Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0375: implementing `CoerceUnsized` does not allow multiple fields to be coerced
What it meansCoerceUnsized or DispatchFromDyn was implemented on a struct which contains more than one field that is being unsized.
What rustc prints
error[E0375]: implementing `CoerceUnsized` does not allow multiple fields to be coerced --> main.rs:11:12 | 11 | impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {} | ^^^^^^^^^^^^^^^^^^^^^^^^ | note: the trait `CoerceUnsized` may only be implemented when a single field is being coerced --> main.rs:6:5 | 6 | b: T, | ^^^^ 7 | c: U, | ^^^^
Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.
The compiler team's explanation
CoerceUnsized or DispatchFromDyn was implemented on a struct which contains more than one field that is being unsized.
Erroneous code example:
#![feature(coerce_unsized)]
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> {}CoerceUnsized is used to coerce structs that have a field that can be unsized, like a custom MyBox<T> being unsized to MyBox<dyn Trait>. DispatchFromDyn is used to dispatch from MyBox<dyn Trait> to MyBox<Self> in a dyn-compatible trait.
If the struct has multiple fields that must be unsized, then the compiler has no way to generate a valid implementation of CoerceUnsized or DispatchFromDyn.
Note that CoerceUnsized and DispatchFromDyn is mainly used by smart pointers like Box, Rc and Arc to be able to mark that they can coerce unsized types that they are pointing at.
From rustc --explain E0375 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.
Related errors
- E0308mismatched types
- E0277the trait bound is not satisfied
- E0599no method named X found in the current scope
- E0433failed to resolve
- E0371the object type `(dyn Baz + 'static)` automatically implements the trait `Bar`
- E0374implementing `CoerceUnsized` requires a field to be coerced
- E0377the trait `CoerceUnsized` may only be implemented for a coercion between structures
- E0378the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, ZST fields with 1 byte alignment that don't mention type/const generics, and nothing else
Up next
Often hit nextE0308: mismatched typesA related error from the same family.Read it next →Did this answer your question?