Fix E0519: Private Type in Function Return Type

Sun, 26 Apr 2026 03:08:04 +0800

1. Symptoms

Error E0519 is a Rust compiler error that appears when you define a public function but the function’s return type references a type that is not accessible outside its defining module. The compiler prevents this because it would create an unusable API—callers would have no way to use the return value if they cannot see its type.

The error message in Rust looks like this:

error[E0519]: private type `InnerStruct` in return type of this function
 --> src/lib.rs:9:5
 |
9 | pub fn get_inner() -> InnerStruct {
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ private type in return type
 |
note: `InnerStruct` is private to this module
 --> src/lib.rs:1:1
 |
1 | struct InnerStruct {
 | ^^^^^^^^^^^^^^^^^^ private struct

The compiler explicitly tells you which type is private and where it is defined. In this example, InnerStruct is marked struct (implicitly private) at line 1, but get_inner() is marked pub fn at line 9. This combination is invalid because external code cannot construct or interact with the return value.

Privacy on ErrorVault — Developer Error Code Dictionary

Fix E0519: Private Type in Function Return Type

1. Symptoms