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//! A macro that generates new structs from the original with each field made optional.
//!
//! # Example
//!
//! ## Basic
//!
//! Add the `#[optional]` and `#[nested]` attributes as follows:
//!
//! ```
//! use umbra::optional;
//!
//! #[optional]
//! #[derive(Default)]
//! struct Foo {
//! id: u32,
//! name: String,
//! #[nested]
//! bar: Bar,
//! }
//!
//! #[optional]
//! #[derive(Default)]
//! struct Bar {
//! name: String,
//! value: Option<i32>,
//! }
//! ```
//!
//! The macro generates following structs:
//!
//! ```
//! # #[derive(Default)]
//! # struct Foo {
//! # id: u32,
//! # name: String,
//! # bar: Bar,
//! # }
//! #
//! # #[derive(Default)]
//! # struct Bar {
//! # name: String,
//! # value: Option<i32>,
//! # }
//! #
//! struct OptionalFoo {
//! id: Option<u32>,
//! name: Option<String>,
//! bar: Option<OptionalBar>,
//! }
//!
//! impl From<OptionalFoo> for Foo {
//! fn from(optional: OptionalFoo) -> Self {
//! let mut base = Self::default(); // create base values by default
//! if let Some(value) = optional.id {
//! base.id = value; // simple field
//! }
//! if let Some(value) = optional.bar {
//! base.bar = value.into(); // nested field
//! }
//! // ...
//! base
//! }
//! }
//!
//! struct OptionalBar {
//! name: Option<String>,
//! value: Option<i32>,
//! }
//!
//! impl From<OptionalBar> for Bar {
//! fn from(optional: OptionalBar) -> Self {
//! let mut base = Self::default();
//! // ...
//! base
//! }
//! }
//! ```
//!
//! ## Derives
//!
//! By using the `derives` attribute, the derive attribute can be added to the generated struct:
//!
//! ```
//! use umbra::optional;
//!
//! #[optional(derives = [Debug, std::clone::Clone])]
//! #[derive(Default)]
//! struct Foo {
//! id: u32,
//! name: String,
//! }
//! ```
//!
//! The macro generates following structs:
//!
//! ```
//! # #[derive(Default)]
//! # struct Foo {
//! # id: u32,
//! # name: String,
//! # }
//! #
//! #[derive(Debug, std::clone::Clone)] // The derive attribute is added
//! struct OptionalFoo {
//! id: Option<u32>,
//! name: Option<String>,
//! }
//!
//! impl From<OptionalFoo> for Foo {
//! fn from(optional: OptionalFoo) -> Self {
//! let mut base = Self::default();
//! // ...
//! base
//! }
//! }
//! ```
//!
//! ## Prefix / Suffix
//!
//! By using the `prefix` and `suffix` attributes, the prefix and suffix of the generated struct are changed:
//!
//! ```
//! use umbra::optional;
//!
//! #[optional(prefix = "Pre", suffix = "Suf")]
//! #[derive(Default)]
//! struct Foo {
//! id: u32,
//! name: String,
//! }
//! ```
//!
//! The macro generates following structs:
//!
//! ```
//! # #[derive(Default)]
//! # struct Foo {
//! # id: u32,
//! # name: String,
//! # }
//! #
//! struct PreFooSuf { // <Prefix><Base name><Suffix> format
//! id: Option<u32>,
//! name: Option<String>,
//! }
//!
//! impl From<PreFooSuf> for Foo {
//! fn from(optional: PreFooSuf) -> Self {
//! let mut base = Self::default();
//! // ...
//! base
//! }
//! }
//! ```
//!
//! ## Visibility
//!
//! By using the `visibility` attribute, the visibility can be added to the generated struct:
//!
//! ```
//! use umbra::optional;
//!
//! #[optional(visibility = pub)]
//! #[derive(Default)]
//! struct Foo {
//! id: u32,
//! name: String,
//! }
//! ```
//!
//! The macro generates following structs:
//!
//! ```
//! #[derive(Default)]
//! struct Foo {
//! id: u32,
//! name: String,
//! }
//!
//! pub struct OptionalFoo { // public
//! id: Option<u32>,
//! name: Option<String>,
//! }
//!
//! impl From<OptionalFoo> for Foo {
//! fn from(optional: OptionalFoo) -> Self {
//! let mut base = Self::default();
//! // ...
//! base
//! }
//! }
//! ```