[wisam]

I don't know about Rust; but in ML family, everything in top level is a binding. Whether it's a variable, function, type or structure.

SML is explicit about it and its syntax makes it clear what type of binding is going on.

val x = 5

fun f = fn x => x * x

datatype weekend = Sat | Sun

So while on top level not everything is an expression, everything on the RHS is an expression.

[qwertyuiop924]

In rust, assignment isn't an expression, but everything else is.