stackable_versioned_macros/

lib.rs

use darling::{FromMeta, ast::NestedMeta};
#[cfg(doc)]
use kube::core::conversion::ConversionReview;
use proc_macro::TokenStream;
use syn::{Error, Item, spanned::Spanned};

use crate::{attrs::module::ModuleAttributes, codegen::module::Module};

#[cfg(test)]
mod test_utils;

mod attrs;
mod codegen;
mod utils;

/// This macro enables generating versioned structs and enums.
///
/// In this guide, code blocks usually come in pairs. The first code block
/// describes how the macro is used. The second expandable block displays the
/// generated piece of code for explanation purposes. It should be noted, that
/// the exact code can diverge from what is being depicted in this guide. Most
/// code is heavily simplified. For example, `#[automatically_derived]` and
/// `#[allow(deprecated)]` are removed in most examples to reduce visual clutter.
///
/// <div class="warning">
///
/// It is **important** to note that this macro must be placed before any other
/// (derive) macros and attributes. Macros supplied before the versioned macro
/// will be erased, because the original struct, enum or module (container) is
/// erased, and new containers are generated. This ensures that the macros and
/// attributes are applied to the generated versioned instances of the
/// container.
///
/// </div>
///
/// # Version Declarations
///
/// Before any of the fields or variants can be versioned, versions need to be
/// declared at the module level. Each version currently supports two
/// parameters: `name` and the `deprecated` flag. The `name` must be a valid
/// (and supported) format.
///
/// <div class="warning">
///
/// Currently, only [Kubernetes API versions][k8s-version-format] are supported.
/// The macro checks each declared version and reports any error encountered
/// during parsing.
///
/// </div>
///
/// It should be noted that the defined struct always represents the **latest**
/// version, eg: when defining three versions `v1alpha1`, `v1beta1`, and `v1`,
/// the struct will describe the structure of the data in `v1`. This behaviour
/// is especially noticeable in the [`changed()`](#changed-action) action which
/// works "backwards" by describing how a field looked before the current
/// (latest) version.
///
/// TODO: Version declarations should eventually be moved back to containers.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(version(name = "v1alpha1"))]
/// mod versioned {
///     struct Foo {
///         bar: usize,
///     }
/// }
/// ```
///
/// <details>
/// <summary>Generated code</summary>
///
/// 1. The `#[automatically_derived]` attribute indicates that the following
///    piece of code is automatically generated by a macro instead of being
///    handwritten by a developer. This information is used by cargo and rustc.
/// 2. For each declared version, a new module containing the containers is
///    generated. This enables you to reference the container by versions via
///    `v1alpha1::Foo`.
/// 3. This `use` statement gives the generated containers access to the imports
///    at the top of the file. This is a convenience, because otherwise you
///    would need to prefix used items with `super::`. Additionally, other
///    macros can have trouble using items referred to with `super::`.
///
/// ```ignore
/// #[automatically_derived] // 1
/// mod v1alpha1 {           // 2
///     use super::*;        // 3
///     pub struct Foo {
///         bar: usize,
///     }
/// }
/// ```
/// </details>
///
/// ## Version Deprecation
///
/// The `deprecated` flag marks the version as deprecated. This currently adds
/// the `#[deprecated]` attribute to the appropriate piece of code. In the
/// future, this will additionally mark the CRD version with `deprecated: true`.
/// See the official docs on [version deprecation][k8s-crd-ver-deprecation].
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(version(name = "v1alpha1", deprecated))]
/// mod versioned {
///     struct Foo {
///         bar: usize,
///     }
/// }
/// ```
///
/// <details>
/// <summary>Generated code</summary>
///
/// 1. The `deprecated` flag will generate a `#[deprecated]` attribute and the
///    note is automatically generated.
///
/// ```ignore
/// #[automatically_derived]
/// #[deprecated = "Version v1alpha1 is deprecated"] // 1
/// mod v1alpha1 {
///     use super::*;
///     pub struct Foo {
///         pub bar: usize,
///     }
/// }
/// ```
/// </details>
///
/// ## Version Sorting
///
/// Additionally, it is ensured that each version is unique. Declaring the same
/// version multiple times will result in an error. Furthermore, declaring the
/// versions out-of-order is prohibited by default. It is possible to opt-out
/// of this check by setting `options(allow_unsorted)`.
///
/// <div class="warning">
///
/// It is **not** recommended to use this setting and instead use sorted versions
/// across all versioned items.
///
/// </div>
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(
///     version(name = "v1beta1"),
///     version(name = "v1alpha1"),
///     options(allow_unsorted)
/// )]
/// mod versioned {
