    Leaf,
    VecNode(Vec<Self>),
}

impl MyTreeNode {
    pub fn iter(&self) -> impl Iterator<Item = &Self> + '_ {
        [self]
            .into_iter()
            .chain(self.nonrecursive_iter().flat_map(|e| e.iter()))
    }

    fn nonrecursive_iter(&self) -> Box<dyn Iterator<Item = &Self> + '_> {
        match self {
            Self::Leaf => Box::new([].into_iter()),
            Self::VecNode(v) => Box::new(v.iter()),
        }
    }
}

fn main() {
    let tree = MyTreeNode::VecNode(vec![
        MyTreeNode::Leaf,
        MyTreeNode::VecNode(vec![MyTreeNode::Leaf, MyTreeNode::Leaf]),
    ]);

    for node in tree.iter() {
        dbg!(node);
    }
}

Current output

Compiling playground v0.0.1 (/playground)
error: concrete type differs from previous defining opaque type use
  --> src/main.rs:9:9
   |
9  | /         [self]
10 | |             .into_iter()
11 | |             .chain(self.nonrecursive_iter().flat_map(|e| e.iter()))
   | |                                                                   ^
   | |                                                                   |
   | |___________________________________________________________________expected `std::iter::Chain<std::array::IntoIter<&MyTreeNode, 1>, FlatMap<Box<dyn Iterator<Item = &MyTreeNode>>, impl Iterator<Item = &MyTreeNode> + '_, {closure@src/main.rs:11:54: 11:57}>>`, got `impl Iterator<Item = &MyTreeNode> + '_`
   |                                                                     this expression supplies two conflicting concrete types for the same opaque type

error: could not compile `playground` (bin "playground") due to 1 previous error

Desired output

Rationale and extra context

Playground

I believe the issue here is not that the expression returns two different types, but that it returns a single recursive type. Changing the iter() function signature to return a boxed iterator makes the code compile. If I'm right, then I think some wording about boxing the output of recursive function calls makes sense (similar to the message you see if you recurse async function calls).

Other cases

Rust Version

rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: aarch64-apple-darwin
release: 1.84.0
LLVM version: 19.1.5

Anything else?

No response