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docs(stackable-versioned): Add section about hint(...) #1128

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35 changes: 35 additions & 0 deletions crates/stackable-versioned-macros/src/lib.rs
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Expand Up @@ -645,6 +645,41 @@ mod utils;
/// ```
/// </details>
///
/// ## Additional Arguments
///
/// In addition to the field actions, the following top-level field arguments
/// are available:
///
/// ### Hinting Wrapper Types
///
/// With `#[versioned(hint(...))]` it is possible to give hints to the macro
/// that the field contains a wrapped type. Currently, these following hints
/// are supported:
///
/// - `hint(option)`: Indicates that the field contains an `Option<T>`.
/// - `hint(vec)`: Indicates that the field contains a `Vec<T>`.
///
/// These hints are especially useful for generated conversion functions. With
/// these hints in place, the types are correctly mapped using `Into::into`.
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@adwk67 adwk67 Dec 22, 2025

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My understanding is that this is done by using whatever is defined for upgrade_with and downgrade_with on the field itself. Is it worth mentioning that here for clarity?

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@Techassi Techassi Dec 22, 2025

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Both upgrade_with and downgrade_with are meant to give the user complete control of the conversion. There are however cases, where the wrapped type already implements conversions (automatically generated or manually implemented), but the macro cannot reason about the wrapped types. An example:

  • Without hint(vec): Generated code will try to convert a Vec<T> by calling old_field.into(). This will fail, because there is no appropriate From implementation for Vec<T>, but only for T.
  • With hint(vec): The generated code now maps each value of the Vec<T>, which will result in the correct Vec<U> afterwards.

Currently, both upgrade_with/downgrade_with and hint kind of "conflict" with each other/produce unexpected code. I've already raised an improvement for this. I think the expected behaviour should be like this:

  • No hint, only upgrade_with/downgrade_with: The conversion function(s) need to deal with the complete type.
  • With hint and upgrade_with/downgrade_with: The inner values are mapped using the defined conversion functions instead of the default Into::into.

Do you want any of this mentioned in the docs?

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@adwk67 adwk67 Dec 22, 2025 •
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What confused me was having nested versioning, whereby I have upgrade_with/downgrade_with on the field itself, and a second conversion (instead of a hint) that basically called those functions in a manual mapping step in the "parent" struct. So it would be a case of I'm now using hint (on the parent), implicitly calling upgrade_with/downgrade_with on the child. At least, I think that is what is happening.

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@adwk67 adwk67 Dec 22, 2025

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(Specifically, upgrade_with/downgrade_with on a field in the gitsync struct, and a hint on the the parent struct that includes the gitsync struct as a field).

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@Techassi Techassi Dec 22, 2025 •
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What confused me was having nested versioning, whereby I have upgrade_with/downgrade_with on the field itself, and a second conversion (instead of a hint) that basically called those functions in a manual mapping step in the "parent" struct.

That's what I basically described above, because the macro cannot reason about wrapped types. Therefor it would complain about missing From impls, which then led you to implement manual conversions, even though they weren't needed (with the usage of hint).

I'm now using hint (on the parent), implicitly calling upgrade_with/downgrade_with on the child. At least, I think that is what is happening.

Both these functions are not needed/currently used by hint. Here is an example of the generated code:

impl From<v1alpha1::AirflowClusterConfig> for v1alpha2::AirflowClusterConfig {
    fn from(__sv_airflowclusterconfig: v1alpha1::AirflowClusterConfig) -> Self {
        Self {
            dags_git_sync: __sv_airflowclusterconfig
                .dags_git_sync
                .into_iter()
                .map(Into::into)
                .collect(),
            // Other fields ...
        }
    }
}

You can expand this code yourself by either using cargo-expand or via rust-analyzer by placing your caret on the top-level #[versioned] macro, opening the command prompt (usually Ctrl+Shift+P in VS Code), and then selecting rust-analyzer: Expand macro recursively at caret.

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@adwk67 adwk67 Dec 23, 2025

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After we discussed this yesterday, I would maybe go with something like this:

With these hints in place, the types are correctly mapped using Into::into (assuming the necessary From trait methods are implemented on the target types for the conversion to be done correctly).

///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(
/// version(name = "v1alpha1"),
/// version(name = "v1beta1")
/// )]
/// mod versioned {
/// pub struct Foo {
/// #[versioned(
/// changed(since = "v1beta1", from_type = "Vec<usize>"),
/// hint(vec)
/// )]
/// bar: Vec<usize>,
/// baz: bool,
/// }
/// }
/// ```
///
/// # Generated Helpers
///
/// This macro generates a few different helpers to enable different operations
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