serde-is-confusing

use std::fmt;

use serde::Deserialize;

extern crate serde;
extern crate serde_json;

struct Foo;

impl<'de> Deserialize<'de> for Foo {
    fn deserialize<D>(deserializer: D) -> Result<Foo, D::Error>
            where D: serde::Deserializer<'de> {
        deserializer.deserialize_map(MyVisitor)
    }
}

struct MyVisitor;

impl<'de> serde::de::Visitor<'de> for MyVisitor {
    type Value = Foo;

    fn expecting(&self, _: &mut fmt::Formatter) -> fmt::Result {
        unimplemented!()
    }

    fn visit_map<A: serde::de::MapAccess<'de>>(self, mut map: A) -> Result<Foo, A::Error> {
        loop {
            match map.next_entry()? {
                Some((key, value)) => saw_entry(key, value),
                None => break,
            }
        }
        Ok(Foo)
    }
}

fn saw_entry(k: &str, v: i32) {
    println!("saw key {}: {}", k, v);
}

fn main() {
    let s = r#"{"foo": 42}"#;
    println!("this works:");
    let _: Foo = serde_json::from_str(s).unwrap();
    println!("this breaks:");
    let _: Foo = serde_json::from_reader(s.as_bytes()).unwrap();
}