Add serde_helper module and document a frequent pattern of enums usage

Changelog.md

@@ -12,10 +12,15 @@
 
 ### New Features
 
+- [#601]: Add `serde_helper` module to the crate root with some useful utility
+  functions and document using of enum's unit variants as a text content of element.
+
 ### Bug Fixes
 
 ### Misc Changes
 
+[#601]: https://github.com/tafia/quick-xml/pull/601
+
 
 ## 0.28.2 -- 2023-04-12
 

src/de/mod.rs

@@ -27,6 +27,7 @@
 //!     - [Enums and sequences of enums](#enums-and-sequences-of-enums)
 //! - [Frequently Used Patterns](#frequently-used-patterns)
 //!   - [`<element>` lists](#element-lists)
+//!   - [Enum::Unit Variants As a Text](#enumunit-variants-as-a-text)
 //!
 //!
 //!
@@ -1579,7 +1580,8 @@
 //! Some XML constructs used so frequent, that it is worth to document the recommended
 //! way to represent them in the Rust. The sections below describes them.
 //!
-//! ## `<element>` lists
+//! `<element>` lists
+//! -----------------
 //! Many XML formats wrap lists of elements in the additional container,
 //! although this is not required by the XML rules:
 //!
@@ -1672,9 +1674,79 @@
 //!
 //! Instead of writing such functions manually, you also could try <https://lib.rs/crates/serde-query>.
 //!
+//! Enum::Unit Variants As a Text
+//! -----------------------------
+//! A one frequent task and a typical mistake is to creation of mapping a text
+//! content of some tag to a Rust `enum`. For example, for the XML:
+//!
+//! ```xml
+//! <some-container>
+//!   <field>EnumValue</field>
+//! </some-container>
+//! ```
+//! one could create an _incorrect_ mapping
+//!
+//! ```
+//! # use serde::{Deserialize, Serialize};
+//! #
+//! #[derive(Serialize, Deserialize)]
+//! enum SomeEnum {
+//!     EnumValue,
+//! # /*
+//!     ...
+//! # */
+//! }
+//!
+//! #[derive(Serialize, Deserialize)]
+//! #[serde(rename = "some-container")]
+//! struct SomeContainer {
+//!     field: SomeEnum,
+//! }
+//! ```
+//!
+//! Actually, those types will be serialized into:
+//! ```xml
+//! <some-container>
+//!   <EnumValue/>
+//! </some-container>
+//! ```
+//! and will not be able to be deserialized.
+//!
+//! You can easily see what's wrong if you think about attributes, which could
+//! be defined in the `<field>` tag:
+//! ```xml
+//! <some-container>
+//!   <field some="attribute">EnumValue</field>
+//! </some-container>
+//! ```
+//!
+//! After that you can find the correct solution, using the principles, explained
+//! above. You should wrap `SomeEnum` into wrapper struct under the [`$text`](#text)
+//! name:
+//! ```
+//! # use serde::{Serialize, Deserialize};
+//! # type SomeEnum = ();
+//! #[derive(Serialize, Deserialize)]
+//! struct Field {
+//!     // Use a special name `$text` to map field to the text content
+//!     #[serde(rename = "$text")]
+//!     content: SomeEnum,
+//! }
+//!
+//! #[derive(Serialize, Deserialize)]
+//! #[serde(rename = "some-container")]
+//! struct SomeContainer {
+//!     field: Field,
+//! }
+//! ```
+//!
+//! If you still want to keep your struct untouched, you can instead use the
+//! helper module [`text_content`].
+//!
 //! [specification]: https://www.w3.org/TR/xmlschema11-1/#Simple_Type_Definition
 //! [`deserialize_with`]: https://serde.rs/field-attrs.html#deserialize_with
 //! [#497]: https://github.com/tafia/quick-xml/issues/497
+//! [`text_content`]: crate::serde_helpers::text_content
 