///     struct Foo {
///         bar: usize,
///     }
/// }
/// ```
///
/// # Versioning Module
///
/// The purpose of the macro is to version Kubernetes CustomResourceDefinitions
/// (CRDs). As such, the design and how it works is focused on defining and
/// versioning these CRDs. These CRDs are defined as a top-level struct which
/// can them self contain many sub structs.
///
/// To be able to maximize the visibility on items comprising the CRD, the macro
/// needs to be applied to module blocks. The name of the module can be freely
/// chosen. Throughout this guide, the name `versioned` is used. The module is
/// erased in the generated code. This behaviour can however be
/// [customized](#preserve-module).
///
/// TODO: Mention visibility of module
///
/// ## Preserve Module
///
/// The previous examples completely replaced the `versioned` module with
/// top-level version modules. This is the default behaviour. Preserving the
/// module can however be enabled by setting the `preserve_module` flag.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(
///     version(name = "v1alpha1"),
///     version(name = "v1"),
///     options(preserve_module)
/// )]
/// mod versioned {
///     struct Foo {
///         bar: usize,
///     }
///
///     struct Bar {
///         baz: String,
///     }
/// }
/// ```
///
/// ## Crate Overrides
///
/// Override the import path of specific crates which is especially useful if
/// the crates are brought into scope through re-exports. The following code
/// block depicts supported overrides and their default values.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(
///     version(name = "v1alpha1"),
///     version(name = "v1beta1"),
///     crates(
///         versioned = "::stackable_versioned",
///         kube_client = "::kube::client",
///         k8s_openapi = "::k8s_openapi",
///         serde_json = "::serde_json",
///         kube_core = "::kube::core",
///         schemars = "::schemars",
///         serde = "::serde",
///     )
/// )]
/// mod versioned {
///     // ...
/// }
/// ```
///
/// ## Additional Options
///
/// This section contains optional options which influence parts of the code
/// generation.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(
///     version(name = "v1alpha1"),
///     version(name = "v1beta1"),
///     options(k8s(
///         // Highly experimental conversion tracking. Opting into this feature will
///         // introduce frequent breaking changes.
///         experimental_conversion_tracking,
///         // Enables instrumentation and log events via the tracing crate.
///         enable_tracing,
///     ))
/// )]
/// mod versioned {
///     // ...
/// }
/// ```
///
/// ## Merging Submodules
///
/// Modules defined in the versioned module will be re-emitted. This allows for
/// composition of re-exports to compose easier to use imports for downstream
/// consumers of versioned containers. The following rules apply:
///
/// 1. Only modules named the same like defined versions will be re-emitted.
///    Using modules with invalid names will return an error.
/// 2. Only `use` statements defined in the module will be emitted. Declaring
///    other items will return an error.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// # mod a {
/// #     pub mod v1alpha1 {}
/// # }
/// # mod b {
/// #     pub mod v1alpha1 {}
/// # }
/// #[versioned(version(name = "v1alpha1"), version(name = "v1"))]
/// mod versioned {
///     mod v1alpha1 {
///         pub use a::v1alpha1::*;
///         pub use b::v1alpha1::*;
///     }
///
///     struct Foo {
///         bar: usize,
///     }
/// }
/// # fn main() {}
/// ```
///
/// <details>
/// <summary>Expand Generated Code</summary>
///
/// ```ignore
/// mod v1alpha1 {
///     use super::*;
///     pub use a::v1alpha1::*;
///     pub use b::v1alpha1::*;
///     pub struct Foo {
///         pub bar: usize,
///     }
/// }
///
/// mod v1 {
///     use super::*;
///     pub struct Foo {
///         pub bar: usize,
///     }
/// }
/// ```
///
/// </details>
///
/// # CRD Spec Definition
///
/// ## Arguments
///
/// <div class="warning">
///
/// It should be noted that not every `#[kube]` argument is supported or
/// forwarded without changes.
///
/// </div>
///
/// Structs annotated with `#[versioned(crd()]` are treated as top-level CRD
/// spec definitions. This section lists all currently supported arguments.
/// Most of these arguments are directly forwarded to the underlying `#[kube]`
/// attribute. Some arguments are specific to this macro and don't exist in the
/// upstream [`kube`] crate.
///
/// ```
/// # use kube::CustomResource;
/// # use schemars::JsonSchema;
/// # use serde::{Deserialize, Serialize};
/// # use stackable_versioned_macros::versioned;
/// #[versioned(version(name = "v1alpha1"), version(name = "v1beta1"))]
/// mod versioned {
///     #[versioned(crd(
///         // **Required.** Set the group of the CRD, usually the domain of the
///         // company, like `example.com`.
///         group = "example.com",
///         // Override the kind field of the CRD. This defaults to the struct
