Understanding Rust Error E0375

Introduction to Rust Error E0375

In Rust, an error that developers may encounter is Rust Error E0375. This error occurs when implementing CoerceUnsized on a struct containing more than one field with an unsized type. An unsized type is any type that the compiler doesn't know the length or alignment of at compile time. Any struct containing an unsized type is also unsized.

Rust Error E0375 Example

Consider the following erroneous code example that triggers Rust Error E0375:

#![allow(unused)]
#![feature(coerce_unsized)]
fn main() {
  use std::ops::CoerceUnsized;

  struct Foo<T: ?Sized, U: ?Sized> {
      a: i32,
      b: T,
      c: U,
  }

  // error: Struct `Foo` has more than one unsized field.
  impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
}

In this code, struct Foo has two unsized type fields, T and U. CoerceUnsized is implemented on Foo, leading to Rust Error E0375 because there are more than one unsized type fields.

Fixing Rust Error E0375

To resolve Rust Error E0375, use only one unsized type field in the struct, and ensure that it is the last field in the struct. If needed, utilize multiple structs to manage each unsized type field separately. Here's a corrected version of the code:

#![allow(unused)]
#![feature(coerce_unsized)]
fn main() {
  use std::ops::CoerceUnsized;

  struct Foo<T: ?Sized> {
      a: i32,
      b: T,
  }

  impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
      where T: CoerceUnsized<U> {}

  fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
      Foo { a: 12i32, b: t } \// use coercion to get the `Foo<U>` type
  }
}

This revised code implements struct Foo with a single unsized type field, T. The CoerceUnsized implementation now correctly works and does not trigger Rust Error E0375.