`utoipa-gen` breakage

[lcnr]

https://crater-reports.s3.amazonaws.com/pr-133502-13/try%233cddd79d4be3c8f1e937d09ca618d192abc69373/gh/Hemp-bandit.rs_service_util/log.txt

With this done, we should have triaged all major regressions (as in, with more than a few dependencies)

[lcnr]

sus

fn foo<I>(x: I) -> Result<impl Iterator, u32> {
    if false {
        foo(x)?; // ok
        foo(())?; // err
    }
    Ok(vec![1u32].into_iter())
} 

[lcnr]

trait Trait {}
impl Trait for u32 {}
fn foo<I>() -> Option<impl Trait> {
    if false {
        foo::<i32>()?; // err
    }
    Some(1u32)
}

[lcnr]

fallback happens before we constrain revealing uses

fn main() {}
fn foo<I>() -> Option<impl Sized> {
    if false {
        let _ = match foo::<i32>() {
            Some(val) => val,
            None => panic!(),
        };
    }
    Some(1u32)
}