zeroable 0.2.0

/*!
Documentation for the `Zeroable` derive macro.

This macro is for deriving the
[`bytemuck::Zeroable` trait](https://docs.rs/bytemuck/1/bytemuck/trait.Zeroable.html).


# Restrictions

All of these restrictions are enforced at compile-time.

### Structs

All fields are required to implement Zeroable.

```rust
use zeroable::Zeroable;

#[derive(Zeroable)]
struct AStruct{
    left:u32,
    right:u32,
}
```

### Enums

Enums must satisfy one of these:

- Having a `#[repr(C/u8/i8/u16/i16/u32/i32/u64/i64/u128/i128/usize/isize)]` attribute,
    with either an implicit discriminant for the first variant (which is always `0`),
    or an explicit `0` discriminant for some variant.
    <br>
    The fields of the variant with a `0` discriminant will then be required to
    implement Zeroable,while the fields of other variants won't be.

- Having a `#[repr(transparent)]` attribute,with a single variant and field,
    which must implement Zeroable.

### Unions

All fields are required to implement Zeroable by default,
opting out of Zeroable for fields individually with `#[zero(nonzero)]`.

The alternative to using the `#[zero(nonzero)]` attribute on fields is
to use the `#[zero(nonzero_fields)]` attribute on the union
(which makes not requiring zeroable for fields the default for that union),
then using the `#[zero(zeroable)]` attribute on zeroable fields.

Zeroable impls for unions have documentation mentioning
which fields were marked as zeroable,and which are not.

# Attributes

These are all the attributes for the derive macro,grouped by where they can be used.

## Container attributes

##### `#[zero(bound="Type:ATrait")]`

Adds a contraint to the `Zeroable` impl.

##### `#[zero(not_zeroable(TypeParamA,TypeParamB,TypeParamC))]`

Removes the default `Zeroable` bound for one/many type parameters.

##### `#[zero(nonzero_fields)]`

For unions only.

Marks all the fields as not being zeroable,
removing the assertion that they implement Zeroable,
requiring some fields to have a `#[zero(zeroable)]` attribute.

##### `#[zero(debug_print)]`

Prints the generated code,stopping compilation.

## Field attributes

##### `#[zero(zeroable)]`

For unions only.

Marks the field as being initializable with zeroes,
adding an assertion that it implements Zeroable.

The field is then mentioned in the generated documentation for
the Zeroable impl under `Zeroable Fields`.

##### `#[zero(nonzero)]`

For unions only.

Marks the field as not being initializable with zeroes,
removing the assertion that it implements Zeroable.

The field is then mentioned in the generated documentation for
the Zeroable impl under `NonZero Fields`.

# Examples

### Enum

A Result-like enum,with `Ok` as the variant instantiated by `zeroed`.

```rust
use zeroable::Zeroable;

#[derive(Debug,PartialEq,Zeroable)]
#[repr(u8)]
#[zero(not_zeroable(E))]
enum MyResult<T,E>{
    Ok(T),
    Err(E),
}

assert_eq!( MyResult::<(),String>::zeroed(), MyResult::Ok(()) );
assert_eq!( MyResult::<bool,Vec<()>>::zeroed(), MyResult::Ok(false) );

// This won't compile because Vec is not zeroable.
// assert_eq!( MyResult::<Vec<()>,String>::zeroed(), MyResult::Ok(vec![]) );

```

### Enum

A simple Option-like enum.

In this the None variant is the one instantiated by `zeroed`.

`#[zero(not_zeroable(T))]` doesn't cause an error because `T` is not in
the variant instantiated by `zeroed`
(if `None` contained a `T`,then it would be an error).

```rust
use zeroable::Zeroable;

#[derive(Debug,PartialEq,Zeroable)]
#[repr(u8)]
#[zero(not_zeroable(T))]
enum MyOption<T>{
    None,
    Some(T)
}


assert_eq!( MyOption::<String>::zeroed(), MyOption::None );

