Crate is_default 

A fast trait for checking whether a value is the default, with convenient derive support for custom types.

The default value is defined as the value returned by the Default trait. Therefore, any implementation of IsDefault must ensure that &self == &Self::default() holds true.

Unlike the naive comparison &self == &Self::default(), it directly evaluates the type’s internal fields without the overhead of constructing a new instance.

Features

Feature	Default	Description
derive	yes	Derive trait for a type
std	yes	Implements for std-types

Nightly-only:

Nightly Feature	Default	Description
nightly	no	Enable all below nightly features
ascii_char	no	Core ascii_char
bstr	no	Std bstr
f16	no	Core f16
f128	no	Core f128

Derive

The IsDefault trait is already implemented for most core and std types that implement Default. For custom types, you can derive IsDefault using derive.

Structs

A struct can derive IsDefault if all of its fields implement IsDefault:

#[derive(IsDefault)]
struct Unit;
assert!(Unit.is_default());

#[derive(IsDefault)]
struct Wrapper(u8);
assert!(Wrapper(0).is_default());
assert!(!Wrapper(1).is_default());

#[derive(IsDefault)]
struct Point { x: i16, y: f32 }
assert!(Point{ x: 0, y: 0.0 }.is_default());
assert!(!Point{ x: 1, y: 0.0 }.is_default());
assert!(!Point{ x: 0, y: 1.1 }.is_default());

Enums

When deriving IsDefault for an enum, you must specify which unit variant should be considered the default. This is done by applying the #[is_default] or #[default] attribute to the variant:

#[derive(IsDefault)]
enum A {
    #[is_default]
    X,
    Y,
}
assert!(A::X.is_default());
assert!(!A::Y.is_default());

#[default] attribute possible to derive both Default and IsDefault:

#[derive(Default, IsDefault)]
enum B {
    X,
    #[default]
    Y,
}
assert!(!B::X.is_default());
assert!(B::Y.is_default());
assert!(matches!(B::default(), B::Y));

You can also derive IsDefault for enums that implement both Default and PartialEq. This approach is more general but may be less efficient, since a new value must be allocated for comparison:

#[derive(PartialEq, IsDefault)]
enum C {
    X(u8),
    Y,
}
impl Default for C {
    fn default() -> C {
        C::X(0)
    }
}

assert!(C::X(0).is_default());
assert!(!C::X(1).is_default());

no_std

For no_std builds, add is_default to your Cargo.toml with default features disabled:

# Cargo.toml

[dependencies]
is_default = { version = "1", default-features = false, features = ["derive"] }

Traits

IsDefault

Checks whether a value is equal to its type’s default.

Derive Macros

IsDefault

Derive macro generating an impl of the trait IsDefault.