///         // name (without the `Spec` suffix). Overriding this value will also
///         // influence the names of other generated items, like the status
///         // struct (if used) or the version enum.
///         kind = "CustomKind",
///         // Set the singular name. Defaults to lowercased `kind` value.
///         singular = "...",
///         // Set the plural name. Defaults to inferring from singular.
///         plural = "...",
///         // Indicate that this is a namespaced scoped resource rather than a
///         // cluster scoped resource.
///         namespaced,
///         // Set the specified struct as the status subresource. If conversion
///         // tracking is enabled, this struct will be automatically merged into
///         // the generated tracking status struct.
///         status = "FooStatus",
///         // Set a shortname. This can be specified multiple times.
///         shortname = "..."
///     ))]
/// #   #[derive(Clone, Debug, Deserialize, Serialize, CustomResource, JsonSchema)]
///     pub struct FooSpec {
///         #[versioned(deprecated(since = "v1beta1"))]
///         deprecated_bar: usize,
///         baz: bool,
///     }
/// }
/// # #[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
/// # pub struct FooStatus {}
/// # fn main() {}
/// ```
///
/// ## Field Actions
///
/// This crate currently supports three different item actions. Items can
/// be added, changed, and deprecated. The macro ensures that these actions
/// adhere to the following set of rules:
///
/// 1. Items cannot be added and deprecated in the same version.
/// 2. Items cannot be added and changed in the same version.
/// 3. Items cannot be changed and deprecated in the same version.
/// 4. Items added in version _a_, renamed _0...n_ times in versions
///    b<sub>1</sub>, ..., b<sub>n</sub> and deprecated in
///    version _c_ must ensure _a < b<sub>1</sub>, ..., b<sub>n</sub> < c_.
/// 5. All item actions must use previously declared versions. Using versions
///    not present at the container level will result in an error.
///
/// For items marked as deprecated, one additional rule applies:
///
/// - Fields must start with the `deprecated_` and variants with the
///   `Deprecated` prefix. This is enforced because Kubernetes doesn't allow
///   removing fields in CRDs entirely. Instead, they should be marked as
///   deprecated. By convention this is done with the `deprecated` prefix.
///
/// ### Added Action
///
/// This action indicates that an item is added in a particular version.
/// Available arguments are:
///
/// - `since` to indicate since which version the item is present.
/// - `default` to customize the default function used to populate the item
///   in auto-generated conversion implementations.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(version(name = "v1alpha1"), version(name = "v1beta1"))]
/// mod versioned {
///     pub struct Foo {
///         #[versioned(added(since = "v1beta1"))]
///         bar: usize,
///         baz: bool,
///     }
/// }
/// ```
///
/// <details>
/// <summary>Expand Generated Code</summary>
///
/// 1. The field `bar` is not yet present in version `v1alpha1` and is therefore
///    not generated.
/// 2. Now the field `bar` is present and uses `Default::default()` to populate
///    the field during conversion. This function can be customized as shown
///    later in this guide.
///
/// ```ignore
/// pub mod v1alpha1 {
///     use super::*;
///     pub struct Foo {                 // 1
///         pub baz: bool,
///     }
/// }
///
/// impl From<v1alpha1::Foo> for v1beta1::Foo {
///     fn from(foo: v1alpha1::Foo) -> Self {
///         Self {
///             bar: Default::default(), // 2
///             baz: foo.baz,
///         }
///     }
/// }
///
/// pub mod v1beta1 {
///     use super::*;
///     pub struct Foo {
///         pub bar: usize,              // 2
///         pub baz: bool,
///     }
/// }
/// ```
/// </details>
///
/// #### Custom Default Function
///
/// To customize the default function used in the generated conversion
/// implementations the `added` action provides the `default` argument. It
/// expects a path to a function without braces. This path can for example point
/// at free-standing or associated functions.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(version(name = "v1alpha1"), version(name = "v1beta1"))]
/// mod versioned {
///     pub struct Foo {
///         #[versioned(added(since = "v1beta1", default = "default_bar"))]
///         bar: usize,
///         baz: bool,
///     }
/// }
///
/// fn default_bar() -> usize {
///     42
/// }
/// ```
///
/// <details>
/// <summary>Expand Generated Code</summary>
///
/// 1. Instead of `Default::default()`, the provided function `default_bar()` is
///    used. It is of course fully type checked and needs to return the expected
///    type (`usize` in this case).
///
/// ```ignore
/// // Snip
///
/// impl From<v1alpha1::Foo> for v1beta1::Foo {
///     fn from(foo: v1alpha1::Foo) -> Self {
///         Self {
///             bar: default_bar(), // 1
///             baz: foo.baz,
