add const generic numeric compact deserialization helpers (#37)

* add const generic numeric compact deserialization helpers

* add u16 and i16; fix docs

src/field_attributes.rs

@@ -31,6 +31,151 @@ pub fn bool_true() -> bool {
     true
 }
 
+/// Allows compact setting `u16` field default value.
+///
+/// # Example
+/// ```rust
+/// use serde_aux::prelude::*;
+///
+/// #[derive(serde::Deserialize, Debug)]
+/// struct MyStruct {
+///     #[serde(default = "default_u16::<30>")]
+///     default_30: u16,
+///     #[serde(default)]
+///     default_zero: u16,
+/// }
+///
+/// let s = r#"{}"#;
+/// let a: MyStruct = serde_json::from_str(s).unwrap();
+/// assert_eq!(a.default_30, 30);
+/// assert_eq!(a.default_zero, 0);
+/// ```
+#[inline]
+pub const fn default_u16<const V: u16>() -> u16 {
+    V
+}
+
+
+/// Allows compact setting `u32` field default value.
+///
+/// # Example
+/// ```rust
+/// use serde_aux::prelude::*;
+///
+/// #[derive(serde::Deserialize, Debug)]
+/// struct MyStruct {
+///     #[serde(default = "default_u32::<30>")]
+///     default_30: u32,
+///     #[serde(default)]
+///     default_zero: u32,
+/// }
+///
+/// let s = r#"{}"#;
+/// let a: MyStruct = serde_json::from_str(s).unwrap();
+/// assert_eq!(a.default_30, 30);
+/// assert_eq!(a.default_zero, 0);
+/// ```
+#[inline]
+pub const fn default_u32<const V: u32>() -> u32 {
+    V
+}
+
+/// Allows compact setting `u64` field default value.
+///
+/// # Example
+/// ```rust
+/// use serde_aux::prelude::*;
+///
+/// #[derive(serde::Deserialize, Debug)]
+/// struct MyStruct {
+///     #[serde(default = "default_u64::<30>")]
+///     default_30: u64,
+///     #[serde(default)]
+///     default_zero: u64,
+/// }
+///
+/// let s = r#"{}"#;
+/// let a: MyStruct = serde_json::from_str(s).unwrap();
+/// assert_eq!(a.default_30, 30);
+/// assert_eq!(a.default_zero, 0);
+/// ```
+#[inline]
+pub const fn default_u64<const V: u64>() -> u64 {
+    V
+}
+
+/// Allows compact setting `i16` field default value.
+///
+/// # Example
+/// ```rust
+/// use serde_aux::prelude::*;
+///
+/// #[derive(serde::Deserialize, Debug)]
+/// struct MyStruct {
+///     #[serde(default = "default_i16::<-30>")]
+///     default_30: i16,
+///     #[serde(default)]
+///     default_zero: i16,
+/// }
+///
+/// let s = r#"{}"#;
+/// let a: MyStruct = serde_json::from_str(s).unwrap();
+/// assert_eq!(a.default_30, -30);
+/// assert_eq!(a.default_zero, 0);
+/// ```
+#[inline]
+pub const fn default_i16<const V: i16>() -> i16 {
+    V
+}
+
+/// Allows compact setting `i32` field default value.
+///
+/// # Example
+/// ```rust
+/// use serde_aux::prelude::*;
+///
+/// #[derive(serde::Deserialize, Debug)]
+/// struct MyStruct {
+///     #[serde(default = "default_i32::<-30>")]
+///     default_30: i32,
+///     #[serde(default)]
+///     default_zero: i32,
+/// }
+///
+/// let s = r#"{}"#;
+/// let a: MyStruct = serde_json::from_str(s).unwrap();
+/// assert_eq!(a.default_30, -30);
+/// assert_eq!(a.default_zero, 0);
+/// ```
+#[inline]
+pub const fn default_i32<const V: i32>() -> i32 {
+    V
+}
+
+/// Allows compact setting `i64` field default value.
+///
+/// # Example
+/// ```rust
+/// use serde_aux::prelude::*;
+///
+/// #[derive(serde::Deserialize, Debug)]
+/// struct MyStruct {
+///     #[serde(default = "default_i64::<-30>")]
+///     default_30: i64,
+///     #[serde(default)]
+///     default_zero: i64,
+/// }
+///
+/// let s = r#"{}"#;
+/// let a: MyStruct = serde_json::from_str(s).unwrap();
+/// assert_eq!(a.default_30, -30);
+/// assert_eq!(a.default_zero, 0);
+/// ```
+#[inline]
+pub const fn default_i64<const V: i64>() -> i64 {
+    V
+}
+
 /// Deserializes a `chrono::DateTime<Utc>` from a milliseconds time stamp. Useful when the data is coming from a number
 /// which is not a seconds time stamp but milliseconds one. It also handles the string to number conversion if the
 /// data was passed as a string with number inside like **"1519927261900"**.