Static TYPE_ALIAS_BOUNDS

static TYPE_ALIAS_BOUNDS: &Lint

The type_alias_bounds lint detects bounds in type aliases.

Example

type SendVec<T: Send> = Vec<T>;

{{produces}}

Explanation

Trait and lifetime bounds on generic parameters and in where clauses of type aliases are not checked at usage sites of the type alias. Moreover, they are not thoroughly checked for correctness at their definition site either similar to the aliased type.

This is a known limitation of the type checker that may be lifted in a future edition. Permitting such bounds in light of this was unintentional.

While these bounds may have secondary effects such as enabling the use of “shorthand” associated type paths¹ and affecting the default trait object lifetime² of trait object types passed to the type alias, this should not have been allowed until the aforementioned restrictions of the type checker have been lifted.

Using such bounds is highly discouraged as they are actively misleading.

---

1. I.e., paths of the form T::Assoc where T is a type parameter bounded by trait Trait which defines an associated type called Assoc as opposed to a fully qualified path of the form <T as Trait>::Assoc. ↩

2. https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes ↩