is_default 0.1.2

A 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.

Features

Nightly-only:

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:

# Cargo.toml

[dependencies]
is_default = { version = "0.1", features = ["derive"] }

Structs

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

# #[cfg(feature = "derive")] {
# use is_default::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:

# #[cfg(feature = "derive")] {
# use is_default::IsDefault;
#
#[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:

# #[cfg(feature = "derive")] {
# use is_default::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:

# #[cfg(all(feature = "derive", not(feature = "via_default_eq")))] {
# use is_default::IsDefault;
#
#[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());
# }

via_default_eq

By default, IsDefault is manually implemented for core and std types. This approach is fast and has no trait dependencies but requires manual implementation for custom types.

Alternatively, you can enable a generic implementation of IsDefault for all types that implement both Default and PartialEq. This is the simplest option, but it may be less efficient, as it allocates a new value for comparison:

# Cargo.toml

[dependencies]
is_default = { version = "0.1.1", features = ["via_default_eq"] }

no_std

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

# Cargo.toml

[dependencies]
is_default = { version = "0.1.1", default-features = false, features = ["derive"] }