Missing type hint on closure arg blows up.

[jfager]

This works:

use std::collections::hash_map::HashMap;

fn main() {
    let mut cmds: HashMap<String, Box<FnMut(&str)->()>> = HashMap::new();
    cmds.insert("ping".to_string(), Box::new(|&mut: s: &str| { println!("{}", s); }));
    cmds.get_mut("ping").map(|f| f("pong"));
}

While this doesn't:

use std::collections::hash_map::HashMap;

fn main() {
    let mut cmds: HashMap<String, Box<FnMut(&str)->()>> = HashMap::new();
    cmds.insert("ping".to_string(), Box::new(|&mut: s| { println!("{}", s); }));
                                                  //^ no type hint
    match cmds.get_mut("ping").map(|f| f("pong"));
}

<anon>:6:37: 6:79 error: type mismatch: the type `closure[<anon>:6:46: 6:78]` implements the trait `core::ops::FnMut(_)`, but the trait `for<'r> core::ops::FnMut(&'r str)` is required (expected concrete lifetime, found bound lifetime parameter )
<anon>:6     cmds.insert("ping".to_string(), Box::new(|&mut: s| { println!("{}", s); }));
                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:6:37: 6:79 note: required for the cast to the object type `for<'r> core::ops::FnMut(&'r str)`
<anon>:6     cmds.insert("ping".to_string(), Box::new(|&mut: s| { println!("{}", s); }));
                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

It would be nice if the argument type could be inferred; having to hint every arg can be very verbose.

Possibly related to #20558, #20254, #19499, but this is a different error message.

[jfager]

Works as of a recent nightly.