Unclear error : unable to infer enough type information about `_`

[BigEpsilon]

This code producesses an unclear error

pub trait ValueGiver<T> {
    fn give(_: Option<Self>) -> T;
}

struct m;

impl<f64> ValueGiver<f64> for m {
    fn give(_:Option<m>) -> f64 {
        0.5f64
    } 
}

fn main() {
    let y : f64 = ValueGiver::give(None::<m>);
}

Here the error of course is the impl<f64> that should be impl, but the error message is not clear :

c.rs:64:5: 68:6 error: unable to infer enough type information about `_`; type annotations required
c.rs:64     impl<f64> ValueGiver<f64> for m {
c.rs:65         fn give(_:Option<m>) -> f64 {
c.rs:66             0.5f64
c.rs:67         } 
c.rs:68     }

Could this error message be more helpful ?

[apasel422]

This now gives the error:

foo.rs:7:6: 7:9 error: user-defined types or type parameters cannot shadow the primitive types [E0317]
foo.rs:7 impl<f64> ValueGiver<f64> for m {
              ^~~

[Manishearth]

Triage: Mostly fixed, would be nice if it had a span_suggestion