compiler/rustc_error_codes/src/error_codes/E0374.md

   1 `CoerceUnsized` was implemented on a struct which does not contain a field with
   2 an unsized type.
   3
   4 Example of erroneous code:
   5
   6 ```compile_fail,E0374
   7 #![feature(coerce_unsized)]
   8 use std::ops::CoerceUnsized;
   9
  10 struct Foo<T: ?Sized> {
  11     a: i32,
  12 }
  13
  14 // error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
  15 impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
  16     where T: CoerceUnsized<U> {}
  17 ```
  18
  19 An [unsized type][1] is any type where the compiler does not know the length or
  20 alignment of at compile time. Any struct containing an unsized type is also
  21 unsized.
  22
  23 [1]: https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait
  24
  25 `CoerceUnsized` is used to coerce one struct containing an unsized type
  26 into another struct containing a different unsized type. If the struct
  27 doesn't have any fields of unsized types then you don't need explicit
  28 coercion to get the types you want. To fix this you can either
  29 not try to implement `CoerceUnsized` or you can add a field that is
  30 unsized to the struct.
  31
  32 Example:
  33
  34 ```
  35 #![feature(coerce_unsized)]
  36 use std::ops::CoerceUnsized;
  37
  38 // We don't need to impl `CoerceUnsized` here.
  39 struct Foo {
  40     a: i32,
  41 }
  42
  43 // We add the unsized type field to the struct.
  44 struct Bar<T: ?Sized> {
  45     a: i32,
  46     b: T,
  47 }
  48
  49 // The struct has an unsized field so we can implement
  50 // `CoerceUnsized` for it.
  51 impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
  52     where T: CoerceUnsized<U> {}
  53 ```
  54
  55 Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
  56 and `Arc` to be able to mark that they can coerce unsized types that they
  57 are pointing at.