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Understanding Rust Error E0055 - Recursion Limit in Auto-Dereferencing

Recursion Limit and Auto-Dereferencing in Rust - E0055

Rust error E0055 occurs when the compiler reaches the recursion limit while auto-dereferencing values. This can be resolved by increasing the recursion limit or refactoring the code to break the infinite recursion.

Insight into Rust Error E0055

To understand the error, consider the official example given:

#![recursion_limit="4"]

struct Foo;

impl Foo {
    fn foo(&self) {}
}

fn main() {
    let foo = Foo;
    let ref_foo = &&&&&Foo;

    // error, reached the recursion limit while auto-dereferencing `&&&&&Foo`
    ref_foo.foo();
}

In this example, the recursion limit is set to 4 using the recursion_limit attribute, and the structure Foo has a method foo(). During the method call ref_foo.foo(), Rust automatically dereferences the value of ref_foo as many times as needed to match the type of the method's receiver. However, the compiler will only attempt to dereference up to the recursion limit, which in this case is 4. Since the type of ref_foo is `&&&&&Foo`, it requires 5 dereferences to match the type of the method's receiver. This causes the E0055 error as the recursion limit is exceeded.

Solutions to Fix Rust Error E0055

1. Increase the recursion limit: This can be done using the recursion_limit attribute. This only offers a temporary solution and may not be the best long-term fix. Moreover, it may result in increased memory usage.

Example:

#![recursion_limit="5"]

struct Foo;

impl Foo {
    fn foo(&self) {}
}

fn main() {
    let foo = Foo;
    let ref_foo = &&&&&Foo;

    ref_foo.foo(); // No error since recursion_limit is set to 5
}

2. Refactor the code to break infinite recursion: This involves carefully analyzing your code to identify areas where you can break the loop. This might require you to rewrite certain parts or restructure your code.

Conclusion

Rust error E0055 occurs when the recursion limit is exceeded during auto-dereferencing. It can be resolved either by increasing the recursion limit or by refactoring the code to break infinite recursion. Understanding these concepts and how they apply in your projects is crucial for writing efficient Rust code.

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