```

### Enum

Here is an Ordering-like enum.

```rust
use zeroable::Zeroable;

#[derive(Debug,PartialEq,Zeroable)]
#[repr(i8)]
enum Ordering{
    Less=-1,
    Equal=0,
    Greater=1,
}

assert_eq!( Ordering::zeroed(), Ordering::Equal );

```

### Enum (non-compiling)

This doesn't compile because there is no variant with a `0` discriminant.

```compile_fail
use zeroable::Zeroable;

#[derive(Debug,PartialEq,Zeroable)]
#[repr(u8)]
enum Directions{
    Left=1,
    Right,
    Up,
    Down,
}

```

### Enum (non-compiling)

This doesn't compile because the first variant contains a `NonZeroU8`,
which is not zeroable.

```compile_fail
use zeroable::Zeroable;

use core::num::NonZeroU8;

#[derive(Debug,PartialEq,Zeroable)]
#[repr(u8)]
enum NonZeroOrZeroable{
    NonZero(NonZeroU8),
    Zeroable(u8),
}

```

It compiles if you swap the variants:
```rust
use zeroable::Zeroable;

use core::num::NonZeroU8;

#[derive(Debug,PartialEq,Zeroable)]
#[repr(u8)]
enum NonZeroOrZeroable{
    Zeroable(u8),
    NonZero(NonZeroU8),
}

assert_eq!( NonZeroOrZeroable::zeroed(), NonZeroOrZeroable::Zeroable(0) );

```
this is because the first variant of an enum implicitly has a `0` discriminant,
and `u8` is zeroable.

### Enum (requires nightly)

This is an example of a `#[repr(transparent)]` enum.

*/
#![cfg_attr(feature="nightly_docs",doc="```rust")]
#![cfg_attr(not(feature="nightly_docs"),doc="```ignore")]
/*!
#![feature(transparent_enums)]

use zeroable::Zeroable;

#[derive(Debug,PartialEq,Zeroable)]
#[repr(transparent)]
enum Wrapper<T>{
    Value(T),
}

assert_eq!( Wrapper::<isize>::zeroed(), Wrapper::Value(0_isize) );
assert_eq!( Wrapper::<usize>::zeroed(), Wrapper::Value(0_usize) );
assert_eq!( Wrapper::<(usize,usize)>::zeroed(), Wrapper::Value((0_usize,0_usize)) );
```

### Enum (requires nightly) (non-compiling)

This is an example that fixes a non-compiling enum by setting the discriminant
of a variant to 0.

*/
#![cfg_attr(feature="nightly_docs",doc="```compile_fail")]
#![cfg_attr(not(feature="nightly_docs"),doc="```ignore")]
/*!
use zeroable::Zeroable;

use std::error::Error;

#[derive(Debug,Zeroable)]
#[repr(i8)]
enum MyError{
    PageNotFound{name:String},
    Undefined,
    Other(Box<dyn Error>)
}
```
This fails to compile because String isn't zeroable,
so let's change the variant with a zero discriminant to `Undefined`

*/
#![cfg_attr(feature="nightly_docs",doc="```rust")]
#![cfg_attr(not(feature="nightly_docs"),doc="```ignore")]
/*!
#![feature(arbitrary_enum_discriminant)]

use zeroable::Zeroable;

use std::error::Error;

#[derive(Debug,Zeroable)]
#[repr(i8)]
enum MyError{
    PageNotFound{name:String}=-1,
    Undefined=0,
    Other(Box<dyn Error>),
}
```
The first variant has to have an explicit discriminant,
because otherwise it uses `0` as its discriminant,
causing a compile time error.

### Struct

A Rectangle type.

```rust
use zeroable::Zeroable;

#[derive(Debug,PartialEq,Zeroable)]
struct Rectangle<T>{
    x:T,
    y:T,
    w:T,
    h:T,
}

assert_eq!( Rectangle::zeroed(), Rectangle{ x:0, y:0, w:0, h:0 } );

```

### Struct

Here we define a binary tree of zeroable with indices instead of pointers:

```
use zeroable::Zeroable;

use core::num::NonZeroU32;

#[derive(Debug,PartialEq)]
struct Tree<T>{
    list:Vec<TreeNode<T>>,
}

#[derive(Debug,PartialEq,Zeroable)]
struct TreeNode<T>{
    value:T,
    left:Option<NonZeroU32>,
    right:Option<NonZeroU32>,
}

assert_eq!(
    TreeNode::<[u8;32]>::zeroed(),
    TreeNode{
        value:[0;32],
        left:None,
        right:None,
    },
);

```

### Struct (non-compiling)

This doesn't compile because `&[T]` is not zeroable.

