"type annotations needed" on `#[derive(serde::Deserialize)]`

[ChayimFriedman2]

Every #[derive(serde::Deserialize)] on a non-empty struct with experimental diagnostics enabled triggers a "type annotations needed" error, e.g.:

#[derive(serde::Deserialize)]
struct Foo {
    v: i32,
}

Minimized:

pub fn missing_field<E>() -> Result<(), E> {
    loop {}
}

fn visit_map() -> Result<(), ()> {
    missing_field()?;
    Ok(())
}

[ChayimFriedman2]

More minimization:

fn any<T>() -> T {
    loop {}
}
fn foo<T, U: From<T>>(_: T) -> U {
    loop {}
}
fn bar() {
    let _: () = foo(any());
}

And this does not work in tests. If you define trait From<T> with a single impl impl<T> From<T> for T it works. So this is some impl in std that causes us troubles.