Understanding Rust Error E0109

Introduction to Rust Error E0109

Rust Error E0109 occurs when you try to provide a generic argument to a type that does not require it. This article will explain the error's cause, show some erroneous code examples, and demonstrate the correct approach to fix the problem.

Erroneous Code Examples

Let's take a look at some examples that cause Rust Error E0109:

fn main() {
    type X = u32<i32>; // error: type arguments are not allowed for this type
    type Y = bool<'static>; // error: lifetime parameters are not allowed on
                            //        this type

In the above example, we attempt to use generic type arguments for u32 and bool types. However, these types do not take generic arguments, and the code will result in Rust Error E0109.

Fixing the Error

To fix Rust Error E0109, we need to ensure that we use the correct types without unnecessary generic arguments. The corrected version of the previous example would look like this:

fn main() {
    type X = u32; // ok!
    type Y = bool; // ok!

Importance of Proper Placement of Generic Arguments

It is worth noting that the correct placement of generic arguments is crucial. For example, when working with an enum variant, the generic arguments should be placed after the variant, not the enum. In the case of the Option enum, the proper usage would be Option::None::<u32>, instead of Option::<u32>::None.


Rust Error E0109 is caused by attempting to use generic arguments with types that do not require them. Ensuring you are using the correct types without extraneous arguments, and placing generic arguments in the appropriate position for enums, will help avoid this error.

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