Build error with schema name "Value"

[HollayHorvath]

When I have a schema component with the name Value the #[derive(OpenApi)] produces unparsable tokens:

error: expected expression, found `,`
 --> src/main.rs:8:10
  |
8 | #[derive(OpenApi)]
  |          ^^^^^^^ expected expression
  |
  = note: this error originates in the derive macro `OpenApi` (in Nightly builds, run with -Z macro-backtrace for more info)

error: expected one of `.`, `;`, `?`, `else`, or an operator, found `)`
 --> src/main.rs:8:10
  |
8 | #[derive(OpenApi)]
  |          ^^^^^^^ expected one of `.`, `;`, `?`, `else`, or an operator
  |
  = note: this error originates in the derive macro `OpenApi` (in Nightly builds, run with -Z macro-backtrace for more info)

error: proc-macro derive produced unparsable tokens
 --> src/main.rs:8:10
  |
8 | #[derive(OpenApi)]
  |          ^^^^^^^

The repro itself is shorter than the error:

use utoipa::{OpenApi, ToSchema};

#[derive(ToSchema)]
struct Value {
    val: &'static str,
}

#[derive(OpenApi)]
#[openapi(components(schemas(Value)))]
struct Api;

fn main() {
    println!("Hello, world!");
}

Renaming the Value struct fixes the issue.

The error is quite hard to decipher as the error reported has nothing to do with the actual problem.
It was working fine under Utoipa 4.x but it's broken under 5.0.

[juhaku]

Utoipa has Value and Object as reserved type names that will clash with other types if named similarly. Probably before utoipa has been lenient on that, but now everything needs explicit implementation for ToSchema which might have something to do with the error. This is due to fully support generics.

Value and Object types are used as meta names to indicate if type is just a json value or plain OpenAPI object. Changing this behavior would be a breaking change and needs further assessment what could be reasonable alternative for current behavior.

[ChristianBeilschmidt]

I have the same error, but in my repo a rather… maximal example.
Do you have any idea how to get to the root cause or debug it?

[ChristianBeilschmidt]

Update: It seems that not only Object and Value lead to this cryptic error.
DateTime is also a forbidden struct word.

[d-k-bo]

When using utoipa_axum::routes!() this error is completely silent, your custom Value component isn't included and is silently replaced with SchemaType::AnyValue. I think this special case should at least be explicitly documented, so that there aren't any unexpected situations.

For now, using a simple type alias circumvents the problem.