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//! This module provides the `assign!` macro to allow mutating a struct value in
//! a declarative style.
//!
//! It is an alternative to [struct update syntax][] that works with
//! [non-exhaustive][] structs.
//!
//! [struct update syntax]: https://doc.rust-lang.org/book/ch05-01-defining-structs.html#creating-instances-from-other-instances-with-struct-update-syntax
//! [non-exhaustive]: https://doc.rust-lang.org/reference/attributes/type_system.html#the-non_exhaustive-attribute
//!
//! It is used as
//!
//! ```
//! # use assign::assign;
//! # struct Struct { field: u8 }
//! # let init_expression = Struct { field: 0 };
//! # let new_field_value = 1;
//! let foo = assign!(init_expression, {
//! field: new_field_value,
//! // other_field: new_other_field_value,
//! // ...
//! });
//! ```
//!
//! For details and examples, see the documentation for the macro itself.
#![no_std]
/// Mutate a struct value in a declarative style.
///
/// # Basic usage
///
/// ```
/// # use assign::assign;
/// #
/// #[non_exhaustive]
/// #[derive(Debug, PartialEq)]
/// struct SomeStruct {
/// a: u32,
/// b: Option<f32>,
/// c: String,
/// }
///
/// impl SomeStruct {
/// fn new() -> Self {
/// // ...
/// # SomeStruct {
/// # a: 1u32,
/// # b: None,
/// # c: String::from("old"),
/// # }
/// }
/// }
///
/// let instance1 = assign!(SomeStruct::new(), {
/// a: 2,
/// c: "new".into(),
/// });
///
/// // The same thing using mutation explicitly.
/// // This is what the above expands to.
/// let instance2 = {
/// let mut item = SomeStruct::new();
/// item.a = 2;
/// item.c = "new".into();
/// item
/// };
///
/// // The same thing using struct update syntax (does not work for
/// // non-exhaustive structs defined in external crates).
/// let instance3 = SomeStruct {
/// a: 2,
/// c: "new".into(),
/// ..SomeStruct::new()
/// };
///
/// assert_eq!(instance1, instance2);
/// assert_eq!(instance1, instance3);
/// ```
///
/// # Slightly more realistic example
///
/// ```
/// # struct Arg {}
/// # impl Arg { fn new(_opt: ArgOptions) -> Self { Self {} } }
/// // in awesome_cli_lib
/// #[non_exhaustive]
/// # #[derive(Default)]
/// struct ArgOptions {
/// pub name: String,
/// pub short: Option<String>,
/// pub long: Option<String>,
/// pub help: Option<String>,
/// pub required: bool,
/// pub takes_value: bool,
/// pub multiple: bool,
/// pub default_value: Option<String>,
/// }
///
/// impl ArgOptions {
/// pub fn new(name: String) -> Self {
/// // ...
/// # Self { name, ..Default::default() }
/// }
/// }
///
/// // your crate
/// use assign::assign;
///
/// let arg = Arg::new(assign!(ArgOptions::new("version".into()), {
/// short: Some("V".into()),
/// long: Some("version".into()),
/// help: Some("prints the version and quits.".into()),
/// }));
/// ```
#[macro_export]
macro_rules! assign {
($initial_value:expr, {
$( $field:ident $( : $value:expr )? ),+ $(,)?
}) => ({
let mut item = $initial_value;
$( $crate::assign!(@assign item $field $( : $value )?); )+
item
});
(@assign $item:ident $field:ident : $value:expr) => {
$item.$field = $value;
};
(@assign $item:ident $field:ident) => {
$item.$field = $field;
};
}
#[cfg(test)]
mod tests {
#[derive(Debug, Default, PartialEq)]
struct SomeStruct {
a: u32,
b: Option<f32>,
c: Option<u64>,
}
#[test]
fn basic() {
let res = assign!(SomeStruct::default(), {
a: 5,
b: None,
});
assert_eq!(
res,
SomeStruct {
a: 5,
b: None,
c: None
}
);
}
#[test]
fn shorthand() {
let def = SomeStruct::default();
let a = 5;
let res = assign!(def, { a });
assert_eq!(
res,
SomeStruct {
a: 5,
b: None,
c: None
}
);
}
#[test]
fn field_expr_inference() {
let b = 0.0.into();
let res = assign!(SomeStruct::default(), {
b,
c: 1.into(),
});
assert_eq!(
res,
SomeStruct {
a: 0,
b: Some(0.0),
c: Some(1)
}
);
}
#[test]
fn all_fields() {
let a = 1;
let b = Some(1.0);
let res = assign!(SomeStruct::default(), {
a,
b,
c: 1.into(),
});
assert_eq!(
res,
SomeStruct {
a: 1,
b: Some(1.0),
c: Some(1),
}
);
}
}