1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
//! A framework for Rust wrappers over C APIs.
//!
//! Ownership is as important in C as it is in Rust, but the semantics are often implicit. In
//! particular, pointer-to-value is commonly used to pass C values both when transferring ownership
//! or a borrow.
//!
//! This crate provides a framework to define a Rust wrapper over these kinds of raw C APIs in a way
//! that allows ownership semantics to be expressed in an ergonomic manner. The framework takes a
//! dual-type approach similar to APIs in the standard library such as `PathBuf`/`Path` or `String`/
//! `&str`. One type represents an owned value and references to the other represent borrowed
//! values.
//!
//! # Examples
//!
//! ```
//! use foreign_types::{ForeignType, ForeignTypeRef, Opaque};
//! use std::ops::{Deref, DerefMut};
//!
//! mod foo_sys {
//!     pub enum FOO {}
//!
//!     extern {
//!         pub fn FOO_free(foo: *mut FOO);
//!     }
//! }
//!
//! // The borrowed type is a newtype wrapper around an `Opaque` value.
//! //
//! // `FooRef` values never exist; we instead create references to `FooRef`s from raw C pointers.
//! pub struct FooRef(Opaque);
//!
//! impl ForeignTypeRef for FooRef {
//!     type CType = foo_sys::FOO;
//! }
//!
//! // The owned type is simply a newtype wrapper around the raw C type.
//! //
//! // It dereferences to `FooRef`, so methods that do not require ownership should be defined
//! // there.
//! pub struct Foo(*mut foo_sys::FOO);
//!
//! impl Drop for Foo {
//!     fn drop(&mut self) {
//!         unsafe { foo_sys::FOO_free(self.0) }
//!     }
//! }
//!
//! impl ForeignType for Foo {
//!     type CType = foo_sys::FOO;
//!     type Ref = FooRef;
//!
//!     unsafe fn from_ptr(ptr: *mut foo_sys::FOO) -> Foo {
//!         Foo(ptr)
//!     }
//!
//!     fn as_ptr(&self) -> *mut foo_sys::FOO {
//!         self.0
//!     }
//! }
//!
//! impl Deref for Foo {
//!     type Target = FooRef;
//!
//!     fn deref(&self) -> &FooRef {
//!         unsafe { FooRef::from_ptr(self.0) }
//!     }
//! }
//!
//! impl DerefMut for Foo {
//!     fn deref_mut(&mut self) -> &mut FooRef {
//!         unsafe { FooRef::from_ptr_mut(self.0) }
//!     }
//! }
//! ```
//!
//! The `foreign_type!` macro can generate this boilerplate for you:
//!
//! ```
//! #[macro_use]
//! extern crate foreign_types;
//!
//! mod foo_sys {
//!     pub enum FOO {}
//!
//!     extern {
//!         pub fn FOO_free(foo: *mut FOO);
//!     }
//! }
//!
//! foreign_type! {
//!     type CType = foo_sys::FOO;
//!     fn drop = foo_sys::FOO_free;
//!     /// A Foo.
//!     pub struct Foo;
//!     /// A borrowed Foo.
//!     pub struct FooRef;
//! }
//!
//! # fn main() {}
//! ```
//!
//! Say we then have a separate type in our C API that contains a `FOO`:
//!
//! ```
//! mod foo_sys {
//!     pub enum FOO {}
//!     pub enum BAR {}
//!
//!     extern {
//!         pub fn FOO_free(foo: *mut FOO);
//!         pub fn BAR_free(bar: *mut BAR);
//!         pub fn BAR_get_foo(bar: *mut BAR) -> *mut FOO;
//!     }
//! }
//! ```
//!
//! The documentation for the C library states that `BAR_get_foo` returns a reference into the `BAR`
//! passed to it, which translates into a reference in Rust. It also says that we're allowed to
//! modify the `FOO`, so we'll define a pair of accessor methods, one immutable and one mutable:
//!
//! ```
//! #[macro_use]
//! extern crate foreign_types;
//!
//! use foreign_types::ForeignTypeRef;
//!
//! mod foo_sys {
//!     pub enum FOO {}
//!     pub enum BAR {}
//!
//!     extern {
//!         pub fn FOO_free(foo: *mut FOO);
//!         pub fn BAR_free(bar: *mut BAR);
//!         pub fn BAR_get_foo(bar: *mut BAR) -> *mut FOO;
//!     }
//! }
//!
//! foreign_type! {
//!     type CType = foo_sys::FOO;
//!     fn drop = foo_sys::FOO_free;
//!     /// A Foo.
//!     pub struct Foo;
//!     /// A borrowed Foo.
//!     pub struct FooRef;
//! }
//!
//! foreign_type! {
//!     type CType = foo_sys::BAR;
//!     fn drop = foo_sys::BAR_free;
//!     /// A Foo.
//!     pub struct Bar;
//!     /// A borrowed Bar.
//!     pub struct BarRef;
//! }
//!
//! impl BarRef {
//!     fn foo(&self) -> &FooRef {
//!         unsafe { FooRef::from_ptr(foo_sys::BAR_get_foo(self.as_ptr())) }
//!     }
//!
//!     fn foo_mut(&mut self) -> &mut FooRef {
//!         unsafe { FooRef::from_ptr_mut(foo_sys::BAR_get_foo(self.as_ptr())) }
//!     }
//! }
//!
//! # fn main() {}
//! ```
#![no_std]
#![warn(missing_docs)]
#![doc(html_root_url="https://docs.rs/foreign-types/0.2.0")]

