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/*!
Prefix-types are types that derive StableAbi along with the
`#[sabi(kind(Prefix(prefix_struct="PrefixEquivalent")))]` attribute.
This is mostly intended for **vtables** and **modules**.
Prefix-types cannot directly be passed through ffi,
instead they must be converted to the type declared with `prefix_struct="PrefixEquivalent"`,
and then pass `&PrefixEquivalent` instead.
To convert `T` to `&PrefixEquivalent` use either:
- `PrefixTypeTrait::leak_into_prefix`:<br>
Which does the conversion directly,but leaks the value.
- `prefix_type::WithMetadata::new` and then `WithMetadata::as_prefix`:<br>
Use this if you need a compiletime constant.
First create a `&'a WithMetadata<Self>` constant,
then use the `WithMetadata::as_prefix` method at runtime
to cast it to `&PrefixEquivalent`.
All fields on `&PrefixEquivalent` are accessed through accessor methods
with the same name as the fields.
To ensure that libraries stay abi compatible,
the first minor version of the library must apply the `#[sabi(last_prefix_field)]` to some
field and every minor version after that must add fields at the end (never moving that attribute).
Changing the field that `#[sabi(last_prefix_field)]` is applied to is a breaking change.
Getter methods for fields after the one to which `#[sabi(last_prefix_field)]` was applied to
will return `Option<FieldType>` by default,because those fields might not exist
(the struct might come from a previous version of the library).
To override how to deal with nonexistent fields,
use the `#[sabi(missing_field())]` attribute,
applied to either the struct or the field.
# Grammar Reference
For the grammar reference,you can look at the documentation for
[`#[derive(StableAbi)]`](../stable_abi_derive/index.html).
# Examples
### Example 1
Declaring a Prefix-type.
```
use abi_stable::{
StableAbi,
std_types::{RDuration,RStr},
};
#[repr(C)]
#[derive(StableAbi)]
#[sabi(kind(Prefix(prefix_struct="Module")))]
#[sabi(missing_field(panic))]
pub struct ModuleVal {
pub lib_name:RStr<'static>,
#[sabi(last_prefix_field)]
pub elapsed:extern fn()->RDuration,
pub description:RStr<'static>,
}
# fn main(){}
```
### Example 2:Declaring a type with a VTable
Here is the implementation of a Box-like type,which uses a VTable that is itself a Prefix.
```
use std::{
ops::{Deref,DerefMut},
marker::PhantomData,
mem::ManuallyDrop,
};
use abi_stable::{
StableAbi,
extern_fn_panic_handling,
pointer_trait::{CallReferentDrop, StableDeref, TransmuteElement},
prefix_type::{PrefixTypeTrait,WithMetadata},
};
/// An ffi-safe `Box<T>`
#[repr(C)]
#[derive(StableAbi)]
pub struct BoxLike<T> {
data: *mut T,
// This can't be a `&'static BoxVtable<T>` because Rust will complain that
// `T` does not live for the `'static` lifetime.
vtable: *const BoxVtable<T>,
_marker: PhantomData<T>,
}
impl<T> BoxLike<T>{
pub fn new(value:T)->Self{
let box_=Box::new(value);
Self{
data:Box::into_raw(box_),
vtable:unsafe{ (*BoxVtableVal::VTABLE).as_prefix() },
_marker:PhantomData,
}
}
// This is to get around a limitation of the type system where
// vtables of generic types can't just be `&'static VTable<T>`
// because it complains that T doesn't live forever.
fn vtable<'a>(&self)->&'a BoxVtable<T>{
unsafe{ &(*self.vtable) }
}
/// Extracts the value this owns.
pub fn into_inner(self)->T{
let this=ManuallyDrop::new(self);
let vtable=this.vtable();
unsafe{
// Must copy this before calling `vtable.destructor()`
// because otherwise it would be reading from a dangling pointer.
