expectations from fudging are a mess

At its core we use fudging for the following pattern

fn foo<T>(x: T) -> Box<T> { Box::new(x) }
fn main() {
    let _: Box<dyn Fn(&str) -> usize> = foo(|s| s.len());
}

We end up with the expectation that foos argument is dyn Fn(&str) -> usize even though that type is not Sized. This is why fudge_inference_if_ok exists and why it's used for function calls when checkig their arguments.

We try to deal with this by weakening the expectation in case it's not sized

https://github.com/rust-lang/rust/blob/7b9905edb4df3aeaabd0d6cd0d4c09b183d7d965/compiler/rustc_hir_typeck/src/expectation.rs#L77-L89

This is widely insufficient for multiple reasons

Start with compiler/rustc_hir_typeck/src/expectation.rs around lines 77–89, then inspect fudge_inference_if_ok and its use when checking function-call arguments. Reproduce the generic function example from the issue and trace how the unsized dyn Fn expectation is formed. The issue does not yet define the remaining cases or a concrete completion criterion.