///         }
///     }
/// }
///
/// // Snip
/// ```
/// </details>
///
/// ### Changed Action
///
/// This action indicates that an item is changed in a particular version. It
/// combines renames and type changes into a single action. You can choose to
/// change the name, change the type or do both. Available arguments are:
///
/// - `since` to indicate since which version the item is changed.
/// - `from_name` to indicate from which previous name the field is renamed.
/// - `from_type` to indicate from which previous type the field is changed.
/// - `upgrade_with` to provide a custom upgrade function. This argument can
///   only be used in combination with the `from_type` argument. The expected
///   function signature is: `fn (OLD_TYPE) -> NEW_TYPE`. This function must
///   not fail.
/// - `downgrade_with` to provide a custom downgrade function. This argument can
///   only be used in combination with the `from_type` argument. The expected
///   function signature is: `fn (NEW_TYPE) -> OLD_TYPE`. This function must
///   not fail.
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(version(name = "v1alpha1"), version(name = "v1beta1"))]
/// mod versioned {
///     pub struct Foo {
///         #[versioned(changed(
///             since = "v1beta1",
///             from_name = "prev_bar",
///             from_type = "u16",
///             downgrade_with = usize_to_u16
///         ))]
///         bar: usize,
///         baz: bool,
///     }
/// }
///
/// fn usize_to_u16(input: usize) -> u16 {
///     input.try_into().unwrap()
/// }
/// ```
///
/// <details>
/// <summary>Expand Generated Code</summary>
///
/// 1. In version `v1alpha1` the field is named `prev_bar` and uses a `u16`.
/// 2. In the next version, `v1beta1`, the field is now named `bar` and uses
///    `usize` instead of a `u16`. The conversion implementations transforms the
///    type automatically.
///
/// ```ignore
/// pub mod v1alpha1 {
///     use super::*;
///     pub struct Foo {
///         pub prev_bar: u16,            // 1
///         pub baz: bool,
///     }
/// }
///
/// impl From<v1alpha1::Foo> for v1beta1::Foo {
///     fn from(foo: v1alpha1::Foo) -> Self {
///         Self {
///             bar: foo.prev_bar.into(), // 2
///             baz: foo.baz,
///         }
///     }
/// }
///
/// pub mod v1beta1 {
///     use super::*;
///     pub struct Foo {
///         pub bar: usize,               // 2
///         pub baz: bool,
///     }
/// }
/// ```
/// </details>
///
/// ### Deprecated Action
///
/// This action indicates that an item is deprecated in a particular version.
/// Deprecated items are not removed. Available arguments are:
///
/// - `since` to indicate since which version the item is deprecated.
/// - `note` to specify an optional deprecation note.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// #[versioned(version(name = "v1alpha1"), version(name = "v1beta1"))]
/// mod versioned {
///     pub struct Foo {
///         #[versioned(deprecated(since = "v1beta1"))]
///         deprecated_bar: usize,
///         baz: bool,
///     }
/// }
/// ```
///
/// <details>
/// <summary>Expand Generated Code</summary>
///
/// 1. In version `v1alpha1` the field `bar` is not yet deprecated and thus uses
///    the name without the `deprecated_` prefix.
/// 2. In version `v1beta1` the field is deprecated and now includes the
///    `deprecated_` prefix. It also uses the `#[deprecated]` attribute to
///    indicate to Clippy this part of Rust code is deprecated. Therefore, the
///    conversion implementations include `#[allow(deprecated)]` to allow the
///    usage of deprecated items in automatically generated code.
///
/// ```ignore
/// pub mod v1alpha1 {
///     use super::*;
///     pub struct Foo {
///         pub bar: usize,                     // 1
///         pub baz: bool,
///     }
/// }
///
/// #[allow(deprecated)]                        // 2
/// impl From<v1alpha1::Foo> for v1beta1::Foo {
///     fn from(foo: v1alpha1::Foo) -> Self {
///         Self {
///             deprecated_bar: foo.bar,        // 2
///             baz: foo.baz,
///         }
///     }
/// }
///
/// pub mod v1beta1 {
///     use super::*;
///     pub struct Foo {
///         #[deprecated]                       // 2
///         pub deprecated_bar: usize,
///         pub baz: bool,
///     }
/// }
/// ```
/// </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`
/// (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
/// around CRD versioning and conversion. The following sections explain these
/// helpers and (some) of the code behind them in detail.
///
/// All these helpers are generated as associated functions on what this macro
/// calls an entry enum. When defining the following three versions: `v1alpha1`,
/// `v1beta1`, and `v1` the following entry enum will be generated:
///
/// ```ignore
/// pub enum Foo {
///     V1Alpha1(v1alpha1::Foo),
///     V1Beta1(v1beta1::Foo),
///     V1(v1::Foo),
/// }
/// ```
///
/// ## Merge CRD Versions
///
/// The generated `merged_crd` method is a wrapper around [kube's `merge_crds`][2]
