Indexing using Into<usize>

Im kinda sick of writing \as usize\\ everytime I index a slice, vec, etc.

I don't understand why can't the compiler automatically attempt to cast the expression in \\\[\\]\\ as \usize\\.

I tried implementing it myself but I got some errors I don't really understand:

Error: Type parameter F must be used as the type parameter for some local type

Does anyone know of a comfortable way to get around this?