3 stable releases
1.2.0 | Feb 24, 2023 |
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1.1.0 | Jan 3, 2023 |
1.0.0 | Aug 12, 2022 |
#2102 in Rust patterns
24 downloads per month
7KB
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.