pub fn merge_type(&self) -> WeldResult<Type> {
        use self::Type::Builder;
        if let Builder(ref kind, _) = *self {
            Ok(kind.merge_type())
        } else {
            compile_err!("merge_type called on non-builder type {}", self)
        }
    }
Returns the type merged into a builder.

Returns an error if this `Type` is not a builder type.
weld/src/ast/ast.rs