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/// Macro for use with [`macro_rules_attribute::derive`] to derive a `const fn default()`
/// in addition to `impl Default`.
///
/// Generics are not supported.
///
/// When using this macro on an enum, the default variant must be marked with `#[custom(default)]`,
/// rather than `#[default]` as the built-in derive macro does. This is a limitation of
/// [`macro_rules_attribute::derive`].
macro_rules! ConstDefault {
// Simplify by dropping attributes and visibility from an `enum`.
(
$(#[$attr:meta])+
$vis:vis enum $enum_name:ident {
$($body:tt)*
}
) => {
$crate::macros::ConstDefault!(enum $enum_name { $($body)* });
};
// Simplify by dropping attributes and visibility from a `struct`.
(
$(#[$attr:meta])+
$vis:vis struct $struct_name:ident $($rest:tt)*
) => {
$crate::macros::ConstDefault!(struct $struct_name $($rest)*);
};
// Base case: match an `enum` with `#[default]` on the first variant.
(
enum $enum_name:ident {
#[custom(default)]
$(#[$other_variant_attr:meta])*
$variant_name:ident $(= $discriminant:expr)?,
$( $rest_of_body:tt )*
}
) => {
impl $enum_name {
/// This function is identical to [`Default::default()`] except that it is a `const fn`.
pub const fn default() -> Self {
Self::$variant_name
}
}
impl $crate::macros::ConstDefaultHelper for $enum_name {
const DEFAULT: Self = Self::$variant_name;
}
impl ::core::default::Default for $enum_name {
fn default() -> Self {
Self::$variant_name
}
}
};
// Recursive case: remove an irrelevant attribute from the first variant.
(
enum $enum_name:ident {
#[$attr_that_is_not_default:meta]
$( $rest_of_body:tt )*
}
) => {
$crate::macros::ConstDefault!(enum $enum_name { $( $rest_of_body )* });
};
// Recursive cases: remove the first variant of an enum (which may or may not have fields),
// because we have checked it is not the default.
(
enum $enum_name:ident {
$variant_name:ident $(= $discriminant:expr)?,
$( $rest_of_body:tt )*
}
) => {
$crate::macros::ConstDefault!(enum $enum_name { $( $rest_of_body )* });
};
(
enum $enum_name:ident {
$variant_name:ident ( $( $field_tokens:tt )* ),
$( $rest_of_body:tt )*
}
) => {
$crate::macros::ConstDefault!(enum $enum_name { $( $rest_of_body )* });
};
(
enum $enum_name:ident {
$variant_name:ident { $( $field_tokens:tt )* },
$( $rest_of_body:tt )*
}
) => {
$crate::macros::ConstDefault!(enum $enum_name { $( $rest_of_body )* });
};
// Struct (without generics)
(
$(#[$struct_attr:meta])*
$vis:vis struct $struct_name:ident {
$(
$(#[$field_attr:meta])*
$field_vis:vis $field_name:ident : $field_type:ty,
)*
}
) => {
impl $struct_name {
/// This function is identical to [`Default::default()`] except that it is a `const fn`.
pub const fn default() -> Self {
<Self as $crate::macros::ConstDefaultHelper>::DEFAULT
}
}
impl $crate::macros::ConstDefaultHelper for $struct_name {
const DEFAULT: Self = Self {
$(
$field_name: <$field_type as $crate::macros::ConstDefaultHelper>::DEFAULT,
)*
};
}
impl ::core::default::Default for $struct_name {
fn default() -> Self {
<Self as $crate::macros::ConstDefaultHelper>::DEFAULT
}
}
};
// Tuple struct
(
$(#[$struct_attr:meta])*
$vis:vis struct $struct_name:ident(
$(
$(#[$field_attr:meta])*
$field_vis:vis $field_type:ty
),* $(,)?
);
) => {
impl $struct_name {
/// This function is identical to [`Default::default()`] except that it is a `const fn`.
pub const fn default() -> Self {
<Self as $crate::macros::ConstDefaultHelper>::DEFAULT
}
}
impl $crate::macros::ConstDefaultHelper for $struct_name {
const DEFAULT: Self = Self(
$(
<$field_type as $crate::macros::ConstDefaultHelper>::DEFAULT,
)*
);
}
impl ::core::default::Default for $struct_name {
fn default() -> Self {
<Self as $crate::macros::ConstDefaultHelper>::DEFAULT
}
}
};
}
pub(crate) use ConstDefault;
/// Default value of a type, as a `const` item.
///
/// This trait is not public and is used solely to help [`ConstDefault`] provide *inherent*
/// `const fn default()`s.
///
/// Specifically, it is implemented both for `wgpu-types` types and `std` types,
/// so that the macro [`ConstDefault`] can call it without needing to understand the type.
pub(crate) trait ConstDefaultHelper {
const DEFAULT: Self;
}
impl ConstDefaultHelper for bool {
const DEFAULT: Self = false;
}
impl ConstDefaultHelper for f32 {
const DEFAULT: Self = 0.;
}
impl ConstDefaultHelper for f64 {
const DEFAULT: Self = 0.;
}
impl ConstDefaultHelper for i32 {
const DEFAULT: Self = 0;
}
impl ConstDefaultHelper for i64 {
const DEFAULT: Self = 0;
}
impl ConstDefaultHelper for u32 {
const DEFAULT: Self = 0;
}
impl ConstDefaultHelper for u64 {
const DEFAULT: Self = 0;
}
impl<T> ConstDefaultHelper for Option<T> {
const DEFAULT: Self = None;
}
impl<T: ConstDefaultHelper, const N: usize> ConstDefaultHelper for [T; N] {
const DEFAULT: Self = [<T as ConstDefaultHelper>::DEFAULT; N];
}