Add a check for uninferred type parameter

[Munksgaard]

This fixes #19978. The bug was introduced by 570325d, where if the type
of an Fn has not been inferred (strs[0] is "_") we slice from 1 to 0.

[alexcrichton]

Instead of special casing _ it looks like this sub-slice needs to be a little more robust, could this just check for ending with ,) and have two different branches?

[Munksgaard]

Sure. Something like this? Is it even possible to have strs[0] == "(foo)" ?

if strs[0].ends_with(",)") {
    strs[0][1 .. strs[0].len() - 2] // Remove '(' and ',)'
} else {
    strs[0]
}

[alexcrichton]

Yeah that should be fine.