fn ascribe_expr(&mut self) -> WeldResult<Box<Expr>> {
        let mut expr = self.apply_expr()?;
        if *self.peek() == TColon {
            expr.ty = self.optional_type()?;
        }
        Ok(expr)
    }
Parse a type abscription expression such as 'e: T', or lower-level ones in precedence.
weld/src/syntax/parser.rs