Comment by simonask
Being a langitem means that the compiler can use it in desugaring, and `for ... in ...` happens to be syntactic sugar in Rust, just like `for (auto x: y) {}` is in C++. `Range` et al are also compiler builtins, because `a..b` desugars to that type. It doesn't imply that the type itself is a compiler builtin, for example.
Another example is the `Deref` trait, which roughly corresponds to `operator->()` or `operator*()`, or an implicit reference conversion. It is called implicitly so you can use `smart_ptr.foo` without special syntax for dereferencing.
I think I fundamentally don't understand why any of this is a problem?
I think maybe your fundamental misunderstanding was just a misreading of what I actually wrote?
I don't like @Nullable (or the ? syntax used in C# for example) because they're a special case magic. They handle exactly one narrow idea (Tony's Billion Dollar Mistake, the "null" pointer) and nothing else.
I prefer sum types - both Option<T> and T | null are syntax for sum types. The former, which Rust has, is an explicit generic type, while the latter is an ad hoc typing feature. I don't have a strong preference between them.