```compile_fail
use zeroable::Zeroable;

#[derive(Debug,PartialEq,Zeroable)]
struct NonEmptySlice<'a,T>{
    slice:&'a [T],
}

```

### Union

```rust
use zeroable::Zeroable;

#[derive(Zeroable)]
#[repr(C)] // This isn't necessary for Zeroable
union U32OrArray{
    num:u32,
    arr:[u8;4],
}

unsafe{
    let zeroed=U32OrArray::zeroed();

    assert_eq!( zeroed.num, 0 );

    assert_eq!( zeroed.arr, [0;4] );
}
```

### Union

```rust
use zeroable::Zeroable;

#[derive(Zeroable)]
#[zero(not_zeroable(T))]
#[zero(nonzero_fields)]
union CondValue<T:Copy>{
    #[zero(zeroable)]
    cond:bool,
    value:T,
}

unsafe{
    let zeroed=CondValue::<&'static str>::zeroed();
    assert_eq!( zeroed.cond, false );
    // You can't read from `zeroed.value` because a reference can't be zeroed.
}
unsafe{
    let zeroed=CondValue::<char>::zeroed();
    assert_eq!( zeroed.cond, false );
    assert_eq!( zeroed.value, '\0' );
}
```

### Union (requires nightly)

This is an example of a `#[repr(transparent)]` union.

*/
#![cfg_attr(feature="nightly_docs",doc="```rust")]
#![cfg_attr(not(feature="nightly_docs"),doc="```ignore")]
/*!
#![feature(transparent_unions)]

use zeroable::Zeroable;

#[derive(Zeroable)]
#[repr(transparent)]
union Wrapper<T:Copy>{
    value:T,
}

unsafe{
    assert_eq!( Wrapper::<isize>::zeroed().value, 0_isize );
    assert_eq!( Wrapper::<usize>::zeroed().value, 0_usize );
    assert_eq!( Wrapper::<(usize,usize)>::zeroed().value, (0_usize,0_usize) );
}
```

### Union (non-compiling)

This doesn't compile because `ManuallyDrop<T>` is not zeroable.

```compile_fail
use zeroable::Zeroable;

use core::mem::ManuallyDrop;

#[derive(Zeroable)]
#[zero(not_zeroable(T))]
union MaybeUninitialized<T:Copy>{
    uninit:(),
    init:ManuallyDrop<T>,
}
```

To fix it simply remove the default `Zeroable` bound on the field like this:
```
use zeroable::Zeroable;

use core::mem::ManuallyDrop;

#[derive(Zeroable)]
#[zero(not_zeroable(T))]
union MaybeUninitialized<T:Copy>{
    uninit:(),
    #[zero(nonzero)]
    init:ManuallyDrop<T>,
}
```



### Union (non-compiling)

This doesn't compile because the union has a `#[zero(nonzero_fields)]` attribute,
but no field has a `#[zero(zeroable)]` attribute.

```compile_fail
use zeroable::Zeroable;

use core::mem::ManuallyDrop;

#[derive(Zeroable)]
#[zero(not_zeroable(T))]
#[zero(nonzero_fields)]
union UnsafeEither<T:Copy,U:Copy>{
    left:T,
    right:U,
}
```

To fix this instance simply add a `#[zero(zeroable)]` attribute to a field like this:
```
use zeroable::Zeroable;

use core::mem::ManuallyDrop;

#[derive(Zeroable)]
#[zero(not_zeroable(T))]
#[zero(nonzero_fields)]
union UnsafeEither<T:Copy,U:Copy>{
    left:T,
    #[zero(zeroable)]
    right:U,
}
```



*/