Why are unused type parameters on Rust structs considered an error and not a warning?

Question

Before somebody marks this as a duplicate, I know how to fix it (and that has been answered already), but I'd like to understand why this limitation exists, and I found no answer on here or in the Rust docs.

For example, I wrote something that goes something like this:

struct ItemList<T> {
  items: Vec<T>
}
impl<T> ItemList<T> {
  fn getFirstItem(&self) -> Link<T> { Link { position: 0 } }
}

struct Link<T> {
  position: usize
}
impl<T> Link<T> {
  fn getFromList<'a>(&self, list: &'a ItemList<T>) -> &'a T {
    &list.items[self.position]
  }
}

But rustc rejects my code with this error:

error[E0392]: parameter `T` is never used
 --> src/main.rs:8:13
  |
8 | struct Link<T> {
  |             ^ unused parameter
  |
  = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
  = help: if you intended `T` to be a const parameter, use `const T: usize` instead

For more information about this error, try `rustc --explain E0392`.

Why is that an error and not a warning? The type parameters only reduce the performance during compile time (if I understood it correctly), so why would you enforce removing it or using a PhantomData marker? Does it have some implications that I missed?

Answer 1

Adding to what @mcarton said, this was not this way from the beginning: Rust used to infer unused generic parameters are bivariant. This changed in 2014 (before Rust 1.0). The motivation is explained in the RFC:

Motivation

Today, variance inference for lifetimes includes the notion of bivariance -- which essentially amounts to unconstrained. In principle, this can have some use, but in practice it tends to be a vector for bugs. In fact, there is no known Rust code that intentionally uses bivariance (though there seems to be plenty that does so accidentally and incorrectly). This RFC proposes that we simply make an inference result of bivariance an error.

As an example of where this comes up, imagine a struct with a "phantom" lifetime parameter, meaning one that is not actually used in the fields of the struct itself. One example of such a type is Items, the vector iterator:

struct Items<'vec, T> {
    x: *mut T
}

Here the lifetime 'vec is intended to represent the lifetime of the vector being iterated over and hence to prevent the iterator from outliving the container. However, because it does not appear in the body of Items at all, the compiler would currently consider it irrelevant to subtyping. This means that you could convert from a Items<'a, T> to a Items<'static, T>, causing the iterator to outlive the container it is iterating over.

To prevent this scenario, the actual definition of the iterator in the standard library uses a marker type. The marker type informs the compiler that, although 'vec does not appear to be used, it should act as if it were. For example, Items might be modified as follows:

struct Items<'vec, T> {
    x: *mut T,
    marker: marker::CovariantType<&'vec T>,
}

the CovariantType marker basically informs the compiler that it should act "as though" a reference of type &'vec T were a member of Items, even thought it is not. Another equivalent option here would be ContravariantLifetime.

Currently, the user must know to insert these markers or else silently get the wrong behavior. This RFC makes it an error to have a type or lifetime parameter that is not (transitively) used somewhere in the type. Nothing else is changed.

The code is pretty old (e.g. CovariantType and ContravariantLifetime were replaced by PhantomData), but the concept still applies.