/// function. It automatically calls the `crd` methods of the CRD in all of its
/// versions and additionally provides a strongly typed selector for the stored
/// API version.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// # use kube::CustomResource;
/// # use schemars::JsonSchema;
/// # use serde::{Deserialize, Serialize};
/// #[versioned(version(name = "v1alpha1"), version(name = "v1beta1"))]
/// mod versioned {
///     #[versioned(crd(group = "example.com"))]
///     #[derive(Clone, Debug, Deserialize, Serialize, CustomResource, JsonSchema)]
///     pub struct FooSpec {
///         #[versioned(added(since = "v1beta1"))]
///         bar: usize,
///         baz: bool,
///     }
/// }
///
/// # fn main() {
/// let merged_crd = Foo::merged_crd(FooVersion::V1Beta1).unwrap();
/// println!("{yaml}", yaml = serde_yaml::to_string(&merged_crd).unwrap());
/// # }
/// ```
///
/// The strongly typed version enum looks very similar to the entry enum
/// described above. It additionally provides various associated functions used
/// for parsing and string representations.
///
/// ```
/// #[derive(Copy, Clone, Debug)]
/// pub enum FooVersion {
///     V1Alpha1,
///     V1Beta1,
/// }
/// ```
///
/// ---
///
/// The generation of merging helpers can be skipped if manual implementation
/// is desired. The following piece of code lists all possible locations where
/// this skip flag can be provided.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// # use kube::CustomResource;
/// # use schemars::JsonSchema;
/// # use serde::{Deserialize, Serialize};
/// #
/// # #[versioned(version(name = "v1alpha1"))]
/// #[versioned(skip(merged_crd))] // Skip generation for ALL specs
/// mod versioned {
///     #[versioned(skip(merged_crd))] // Skip generation for specific specs
///
/// #   #[versioned(crd(group = "example.com"))]
/// #   #[derive(Clone, Debug, CustomResource, Deserialize, Serialize, JsonSchema)]
///     pub struct FooSpec {}
/// }
/// #
/// # fn main() {}
/// ```
///
/// ## Convert CustomResources
///
/// The conversion of CRs is tightly integrated with [`ConversionReview`]s, the
/// payload which a conversion webhook receives from the Kubernetes apiserver.
/// Naturally, the `try_convert` function takes in [`ConversionReview`] as a
/// parameter and also returns a [`ConversionReview`] indicating success or
/// failure.
///
/// ```ignore
/// # use stackable_versioned_macros::versioned;
/// # use kube::CustomResource;
/// # use schemars::JsonSchema;
/// # use serde::{Deserialize, Serialize};
/// #[versioned(
///     version(name = "v1alpha1"),
///     version(name = "v1beta1"),
///     version(name = "v1")
/// )]
/// mod versioned {
///     #[versioned(crd(group = "example.com"))]
///     #[derive(Clone, Debug, Deserialize, Serialize, CustomResource, JsonSchema)]
///     pub struct FooSpec {
///         #[versioned(added(since = "v1beta1"))]
///         bar: usize,
///
///         #[versioned(added(since = "v1"))]
///         baz: bool,
///
///         quox: String,
///     }
/// }
///
/// # fn main() {
/// let conversion_review = Foo::try_convert(conversion_review);
/// # }
/// ```
///
/// The generation of conversion helpers can be skipped if manual implementation
/// is desired. The following piece of code lists all possible locations where
/// this skip flag can be provided:
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// # use kube::CustomResource;
/// # use schemars::JsonSchema;
/// # use serde::{Deserialize, Serialize};
/// #
/// # #[versioned(version(name = "v1alpha1"))]
/// #[versioned(skip(try_convert))] // Skip generation for ALL specs
/// mod versioned {
///     #[versioned(skip(try_convert))] // Skip generation for specific specs
///
/// #   #[versioned(crd(group = "example.com"))]
/// #   #[derive(Clone, Debug, CustomResource, Deserialize, Serialize, JsonSchema)]
///     pub struct FooSpec {}
/// }
/// #
/// # fn main() {}
/// ```
///
/// ### Conversion Tracking
///
/// <div class="warning">
///
/// This is a highly experimental feature. To enable it, provide the
/// `experimental_conversion_tracking` flag. See the
/// [additional options](#additional-options) section for more information.
///
/// </div>
///
/// As per recommendation by the Kubernetes project, conversions should aim to
/// be infallible and lossless. The above example perfectly illustrates that
/// achieving this is not as easy as it looks on the surface. Let's assume the
/// following conditions:
///
/// - The CRD's latest version `v1` is marked as the stored version.
/// - A client requests a CR in an earlier version, in this case `v1alpha1`.
///
/// The Kubernetes apiserver retrieves the stored object in `v1` from etcd.
/// It needs to be downgraded to `v1alpha1` to be able to serve the client the
/// correct requested version. As defined above, the field `baz` was only added
/// in `v1` and `bar` was added in `v1beta1` and as such both don't exist in
/// `v1alpha1`. During the downgrade, the conversion would lose these pieces of
/// data when upgrading to `v1` again after the client did it's changes. This
/// macro however provides a mechanism to automatically track values across
