The compiler is telling me a method doesn't exist and I'm sure it does

[doublequartz]

I'm trying to concurrently access two different properties in a serde_json Object.
The first way I went is to use the splitmut crate but it wasn't implemented on serde_json Map, so I forked it.
And I got this error:
no method named `get_mut` found for type `&mut serde_json::Map<String, serde_json::Value>` in the current scope

I have no clue anymore. every trait bound is looking right.
I'm dumping all code I added.

cargo.toml:

[dependencies]
serde_json = "1.0"

lib.rs:

extern crate serde_json;

unsafe impl<'a, String: borrow::Borrow<Q>, Q: Ord + Eq + hash::Hash + ?Sized> SplitMut<&'a Q, serde_json::Value> for serde_json::Map<String, serde_json::Value> {
    #[inline]
    fn get1_mut(&mut self, k: &'a Q) -> Option<&mut serde_json::Value> { self.get_mut(k) }
    #[inline]
    unsafe fn get1_unchecked_mut(&mut self, k: &'a Q) -> &mut serde_json::Value { std::mem::transmute(self.get_mut(k)) }
}

What is wrong in here? is there a better approach to do this?

[mbrubeck]

In this code, String is not the standard library's std::string::String type, but a new type parameter you have declared that happens to have the same name:

unsafe impl<'a, String: borrow::Borrow<Q>, Q: Ord + Eq + hash::Hash + ?Sized>

but to use get_mut you need to use the standard String type, not a generic type variable named String. Something like this should work:

unsafe impl<'a, Q> SplitMut<&'a Q, serde_json::Value>
for serde_json::Map<String, serde_json::Value>
where
    String: borrow::Borrow<Q>,
    Q: Ord + Eq + hash::Hash + ?Sized

[doublequartz]

Oh, I heard where clause was just syntactic sugar for long generic definition but apparently there are situation where it's necessary? anyway that's super helpful for saving my night. thanks!

[mbrubeck]

Yes, where clauses can express things that can't be expressed in the parameter list, like bounds on types that are not generic parameters.