wasmtime_environ::__core::prelude::rust_2021

Trait Default

1.55.0 (const: unstable) · Source

pub trait Default: Sized {
    // Required method
    fn default() -> Self;
}

Expand description

A trait for giving a type a useful default value.

Sometimes, you want to fall back to some kind of default value, and don’t particularly care what it is. This comes up often with structs that define a set of options:

struct SomeOptions {
    foo: i32,
    bar: f32,
}

How can we define some default values? You can use Default:

#[derive(Default)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}

fn main() {
    let options: SomeOptions = Default::default();
}

Now, you get all of the default values. Rust implements Default for various primitive types.

If you want to override a particular option, but still retain the other defaults:

fn main() {
    let options = SomeOptions { foo: 42, ..Default::default() };
}

§Derivable

This trait can be used with #[derive] if all of the type’s fields implement Default. When derived, it will use the default value for each field’s type.

§`enum`s

When using #[derive(Default)] on an enum, you need to choose which unit variant will be default. You do this by placing the #[default] attribute on the variant.

#[derive(Default)]
enum Kind {
    #[default]
    A,
    B,
    C,
}

You cannot use the #[default] attribute on non-unit or non-exhaustive variants.

The #[default] attribute was stabilized in Rust 1.62.0.

§How can I implement `Default`?

Provide an implementation for the default() method that returns the value of your type that should be the default:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() -> Self { Kind::A }
}

§Examples

#[derive(Default)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}

Required Methods§

1.0.0 (const: unstable) · Source

fn default() -> Self

Returns the “default value” for a type.

Default values are often some kind of initial value, identity value, or anything else that may make sense as a default.

§Examples

Using built-in default values:

let i: i8 = Default::default();
let (x, y): (Option<String>, f64) = Default::default();
let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();

Making your own:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() -> Self { Kind::A }
}

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors§

1.10.0 (const: unstable) · Source§

impl Default for &CStr

1.9.0 · Source§

impl Default for &OsStr

1.28.0 (const: unstable) · Source§

impl Default for &mut str

1.0.0 (const: unstable) · Source§

impl Default for &str

1.0.0 (const: unstable) · Source§

impl Default for ()

Default

Trait Default Copy item path

§Derivable

§enums

§How can I implement Default?

§Examples

Required Methods§

fn default() -> Self

§Examples

Dyn Compatibility§

Implementors§

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for &str

impl Default for ()

impl Default for Abbreviations

impl Default for AbbreviationsCache

impl Default for Abi

impl Default for AddressMapSection

impl Default for Alignment

impl Default for AllocVec

impl Default for Arc<CStr>

impl Default for Arc<str>

impl Default for AsciiChar

impl Default for Atom

impl Default for Atomic<bool>

impl Default for Atomic<i8>

impl Default for Atomic<i16>

impl Default for Atomic<i32>

impl Default for Atomic<i64>

impl Default for Atomic<isize>

impl Default for Atomic<u8>

impl Default for Atomic<u16>

impl Default for Atomic<u32>

impl Default for Atomic<u64>

impl Default for Atomic<usize>

impl Default for Augmentation

impl Default for AuxSymbolSection

impl Default for AuxSymbolWeak

impl Default for B0

impl Default for B1

impl Default for BaseAddresses

impl Default for BigEndian

impl Default for BigEndian

impl Default for Box<CStr>

impl Default for Box<OsStr>

impl Default for Box<str>

impl Default for BranchHints

impl Default for BuildMetadata

impl Default for ByteString

impl Default for CString

impl Default for CanonicalAbiInfo

impl Default for CanonicalFunctionSection

impl Default for anyhow::Chain<'_>

impl Default for Class

impl Default for CodeSection

impl Default for ColorChoice

impl Default for ColorSpec

impl Default for Component

impl Default for wasmtime_environ::component::Component

impl Default for ComponentAliasSection

impl Default for ComponentBuilder

impl Default for ComponentDfg

impl Default for ComponentExportSection

impl Default for ComponentImportSection

impl Default for ComponentInstanceSection

impl Default for ComponentNameSection

impl Default for ComponentType

impl Default for ComponentTypeSection

impl Default for ComponentTypes

impl Default for std::sync::nonpoison::condvar::Condvar

impl Default for std::sync::poison::condvar::Condvar

impl Default for Config

impl Default for ConfigTunables

impl Default for ControlStack

impl Default for CopyingTypeLayouts

impl Default for CoreDumpSection

impl Default for CoreDumpStackSection

impl Default for CoreTypeSection

Trait Default

§`enum`s

§How can I implement `Default`?