/// conversations without data loss.
///
/// <div class="warning">
///
/// Currently, only tracking of **added** fields is supported. This will be
/// expanded to removed fields, field type changes, and fields containing
/// collections in the future.
///
/// </div>
///
/// There are many moving parts to enable this mechanism to work automatically
/// with minimal manual developer input. Pretty much all of the required code
/// can be generated based on a simple CRD definition described above. The
/// following paragraphs explain various parts of the system in more detail.
/// For a complete overview, it is however advised to look at the source code
/// of the macro and the code it produces.
///
/// #### Tracking Values in the Status
///
/// Tracking changed values across conversions requires state. In this context,
/// storing this state as close to the CustomResource as possible is essential.
/// This is due to various factors such as avoiding external calls (which are
/// susceptible to network errors), reducing the risk of state drift, and
/// making the use of conversions as easy as possible straight out of the box;
/// for both users and cluster administrators.
///
/// As such, values are tracked using the CustomResource's status via the
/// `changedValues` field. This field contains two sections, one for upgrades
/// and one for downgrades. Both of these sections contain version keys which
/// list all tracked values for a particular version.
///
/// ```yaml
/// status:
///   changedValues:
///     downgrades: null
///     upgrades:
///       v1beta1:
///         - fieldName: "bar"
///           value: 42
///       v1:
///         - fieldName: "baz"
///           value: true
/// ```
///
/// To continue the above example, upgrading the CustomResource from `v1alpha1`
/// back to `v1` requires us to re-hydrate the resource with the tracked values.
/// First, the resource is upgraded to `v1beta1`. During this step, the tracked
/// value for the `bar` field is applied and removed from the status afterwards.
/// The final upgrade to `v1` will apply the tracked value for the field `baz`.
/// Again, it is removed from the status afterwards.
///
/// ### Tracking Nested Changes
///
/// To be able to automatically track values of changed fields in nested sub
/// structs of specs, the fields needs to be marked with `#[versioned(nested)]`.
/// This will indicate the macro to generate the appropriate conversion
/// functions.
///
/// ```
/// # use stackable_versioned_macros::versioned;
/// # use kube::CustomResource;
/// # use schemars::JsonSchema;
/// # use serde::{Deserialize, Serialize};
/// #[versioned(
///     version(name = "v1alpha1"),
///     version(name = "v1beta1"),
///     options(k8s(experimental_conversion_tracking))
/// )]
/// mod versioned {
///     #[versioned(crd(group = "example.com"))]
///     #[derive(Clone, Debug, Deserialize, Serialize, CustomResource, JsonSchema)]
///     struct FooSpec {
///         bar: usize,
///
///         // TODO: This technically needs to be combined with a change, but
///         // we want proper, per-container versioning before we add the correct
///         // attributes here.
///         #[versioned(nested)]
///         baz: Baz,
///     }
///
///     #[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
///     struct Baz {
///         quax: String,
///
///         #[versioned(added(since = "v1beta1"))]
///         quox: bool,
///     }
/// }
/// # fn main() {}
/// ```
///
/// # OpenTelemetry Semantic Conventions
///
/// If tracing is enabled, various traces and events are emitted. The fields of
/// these signals follow the general rules of OpenTelemetry semantic conventions.
/// There are currently no agreed-upon semantic conventions for CRD conversions.
/// In the meantime these fields are used:
///
/// | Field | Type (Example) | Description |
/// | :---- | :------------- | :---------- |
/// | `k8s.crd.conversion.converted_object_count` | usize (6) | The number of successfully converted objects sent back in a conversion review |
/// | `k8s.crd.conversion.desired_api_version` | String (v1alpha1) | The desired api version received via a conversion review |
/// | `k8s.crd.conversion.api_version` | String (v1beta1) | The current api version of an object received via a conversion review |
/// | `k8s.crd.conversion.steps` | usize (2) | The number of steps required to convert a single object from the current to the desired version |
/// | `k8s.crd.conversion.kind` | String (Foo) | The kind of the CRD |
///
/// [1]: https://docs.rs/schemars/latest/schemars/derive.JsonSchema.html
/// [2]: https://docs.rs/kube/latest/kube/core/crd/fn.merge_crds.html
/// [k8s-version-format]: https://kubernetes.io/docs/reference/using-api/#api-versioning
/// [k8s-crd-ver-deprecation]: https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definition-versioning/#version-deprecation
#[proc_macro_attribute]
pub fn versioned(attrs: TokenStream, input: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(input as Item);
    versioned_impl(attrs.into(), input).into()
}

