Rust error E0446: private type `Bar` in public interface

What it meansA private type or trait was used in a public associated type signature.

What rustc prints

error[E0446]: private type `Bar` in public interface
 --> main.rs:8:5
  |
1 | struct Bar;
  | ---------- `Bar` declared as private
...
8 |     type Alias = Bar; // error private type in public interface
  |     ^^^^^^^^^^ can't leak private type

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.

E0446Types and traits

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

A private type or trait was used in a public associated type signature.

Erroneous code example:

struct Bar;

pub trait PubTr {
    type Alias;
}

impl PubTr for u8 {
    type Alias = Bar; // error private type in public interface
}

fn main() {}

There are two ways to solve this error. The first is to make the public type signature only public to a module that also has access to the private type. This is done by using pub(crate) or pub(in crate::my_mod::etc) Example:

struct Bar;

pub(crate) trait PubTr { // only public to crate root
    type Alias;
}

impl PubTr for u8 {
    type Alias = Bar;
}

fn main() {}

The other way to solve this error is to make the private type public. Example:


pub struct Bar; // we set the Bar trait public

pub trait PubTr {
    type Alias;
}

impl PubTr for u8 {
    type Alias = Bar;
}

fn main() {}

From rustc --explain E0446 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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Rust error E0446: private type `Bar` in public interface

What rustc prints

The compiler team's explanation

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