Understanding Rust Error E0632
What is Rust Error E0632?
Rust Error E0632 occurs when an explicit generic argument is provided when calling a function that uses impl Trait in argument position. The solution is to either let the compiler infer the generic arguments at the call site or change the function definition to use explicit generic type parameters instead of impl Trait.
Erroneous code example:
fn foo<T: Copy>(a: T, b: impl Clone) {}
foo::<i32>(0i32, "abc".to_string());Explaining the error:
In the erroneous code example, the function foo is defined with a generic type T and an impl Trait type for the second parameter. The compiler expects to infer the type of T when calling foo, but the code explicitly provides the generic argument i32, causing the E0632 error.
Correcting the error:
You can correct the error by updating the function call without explicitly providing the generic argument or by changing the function definition to use explicit generic type parameters. Here's the corrected code example:
fn foo<T: Copy>(a: T, b: impl Clone) {}
fn bar<T: Copy, U: Clone>(a: T, b: U) {}
foo(0i32, "abc".to_string());
bar::<i32, String>(0i32, "abc".to_string());
bar::<_, _>(0i32, "abc".to_string());
bar(0i32, "abc".to_string());In the corrected code, either the compiler infers the types or the function definition uses explicit generic type parameters. All three bar function calls are valid ways to handle the types without causing the E0632 error.