fn versioned_impl(attrs: proc_macro2::TokenStream, input: Item) -> proc_macro2::TokenStream {
    // TODO (@Techassi): Think about how we can handle nested structs / enums which
    // are also versioned.

    match input {
        Item::Mod(item_mod) => {
            let module_attributes: ModuleAttributes = match parse_outer_attributes(attrs) {
                Ok(ma) => ma,
                Err(err) => return err.write_errors(),
            };

            let module = match Module::new(item_mod, module_attributes) {
                Ok(module) => module,
                Err(err) => return err.write_errors(),
            };

            module.generate_tokens()
        }
        _ => Error::new(
            input.span(),
            "attribute macro `versioned` can be only be applied to modules",
        )
        .into_compile_error(),
    }
}

fn parse_outer_attributes<T>(attrs: proc_macro2::TokenStream) -> Result<T, darling::Error>
where
    T: FromMeta,
{
    let nm = NestedMeta::parse_meta_list(attrs)?;
    T::from_list(&nm)
}

#[cfg(test)]
mod snapshots {
    use insta::{assert_snapshot, glob};

    use super::*;

    #[test]
    fn pass() {
        // TODO (@Techassi): Re-add skip tests
        let _settings_guard = test_utils::set_snapshot_path().bind_to_scope();

        glob!("../tests/inputs/pass", "*.rs", |path| {
            let formatted = test_utils::expand_from_file(path)
                .inspect_err(|err| eprintln!("{err}"))
                .unwrap();
            assert_snapshot!(formatted);
        });
    }
}