let ret=this.data.read();
vtable.destructor()(this.data,CallReferentDrop::No);
ret
}
}
}
impl<T> Deref for BoxLike<T> {
type Target=T;
fn deref(&self)->&T{
unsafe{
&(*self.data)
}
}
}
impl<T> DerefMut for BoxLike<T> {
fn deref_mut(&mut self)->&mut T{
unsafe{
&mut (*self.data)
}
}
}
impl<T> Drop for BoxLike<T>{
fn drop(&mut self){
let vtable=self.vtable();
unsafe{
vtable.destructor()(self.data,CallReferentDrop::Yes)
}
}
}
#[repr(C)]
#[derive(StableAbi)]
#[sabi(kind(Prefix(prefix_struct="BoxVtable")))]
pub(crate) struct BoxVtableVal<T> {
#[sabi(last_prefix_field)]
destructor: unsafe extern "C" fn(*mut T, CallReferentDrop),
}
impl<T> BoxVtableVal<T>{
const TMP0:Self=Self{
destructor:destroy_box::<T>,
};
// This can't be a `&'static WithMetadata<Self>` because Rust will complain that
// `T` does not live for the `'static` lifetime.
const VTABLE:*const WithMetadata<Self>={
&WithMetadata::new(PrefixTypeTrait::METADATA,Self::TMP0)
};
}
unsafe extern "C" fn destroy_box<T>(v: *mut T, call_drop: CallReferentDrop) {
extern_fn_panic_handling! {
let mut box_ = Box::from_raw(v as *mut ManuallyDrop<T>);
if call_drop == CallReferentDrop::Yes {
ManuallyDrop::drop(&mut *box_);
}
drop(box_);
}
}
# fn main(){}
```
### Example 3:module
This declares,initializes,and uses a module.
```
use abi_stable::{
StableAbi,
std_types::RDuration,
prefix_type::PrefixTypeTrait,
};
#[repr(C)]
#[derive(StableAbi)]
#[sabi(kind(Prefix(prefix_struct="PersonMod")))]
pub struct PersonModVal {
// The getter for this field is infallible,defined (approximately) like this:
// ```
// extern fn customer_for(&self)->extern fn(Id)->RDuration {
// self.customer_for
// }
// ```
#[sabi(last_prefix_field)]
pub customer_for: extern fn(Id)->RDuration,
// The default behavior for the getter is to return an Option<FieldType>,
// if the field exists it returns Some(_),
// otherwise it returns None.
pub bike_count: extern fn(Id)->u32,
// The getter for this field panics if the field doesn't exist.
#[sabi(missing_field(panic))]
pub visits: extern fn(Id)->u32,
// The getter for this field returns `default_score()` if the field doesn't exist.
#[sabi(missing_field(with="default_score"))]
pub score: extern fn(Id)->u32,
// The getter for this field returns `Default::default()` if the field doesn't exist.
#[sabi(missing_field(default))]
pub visit_length: Option< extern fn(Id)->RDuration >,
}
fn default_score()-> extern fn(Id)->u32 {
extern fn default(_:Id)->u32{
1000
}
default
}
type Id=u32;
#
# static VARS:&[(RDuration,u32)]=&[
# (RDuration::new(1_000,0),10),
# (RDuration::new(1_000_000,0),1),
# ];
#
# extern fn customer_for(id:Id)->RDuration{
# VARS[id as usize].0
# }
#
# extern fn bike_count(id:Id)->u32{
# VARS[id as usize].1
# }
# extern fn visits(id:Id)->u32{
# VARS[id as usize].1
# }
# extern fn score(id:Id)->u32{
# VARS[id as usize].1
# }
#
/*
...
Elided function definitions
...
*/
# fn main(){
let module:&'static PersonMod=
PersonModVal{
customer_for,
bike_count,
visits,
score,
visit_length:None,
}.leak_into_prefix();
// Getting the value for every field of `module`.
let customer_for: extern fn(Id)->RDuration =
module.customer_for();
let bike_count: Option<extern fn(Id)->u32> =
module.bike_count();
let visits: extern fn(Id)->u32=
module.visits();
let score: extern fn(Id)->u32=
module.score();
let visit_length: Option<extern fn(Id)->RDuration> =
module.visit_length();
# }
```
*/