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Diffstat (limited to 'rust/qemu-api/src/callbacks.rs')
| -rw-r--r-- | rust/qemu-api/src/callbacks.rs | 241 |
1 files changed, 0 insertions, 241 deletions
diff --git a/rust/qemu-api/src/callbacks.rs b/rust/qemu-api/src/callbacks.rs deleted file mode 100644 index 9642a16..0000000 --- a/rust/qemu-api/src/callbacks.rs +++ /dev/null @@ -1,241 +0,0 @@ -// SPDX-License-Identifier: MIT - -//! Utility functions to deal with callbacks from C to Rust. - -use std::{mem, ptr::NonNull}; - -/// Trait for functions (types implementing [`Fn`]) that can be used as -/// callbacks. These include both zero-capture closures and function pointers. -/// -/// In Rust, calling a function through the `Fn` trait normally requires a -/// `self` parameter, even though for zero-sized functions (including function -/// pointers) the type itself contains all necessary information to call the -/// function. This trait provides a `call` function that doesn't require `self`, -/// allowing zero-sized functions to be called using only their type. -/// -/// This enables zero-sized functions to be passed entirely through generic -/// parameters and resolved at compile-time. A typical use is a function -/// receiving an unused parameter of generic type `F` and calling it via -/// `F::call` or passing it to another function via `func::<F>`. -/// -/// QEMU uses this trick to create wrappers to C callbacks. The wrappers -/// are needed to convert an opaque `*mut c_void` into a Rust reference, -/// but they only have a single opaque that they can use. The `FnCall` -/// trait makes it possible to use that opaque for `self` or any other -/// reference: -/// -/// ```ignore -/// // The compiler creates a new `rust_bh_cb` wrapper for each function -/// // passed to `qemu_bh_schedule_oneshot` below. -/// unsafe extern "C" fn rust_bh_cb<T, F: for<'a> FnCall<(&'a T,)>>( -/// opaque: *mut c_void, -/// ) { -/// // SAFETY: the opaque was passed as a reference to `T`. -/// F::call((unsafe { &*(opaque.cast::<T>()) }, )) -/// } -/// -/// // The `_f` parameter is unused but it helps the compiler build the appropriate `F`. -/// // Using a reference allows usage in const context. -/// fn qemu_bh_schedule_oneshot<T, F: for<'a> FnCall<(&'a T,)>>(_f: &F, opaque: &T) { -/// let cb: unsafe extern "C" fn(*mut c_void) = rust_bh_cb::<T, F>; -/// unsafe { -/// bindings::qemu_bh_schedule_oneshot(cb, opaque as *const T as *const c_void as *mut c_void) -/// } -/// } -/// ``` -/// -/// Each wrapper is a separate instance of `rust_bh_cb` and is therefore -/// compiled to a separate function ("monomorphization"). If you wanted -/// to pass `self` as the opaque value, the generic parameters would be -/// `rust_bh_cb::<Self, F>`. -/// -/// `Args` is a tuple type whose types are the arguments of the function, -/// while `R` is the returned type. -/// -/// # Examples -/// -/// ``` -/// # use qemu_api::callbacks::FnCall; -/// fn call_it<F: for<'a> FnCall<(&'a str,), String>>(_f: &F, s: &str) -> String { -/// F::call((s,)) -/// } -/// -/// let s: String = call_it(&str::to_owned, "hello world"); -/// assert_eq!(s, "hello world"); -/// ``` -/// -/// Note that the compiler will produce a different version of `call_it` for -/// each function that is passed to it. Therefore the argument is not really -/// used, except to decide what is `F` and what `F::call` does. -/// -/// Attempting to pass a non-zero-sized closure causes a compile-time failure: -/// -/// ```compile_fail -/// # use qemu_api::callbacks::FnCall; -/// # fn call_it<'a, F: FnCall<(&'a str,), String>>(_f: &F, s: &'a str) -> String { -/// # F::call((s,)) -/// # } -/// let x: &'static str = "goodbye world"; -/// call_it(&move |_| String::from(x), "hello workd"); -/// ``` -/// -/// `()` can be used to indicate "no function": -/// -/// ``` -/// # use qemu_api::callbacks::FnCall; -/// fn optional<F: for<'a> FnCall<(&'a str,), String>>(_f: &F, s: &str) -> Option<String> { -/// if F::IS_SOME { -/// Some(F::call((s,))) -/// } else { -/// None -/// } -/// } -/// -/// assert!