use core::cell::UnsafeCell;

/// An opaque type used to define `ForeignTypeRef` types.
///
/// A type implementing `ForeignTypeRef` should simply be a newtype wrapper around this type.
pub struct Opaque(UnsafeCell<()>);

/// A type implemented by wrappers over foreign types.
pub trait ForeignType: Sized {
    /// The raw C type.
    type CType;

    /// The type representing a reference to this type.
    type Ref: ForeignTypeRef<CType = Self::CType>;

    /// Constructs an instance of this type from its raw type.
    unsafe fn from_ptr(ptr: *mut Self::CType) -> Self;

    /// Returns a raw pointer to the wrapped value.
    fn as_ptr(&self) -> *mut Self::CType;
}

/// A trait implemented by types which reference borrowed foreign types.
pub trait ForeignTypeRef: Sized {
    /// The raw C type.
    type CType;

    /// Constructs a shared instance of this type from its raw type.
    #[inline]
    unsafe fn from_ptr<'a>(ptr: *mut Self::CType) -> &'a Self {
        &*(ptr as *mut _)
    }

    /// Constructs a mutable reference of this type from its raw type.
    #[inline]
    unsafe fn from_ptr_mut<'a>(ptr: *mut Self::CType) -> &'a mut Self {
        &mut *(ptr as *mut _)
    }

    /// Returns a raw pointer to the wrapped value.
    #[inline]
    fn as_ptr(&self) -> *mut Self::CType {
        self as *const _ as *mut _
    }
}

/// A macro to easily define wrappers for foreign types.
///
/// # Examples
///
/// ```
/// #[macro_use]
/// extern crate foreign_types;
///
/// # mod openssl_sys { pub type SSL = (); pub unsafe fn SSL_free(_: *mut SSL) {} }
/// foreign_type! {
///     type CType = openssl_sys::SSL;
///     fn drop = openssl_sys::SSL_free;
///     /// Documentation for the owned type.
///     pub struct Ssl;
///     /// Documentation for the borrowed type.
///     pub struct SslRef;
/// }
///
/// # fn main() {}
/// ```
#[macro_export]
macro_rules! foreign_type {
    (
        type CType = $ctype:ty;
        fn drop = $drop:expr;
        $(#[$owned_attr:meta])*
        pub struct $owned:ident;
        $(#[$borrowed_attr:meta])*
        pub struct $borrowed:ident;
    ) => {
        $(#[$owned_attr])*
        pub struct $owned(*mut $ctype);

        impl $crate::ForeignType for $owned {
            type CType = $ctype;
            type Ref = $borrowed;

            #[inline]
            unsafe fn from_ptr(ptr: *mut $ctype) -> $owned {
                $owned(ptr)
            }

            #[inline]
            fn as_ptr(&self) -> *mut $ctype {
                self.0
            }
        }

        impl Drop for $owned {
            #[inline]
            fn drop(&mut self) {
                unsafe { $drop(self.0) }
            }
        }

        impl ::std::ops::Deref for $owned {
            type Target = $borrowed;

            #[inline]
            fn deref(&self) -> &$borrowed {
                unsafe { $crate::ForeignTypeRef::from_ptr(self.0) }
            }
        }

        impl ::std::ops::DerefMut for $owned {
            #[inline]
            fn deref_mut(&mut self) -> &mut $borrowed {
                unsafe { $crate::ForeignTypeRef::from_ptr_mut(self.0) }
            }
        }

        $(#[$borrowed_attr])*
        pub struct $borrowed($crate::Opaque);

        impl $crate::ForeignTypeRef for $borrowed {
            type CType = $ctype;
        }
    }
}