Remove questionable type conversions

[dtolnay]

Fixes #836.

Type	No longer deserialized from
()	empty seq
bool	"true" and "false" strings
number	FromStr
string	unit
various sequence types	unit
[T; 0]	unit
various map types	unit

These conversions are bad because they make untagged enums confusing.

#[macro_use]
extern crate serde_derive;

extern crate serde;
extern crate serde_json;

#[derive(Deserialize, Debug)]
#[serde(untagged)]
enum E {
    B(bool),
    S(String),
}

fn main() {
    // This should be E::S but is actually E::B because of implicit conversion.
    println!("{:?}", serde_json::from_str::<E>("\"false\"").unwrap());
}

It looks like bool from string and number from string were added only to support XML in #70. @RReverser can you make serde-xml-rs work without those conversions? Basically deserialize_bool and deserialize_u64 etc need to not forward to deserialize_string anymore but have their own logic for handling the appropriate type.

The rest were added before we had deserialize_with. I think deserialize_with changes the game and we no longer want these implicit conversions.

[oli-obk]

Yea, serde XML could have done without these hacks, but they were so common already, I didn't stop to think if it makes sense

[RReverser]

@RReverser can you make serde-xml-rs work without those conversions? Basically deserialize_bool and deserialize_u64 etc need to not forward to deserialize_string anymore but have their own logic for handling the appropriate type.

Sure. Are all deserialize_(integer) forwarded to deserialize_u64 and same for float?

[dtolnay]

Yes all the number types are still interconverted so you only need to write logic for u64, i64, f64.