Specifying a function for #[serde(expecting)]

[Anders429]

#1916 added support for #[serde(expecting = "...")] for providing a custom message to the generated Visitor::expecting() method. This works great for specifying a custom message when deriving Deserialize.

I am wondering if it would be possible to expand this behavior to specify a custom function, rather than just a string. Something like:

fn custom_expecting(formatter: &mut fmt::Formatter) -> fmt::Result {
    // Custom description of expected types.
}

#[derive(Deserialize)]
#[serde(expecting_with = "custom_expecting")]
struct Foo {
    // ...
}

The motivation for this is to allow slightly more involved expecting logic, supporting custom messages based on the flags provided by fmt::Formatter. While this can be accomplished currently with custom Deserialize implementations, I would like to use the derived Deserialize impl with only the expecting() method changed.

Is this something that could be supported in serde?