How to resolve "type annotations required: cannot resolve _" when calling generic static method?

Question

I am trying to call a generic static method within a different static method, but I get a confusing error:

error: type annotations required: cannot resolve `_: Config` [--explain E0283]
  --> src/main.rs:15:38
   |>
15 |>                     "json" => return Config::parse_json::<T>(source, datatype),
   |>                                      ^^^^^^^^^^^^^^^^^^^^^^^
note: required by `Config::parse_json`

When I ran rustc --explain E0283, the error message said:

This error occurs when the compiler doesn't have enough information to unambiguously choose an implementation.

Which is confusing as there is only one implementation of the function.

use rustc_serialize::json;
use std::fs::File;
use std::io::prelude::*;
use std::path::PathBuf;
use rustc_serialize;

pub trait Config {
    fn get_config<T: rustc_serialize::Decodable>(source: PathBuf, datatype: T) -> Option<T> {
        let extension = source.extension().unwrap();
        if let Some(extension) = extension.to_str() {
            match extension {
                "json" => return Config::parse_json::<T>(source, datatype),
                _ => panic!("Unable to parse the specfied extension."),
            }
        } else {
            panic!("No extension was found.");
        }
    }

    fn parse_json<T: rustc_serialize::Decodable>(source: PathBuf, datatype: T) -> Option<T> {
        let mut file = File::open(source).unwrap();
        let mut contents = String::new();
        file.read_to_string(&mut contents).unwrap();
        let decoded: T = json::decode(&contents).unwrap();
        let option: Option<T> = Some(datatype);
        return option;
    }
}

Answer 1

It means Rust couldn't figure out the type of T. Rust generic methods work by generating a separate implementation for each concrete T that you actually use in your code. Meaning you need a concrete type spelled out somewhere.

You could fix it by using something like:

return Config::parse_json(source, datatype as AConcreteDataType);

But to know the problem for sure, we'd need to see the rest of the calling code in main.rs.

Aside from that, the parse_json method looks iffy; why is it returning the datatype instead of the decoded result?