Rust Error E0747: Mismatched Generic Argument Order

Understanding Rust Error E0747

Rust Error E0747 is encountered when generic arguments are not provided in the same order as the corresponding generic parameters are declared. This can lead to confusion and incorrect associations between the type and lifetime arguments.

Erroneous code example

struct S<'a, T>(&'a T);

type X = S<(), 'static>; // error: the type argument is provided before the lifetime argument

Here, the generic argument order is incorrect, as the type argument is provided before the lifetime argument. This causes the Rust compiler to emit the E0747 error.

Fixing the error

To fix the error, the argument order should be changed to match the parameter declaration order. Update the code as follows:

struct S<'a, T>(&'a T);

type X = S<'static, ()>; // ok

Here, the correct order of generic arguments is provided, allowing the Rust compiler to correctly associate the type and lifetime arguments.

Adding parameters

In cases with multiple generic arguments, providing the arguments in the correct order is crucial. Here's an example:

struct S<'a, T, U>(&'a T, U);

type Y = S<'static, (), u32>; // ok

This code has three generic parameters, and their arguments are provided in the correct order, avoiding the E0747 error.

Conclusion