Type inference issue with IntoDeserializer

[ehuss]

With the release of 1.122, IntoDeserializer seems to have type inference problems with strings. For example:

use serde::de::IntoDeserializer;

fn f<'a, T: IntoDeserializer<'a>>(t: T) {
    t.into_deserializer();
}

fn main() {
    let s = String::new();
    f(s.as_ref());  // ERROR: type annotation needed
}

The real-world example is in Cargo as illustrated here: rust-lang/cargo#9101

This seems to be caused by #1898. cc @Mingun

I'm fine if this is closed as WontFix (I can easily add a type annotation), I just wanted to check if this was intended or ok.

EDIT: Updated code to avoid issue in rust-lang/cargo#9102