The actual reason why let … in syntax tends to not use C-style “type var” like syntax is because it’s derived from the syntax type theory uses, and type theorists know about parameterised types. Generics, in C++ parlance, excuse my Haskell:

let foo :: Map Int String = mempty

We have an empty map, and it maps integers to Strings. We call it foo. Compare:

Map Int String foo = mempty

If nothing else, that’s just awkward to read and while it may be grammatically unambiguous (a token is a name if it sits directly in front of =) parser error messages are going to suck. Map<Int,String> is also awkward but alas that’s what we’re stuck with in Rust because they reasoned that it would be cruel to put folks coming from C++ on angle bracket withdrawal. Also Rust has ML ancestry don’t get me started on their type syntax.