(optional(&(), "hello world").is_none()); -/// ``` -/// -/// Invoking `F::call` will then be a run-time error. -/// -/// ```should_panic -/// # use qemu_api::callbacks::FnCall; -/// # fn call_it<F: for<'a> FnCall<(&'a str,), String>>(_f: &F, s: &str) -> String { -/// # F::call((s,)) -/// # } -/// let s: String = call_it(&(), "hello world"); // panics -/// ``` -/// -/// # Safety -/// -/// Because `Self` is a zero-sized type, all instances of the type are -/// equivalent. However, in addition to this, `Self` must have no invariants -/// that could be violated by creating a reference to it. -/// -/// This is always true for zero-capture closures and function pointers, as long -/// as the code is able to name the function in the first place. -pub unsafe trait FnCall<Args, R = ()>: 'static + Sync + Sized { - /// Referring to this internal constant asserts that the `Self` type is - /// zero-sized. Can be replaced by an inline const expression in - /// Rust 1.79.0+. - const ASSERT_ZERO_SIZED: () = { assert!(mem::size_of::<Self>() == 0) }; - - /// Referring to this constant asserts that the `Self` type is an actual - /// function type, which can be used to catch incorrect use of `()` - /// at compile time. - /// - /// # Examples - /// - /// ```compile_fail - /// # use qemu_api::callbacks::FnCall; - /// fn call_it<F: for<'a> FnCall<(&'a str,), String>>(_f: &F, s: &str) -> String { - /// let _: () = F::ASSERT_IS_SOME; - /// F::call((s,)) - /// } - /// - /// let s: String = call_it((), "hello world"); // does not compile - /// ``` - /// - /// Note that this can be more simply `const { assert!(F::IS_SOME) }` in - /// Rust 1.79.0 or newer. - const ASSERT_IS_SOME: () = { assert!(Self::IS_SOME) }; - - /// `true` if `Self` is an actual function type and not `()`. - /// - /// # Examples - /// - /// You can use `IS_SOME` to catch this at compile time: - /// - /// ```compile_fail - /// # use qemu_api::callbacks::FnCall; - /// fn call_it<F: for<'a> FnCall<(&'a str,), String>>(_f: &F, s: &str) -> String { - /// const { assert!(F::IS_SOME) } - /// F::call((s,)) - /// } - /// - /// let s: String = call_it((), "hello world"); // does not compile - /// ``` - const IS_SOME: bool; - - /// `false` if `Self` is an actual function type, `true` if it is `()`. - fn is_none() -> bool { - !Self::IS_SOME - } - - /// `true` if `Self` is an actual function type, `false` if it is `()`. - fn is_some() -> bool { - Self::IS_SOME - } - - /// Call the function with the arguments in args. - fn call(a: Args) -> R; -} - -/// `()` acts as a "null" callback. Using `()` and `function` is nicer -/// than `None` and `Some(function)`, because the compiler is unable to -/// infer the type of just `None`. Therefore, the trait itself acts as the -/// option type, with functions [`FnCall::is_some`] and [`FnCall::is_none`]. -unsafe impl<Args, R> FnCall<Args, R> for () { - const IS_SOME: bool = false; - - /// Call the function with the arguments in args. - fn call(_a: Args) -> R { - panic!("callback not specified") - } -} - -macro_rules! impl_call { - ($($args:ident,)* ) => ( - // SAFETY: because each function is treated as a separate type, - // accessing `FnCall` is only possible in code that would be - // allowed to call the function. - unsafe impl<F, $($args,)* R> FnCall<($($args,)*), R> for F - where - F: 'static + Sync + Sized + Fn($($args, )*) -> R, - { - const IS_SOME: bool = true; - - #[inline(always)] - fn call(a: ($($args,)*)) -> R { - let _: () = Self::ASSERT_ZERO_SIZED; - - // SAFETY: the safety of this method is the condition for implementing - // `FnCall`. As to the `NonNull` idiom to create a zero-sized type, - // see https://github.com/rust-lang/libs-team/issues/292. - let f: &'static F = unsafe { &*NonNull::<Self>::dangling().as_ptr() }; - let ($($args,)*) = a; - f($($args,)*) - } - } - ) -} - -impl_call!(_1, _2, _3, _4, _5,); -impl_call!(_1, _2, _3, _4,); -impl_call!(_1, _2, _3,); -impl_call!(_1, _2,); -impl_call!(_1,); -impl_call!(); - -#[cfg(test)] -mod tests { - use super::*; - - // The `_f` parameter is unused but it helps the compiler infer `F`. - fn do_test_call<'a, F: FnCall<(&'a str,), String>>(_f: &F) -> String { - F::call(("hello world",)) - } - - #[test] - fn test_call() { - assert_eq!(do_test_call(&str::to_owned), "hello world") - } - - // The `_f` parameter is unused but it helps the compiler infer `F`. - fn do_test_is_some<'a, F: FnCall<(&'a str,), String>>(_f: &F) { - assert!(F::is_some()); - } - - #[test] - fn test_is_some() { - do_test_is_some(&str::to_owned); - } -} |