 // Macros should be defined before the modules that using them
 // Also, macros should be imported before using them

src/lib.rs

@@ -22,7 +22,8 @@
 //! Furthermore, quick-xml also contains optional [Serde] support to directly
 //! serialize and deserialize from structs, without having to deal with the XML events.
 //! To get it enable the `serialize` feature. Read more about mapping Rust types
-//! to XML in the documentation of [`de`] module.
+//! to XML in the documentation of [`de`] module. Also check [`serde_helpers`]
+//! module.
 //!
 //! # Examples
 //!
@@ -62,6 +63,8 @@ pub mod name;
 pub mod reader;
 #[cfg(feature = "serialize")]
 pub mod se;
+#[cfg(feature = "serialize")]
+pub mod serde_helpers;
 /// Not an official API, public for integration tests
 #[doc(hidden)]
 pub mod utils;

src/serde_helpers.rs

@@ -0,0 +1,109 @@
+//! Provides helper functions to glue an XML with a serde content model.
+
+/// Provides helper functions to serialization and deserialization of types
+/// (usually enums) as a text content of an element and intended to use with
+/// [`#[serde(with = "...")]`][with], [`#[serde(deserialize_with = "...")]`][de-with]
+/// and [`#[serde(serialize_with = "...")]`][se-with].
+///
+/// When you serialize unit variants of enums, they are serialized as an empty
+/// elements, like `<Unit/>`. At the same time, when enum consist only from unit
+/// variants, it is frequently needed to serialize them as string content of an
+/// element, like `<field>Unit</field>`. To make this possible use this module.
+///
+/// ```
+/// # use pretty_assertions::assert_eq;
+/// use quick_xml::de::from_str;
+/// use quick_xml::se::to_string;
+/// use serde::{Serialize, Deserialize};
+///
+/// #[derive(Serialize, Deserialize, PartialEq, Debug)]
+/// enum SomeEnum {
+///     // Default implementation serializes enum as an `<EnumValue/>` element
+///     EnumValue,
+/// # /*
+///     ...
+/// # */
+/// }
+///
+/// #[derive(Serialize, Deserialize, PartialEq, Debug)]
+/// #[serde(rename = "some-container")]
+/// struct SomeContainer {
+///     #[serde(with = "quick_xml::serde_helpers::text_content")]
+///     field: SomeEnum,
+/// }
+///
+/// let container = SomeContainer {
+///     field: SomeEnum::EnumValue,
+/// };
+/// let xml = "\
+///     <some-container>\
+///         <field>EnumValue</field>\
+///     </some-container>";
+///
+/// assert_eq!(to_string(&container).unwrap(), xml);
+/// assert_eq!(from_str::<SomeContainer>(xml).unwrap(), container);
+/// ```
+///
+/// Using of this module is equivalent to replacing `field`'s type to this:
+///
+/// ```
+/// # use serde::{Deserialize, Serialize};
+/// # type SomeEnum = ();
+/// #[derive(Serialize, Deserialize)]
+/// struct Field {
+///     // Use a special name `$text` to map field to the text content
+///     #[serde(rename = "$text")]
+///     content: SomeEnum,
+/// }
+///
+/// #[derive(Serialize, Deserialize)]
+/// #[serde(rename = "some-container")]
+/// struct SomeContainer {
+///     field: Field,
+/// }
+/// ```
+/// Read about the meaning of a special [`$text`] field.
+///
+/// [with]: https://serde.rs/field-attrs.html#with
+/// [de-with]: https://serde.rs/field-attrs.html#deserialize_with
+/// [se-with]: https://serde.rs/field-attrs.html#serialize_with
+/// [`$text`]: ../../de/index.html#text
+pub mod text_content {
+    use serde::{Deserialize, Deserializer, Serialize, Serializer};
+
+    /// Serializes `value` as an XSD [simple type]. Intended to use with
+    /// `#[serde(serialize_with = "...")]`. See example at [`text_content`]
+    /// module level.
+    ///
+    /// [simple type]: https://www.w3.org/TR/xmlschema11-1/#Simple_Type_Definition
+    pub fn serialize<S, T>(value: &T, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+        T: Serialize,
+    {
+        #[derive(Serialize)]
+        struct Field<'a, T> {
+            #[serde(rename = "$text")]
+            value: &'a T,
+        }
+        Field { value }.serialize(serializer)
+    }
+
+    /// Deserializes XSD's [simple type]. Intended to use with
+    /// `#[serde(deserialize_with = "...")]`. See example at [`text_content`]
+    /// module level.
+    ///
+    /// [simple type]: https://www.w3.org/TR/xmlschema11-1/#Simple_Type_Definition
+    pub fn deserialize<'de, D, T>(deserializer: D) -> Result<T, D::Error>
+    where
+        D: Deserializer<'de>,
+        T: Deserialize<'de>,
+    {
+        #[derive(Deserialize)]
+        struct Field<T> {
+            #[serde(rename = "$text")]
+            value: T,
+        }
+        Ok(Field::deserialize(deserializer)?.value)
+    }
+}