Understanding Rust Error E0207: Unconstrained Type, Const, or Lifetime Parameter

Introduction to Rust Error E0207

Rust error E0207 occurs when a type, const, or lifetime parameter specified for impl is not constrained. In this article, we will discuss what causes this error, its consequences, and how to fix it.

Causes of Rust Error E0207

Any type or const parameter of an impl must meet at least one of the following criteria: 1. it appears in the implementing type of the impl, e.g. impl<T> Foo<T> 2. for a trait impl, it appears in the implemented trait, e.g. impl<T> SomeTrait<T> for Foo 3. it is bound as an associated type, e.g. impl<T, U> SomeTrait for T where T: AnotherTrait<AssocType=U> An unconstrained lifetime parameter of an impl is not supported if the lifetime parameter is used by an associated type.

Error Example 1

Consider the following erroneous code example: #![allow(unused)] fn main() { struct Foo; impl<T: Default> Foo { // error: the type parameter `T` is not constrained by the impl trait, self // type, or predicates [E0207] fn get(&self) -> T { <T as Default>::default() } } } The cause of this error is that the type parameter `T` does not appear in the implementing type `Foo`. To fix the error, we can move the type parameter from the impl to the method `get`.

Error Example 2

Consider the following erroneous code example: #![allow(unused)] fn main() { trait Maker { type Item; fn make(&mut self) -> Self::Item; } struct Foo<T> { foo: T } struct FooMaker; impl<T: Default> Maker for FooMaker { // error: the type parameter `T` is not constrained by the impl trait, self // type, or predicates [E0207] type Item = Foo<T>; fn make(&mut self) -> Foo<T> { Foo { foo: <T as Default>::default() } } } } This error occurs because `T` does not appear in the trait `Maker` or the implementing type `FooMaker`. One way to fix the error is by introducing a phantom type parameter into `FooMaker`. Another solution is to use an input type parameter instead of the associated type in the `Maker` trait.

Error Example 3

Consider the following erroneous code example: #![allow(unused)] fn main() { struct Foo; impl<const T: i32> Foo { // error: the const parameter `T` is not constrained by the impl trait, self // type, or predicates [E0207] fn get(&self) -> i32 { i32::default() } } } The cause of this error is that the const parameter `T` does not appear in the implementing type `Foo`. To fix the error, we can move the const parameter from the impl to the method `get`.

Error Example 4

Consider the following erroneous code example: #![allow(unused)] fn main() { struct Foo; struct Bar<'a>; trait Contains { type B; fn get(&self) -> i32; } impl<'a> Contains for Foo { type B = Bar<'a>; // error: the lifetime parameter `'a` is not constrained by the impl trait, // self type, or predicates [E0207] fn get(&self) -> i32 { i32::default() } } } The lifetime parameter `'a` is not constrained, and unconstrained lifetime parameters are not supported if they are used by an associated type.

Conclusion

Rust Error E0207 occurs when a type, const, or lifetime parameter specified for impl is not constrained. To fix the error, ensure that the parameters meet the criteria specified earlier in the article, or consider modifying the design of your code to avoid the need for unconstrained parameters.