Inference poisoned by presence of unrelated impl

The following code compiles fine:

extern crate serde;
use std::borrow::Cow;

pub fn dothing<'data, V>(
) -> Cow<'data, V>
where
    V: ?Sized + ToOwned ,
    for<'de> Box<V>: serde::Deserialize<'de>,
{
    unimplemented!()
}


pub fn dothing_str<'data>(
) -> Cow<'data, str>
where
{
    dothing()
}

Here, dothing_str() simply uses return type inference to resolve dothing() to dothing::<str>(). Quite straightforward, nothing fancy.

However, if you add in the following type, inference gets poisoned:

pub struct Poison<T: ?Sized> {
    x: T
}


impl<'de, T> serde::Deserialize<'de> for Box<Poison<T>>
where
    T: ?Sized,
    Box<T>: serde::Deserialize<'de>,
{
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        unimplemented!()
    }
}

Instead of trying to resolve Box<str>: Deserialize (reasonable), it tries to resolve Box<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<...>>>>>>>>>>>>>: Deserialize. Note that this happens even if you replace the for<'de> ... Deserialize<'de> with Deserialize<'static>, this isn't Yet Another HRTB issue.

error[E0275]: overflow evaluating the requirement `Box<[_]>: Deserialize<'_>`
  --> src/lib.rs:37:5
   |
37 |     dothing()
   |     ^^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`playground`)
note: required for `Box<Poison<[_]>>` to implement `Deserialize<'_>`
  --> src/lib.rs:9:14
   |
9  | impl<'de, T> serde::Deserialize<'de> for Box<Poison<T>>
   |              ^^^^^^^^^^^^^^^^^^^^^^^     ^^^^^^^^^^^^^^
...
12 |     Box<T>: serde::Deserialize<'de>,
   |             ----------------------- unsatisfied trait bound introduced here
   = note: 126 redundant requirements hidden
   = note: required for `Box<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<Poison<...>>>>>>>>>>>>>` to implement `for<'de> Deserialize<'de>`
note: required by a bound in `dothing`
  --> src/lib.rs:27:22
   |
23 | pub fn dothing<'data, V>(
   |        ------- required by a bound in this function
...
27 |     for<'de> Box<V>: serde::Deserialize<'de>,
   |                      ^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `dothing`
   = note: the full name for the type has been written to '/playground/target/debug/deps/playground-dd3320ba655b84a9.long-type-390055455354675765.txt'
   = note: consider using `--verbose` to print the full type name to the console

The poisoning is fixed by explicitly invoking dothing::<str>(). It's the return type inference that's getting grossly misled here, and it's really unclear as to why.

Start by running the two linked Rust Playground examples and compare inference at the dothing() call in dothing_str(). Investigate why adding the Poison implementation selects recursively nested types instead of str; done means the example compiles and resolves dothing() to str without an inference overflow.