Non-tuple path parameters in axum

[srkaj]

We (me and @lisasvenssonsr) have noticed a discrepancy in how url parameters are detected for the openapi specification. If we use a tuple, the arguments are auto-detected, but if we use a single parameeter, we seem to need an annotation that I think should no longer be needed because of #1224.

These endpoints work as expected:

#[utoipa::path(
    get, path = "/{id}",
    params(MyId),  // This is still needed?
)]
async fn one(
    Path(id): Path<MyId>,
) -> Result<Json<MyDTO>> {
    // ...
}

#[utoipa::path(
    get, path = "/{id}/with/{subid}", // No "params" needed here!
)]
async fn other(
    Path((id, subid)): Path<(MyId, SubId)>,
) -> Result<Json<MyDTO>> {
    // ...
}

Having a single argument and no "params" annotation, like this endpoint, also works as expected in axum, except it does not get any parameters in the openapi specification / swagger guide:

#[utoipa::path(
    get, path = "/{id}",
)]
async fn one(
    Path(id): Path<MyId>,
) -> Result<Json<MyDTO>> {
    // ...
}

As a workaround, it seems wrapping the parameter in a single-value tuple is enogh to get it to work:

#[utoipa::path(
    get, path = "/{id}",
)]
async fn one(
    Path((id,)): Path<(MyId,)>,
) -> Result<Json<MyDTO>> {
    // ...
}

Are we missing anything? Should there be another trait implemented for MyId that is somehow implemented for the tuple? Or is this a bug in utoipa?

Versions used

According to our Cargo.toml:

axum = "0.8.1"
utoipa = { version = "5.2.0", features = ["axum_extras", "openapi_extensions", "preserve_order", "uuid"] }
utoipa-axum = "0.2.0"
utoipa-swagger-ui = { version="9.0.0", features = ["axum"] }