Inline Default

A macro for implementing Default within a struct definition. This is useful when specifying the default values within the struct definition is cleaner than separating them, for example structs with a large number fields, or when many structs are constructed. For example,

use inline_default::inline_default;

inline_default! {
struct KeyMap {
    left:   char = 'h',
    down:   char = 'j',
    up:     char = 'k',
    right:  char = 'l',

    // uses bool::default(),
    flag:   bool, 
    
    // any constant-time expression is allowed
    qwerty: bool = Keyboard::IS_QWERTY,
}}

expands to

struct KeyMap {
    left:   char,
    down:   char,
    up:     char,
    right:  char,
    flag:   bool,
    qwerty: bool,
}

impl Default for KeyMap {
    fn default() {
        KeyMap {
            left:  'h',
            down:  'j',
            up:    'k',
            right: 'l',
            flag:  bool::default(),
            qwerty: Keyboard::IS_QWERTY,
        }
    }
}

Supports:

• Visibility specifiers
• Use Default if not specified
• Attributes (including derives) on both the struct and the fields
• Doc comments on both the struct and the fields
• Multiple struct definitions within each macro
• Lifetimes and generics, with major caveats

Due to the complexity in parsing trait bounds, only a single trait bound without generics is accepted. where clauses are not supported. Specifying lifetimes are accepted, but not the 'outlives' syntax 'a: 'b. For example, the following is accepted,

use inline_default::inline_default;

inline_default! {
/// Example struct
#[derive(Copy, Clone)]
pub(crate) struct Example<'a, T: Default> {
    pub a: &'a str = "example",

    /// field b
    b: T,
}}

but the following are not:

use inline_default::inline_default;

// NOT VALID - too many trait bounds on T
inline_default! {
struct Example1<T: Default + Copy> {
    a: T,
}}

// NOT VALID - Traits bounds cannot be generic
inline_default! {
struct Example2<T: From<f32>> {
    a: T = T::from(0.0),
}}

// NOT VALID - outlives syntax is not supported
inline_default! {
struct Example2<'a, 'b: 'a> {
    a: &'a str = "test",
    b: &'b str = "test2",
}}

Proc Macro?

Making a proc macro would be fuller feature-wise, but I can't be bothered.