Add global_block! macro

Author: madsmtm

block2/CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased - YYYY-MM-DD
 
+### Added
+* `GlobalBlock` and corresponding `global_block!` macro, allowing statically
+  creating blocks that don't reference their environment.
+
 
 ## 0.2.0-alpha.1 - 2021-11-22
 

block2/src/global.rs

@@ -0,0 +1,184 @@
+use core::marker::PhantomData;
+use core::mem;
+use core::ops::Deref;
+use std::os::raw::c_ulong;
+
+use objc2_encode::{Encode, EncodeArguments};
+
+use super::{ffi, Block};
+use crate::BlockArguments;
+
+#[doc(hidden)]
+pub const __GLOBAL_DESCRIPTOR: ffi::Block_descriptor_header = ffi::Block_descriptor_header {
+    reserved: 0,
+    size: mem::size_of::<ffi::Block_layout>() as c_ulong,
+};
+
+/// An Objective-C block that does not capture it's environment.
+///
+/// This is effectively just a glorified function pointer, and can created and
+/// stored in static memory using the [`global_block`][`global_block!`] macro.
+///
+/// If [`ConcreteBlock`] is the [`Fn`]-block equivalent, this is likewise the
+/// [`fn`]-block equivalent.
+#[repr(C)]
+pub struct GlobalBlock<A, R = ()> {
+    layout: ffi::Block_layout,
+    p: PhantomData<(A, R)>,
+}
+
+unsafe impl<A, R> Sync for GlobalBlock<A, R>
+where
+    A: BlockArguments + EncodeArguments,
+    R: Encode,
+{
+}
+unsafe impl<A, R> Send for GlobalBlock<A, R>
+where
+    A: BlockArguments + EncodeArguments,
+    R: Encode,
+{
+}
+
+// Note: We can't put correct bounds on A and R because we have a const fn!
+//
+// Fortunately, we don't need them, since they're present on `Sync`, so
+// constructing the static in `global_block!` with an invalid `GlobalBlock`
+// triggers an error.
+impl<A, R> GlobalBlock<A, R> {
+    /// Use the [`global_block`] macro instead.
+    #[doc(hidden)]
+    pub const unsafe fn from_layout(layout: ffi::Block_layout) -> Self {
+        Self {
+            layout,
+            p: PhantomData,
+        }
+    }
+}
+
+impl<A, R> Deref for GlobalBlock<A, R>
+where
+    A: BlockArguments + EncodeArguments,
+    R: Encode,
+{
+    type Target = Block<A, R>;
+
+    fn deref(&self) -> &Block<A, R> {
+        // TODO: SAFETY
+        unsafe { &*(self as *const Self as *const Block<A, R>) }
+    }
+}
+
+/// Construct a static [`GlobalBlock`].
+///
+/// The syntax is similar to a static closure. Note that the block cannot
+/// capture it's environment, and it's argument types and return type must be
+/// [`Encode`].
+///
+/// # Examples
+///
+/// ```
+/// use block2::global_block;
+/// global_block! {
+///     static MY_BLOCK = || -> i32 {
+///         42
+///     }
+/// };
+/// assert_eq!(unsafe { MY_BLOCK.call(()) }, 42);
+/// ```
+///
+/// ```
+/// use block2::global_block;
+/// global_block! {
+///     static ADDER_BLOCK = |x: i32, y: i32,| -> i32 {
+///         x + y
+///     }
+/// };
+/// assert_eq!(unsafe { ADDER_BLOCK.call((5, 7)) }, 12);
+/// ```
+///
+/// ```
+/// use block2::global_block;
+/// global_block! {
+///     pub static MUTATING_BLOCK = |x: &mut i32| {
+///         *x = *x + 42;
+///     }
+/// };
+/// let mut x = 5;
+/// unsafe { MUTATING_BLOCK.call((&mut x,)) };
+/// assert_eq!(x, 47);
+/// ```
+///
+/// The following does not compile because [`Box`] is not [`Encode`]:
+///
+/// ```compile_fail
+/// use block2::global_block;
+/// global_block! {
+///     pub static INVALID_BLOCK = |b: Box<i32>| {}
+/// };
+/// ```
+#[macro_export]
+macro_rules! global_block {
+    // `||` is parsed as one token
+    (
+        $(#[$m:meta])*
+        $vis:vis static $name:ident = || $(-> $r:ty)? $body:block
+    ) => {
+        $crate::global_block!($(#[$m])* $vis static $name = |,| $(-> $r)? $body);
+    };
+    (
+        $(#[$m:meta])*
+        $vis:vis static $name:ident = |$($a:ident: $t:ty),* $(,)?| $(-> $r:ty)? $body:block
+    ) => {
+        $(#[$m])*
+        #[allow(unused_unsafe)]
+        $vis static $name: $crate::GlobalBlock<($($t,)*) $(, $r)?> = unsafe {
+            $crate::GlobalBlock::from_layout($crate::ffi::Block_layout {
+                isa: &$crate::ffi::_NSConcreteGlobalBlock as *const _ as *mut _,
+                flags: $crate::ffi::BLOCK_IS_GLOBAL | $crate::ffi::BLOCK_USE_STRET,
+                reserved: 0,
+                invoke: {
+                    unsafe extern "C" fn inner(_: *mut $crate::ffi::Block_layout, $($a: $t),*) $(-> $r)? {
+                        $body
+                    }
+                    let inner: unsafe extern "C" fn(*mut $crate::ffi::Block_layout, $($a: $t),*) $(-> $r)? = inner;
+
+                    // TODO: SAFETY
+                    ::core::mem::transmute(inner)
+                },
+                descriptor: &$crate::__GLOBAL_DESCRIPTOR as *const _ as *mut _,
+            })
+        };
+    };
+}
+
+#[cfg(test)]
+mod tests {
+    global_block! {
+        /// Test comments and visibility
+        pub(super) static NOOP_BLOCK = || {}
+    }
+
+    global_block! {
+        /// Multiple arguments + trailing comma
+        #[allow(unused)]
+        static BLOCK = |x: i32, y: i32, z: i32, w: i32,| -> i32 {
+            x + y + z + w
+        }
+    }
+
+    #[test]
+    fn test_noop_block() {
+        unsafe { NOOP_BLOCK.call(()) };
+    }
+
+    #[test]
+    fn test_defined_in_function() {
+        global_block!(
+            static MY_BLOCK = || -> i32 {
+                42
+            }
+        );
+        assert_eq!(unsafe { MY_BLOCK.call(()) }, 42);
+    }
+}

block2/src/lib.rs

@@ -66,6 +66,11 @@ use std::os::raw::{c_int, c_ulong};
 pub use block_sys as ffi;
 use objc2_encode::{Encode, EncodeArguments, Encoding, RefEncode};
 
+#[macro_use]
+mod global;
+
+pub use global::{GlobalBlock, __GLOBAL_DESCRIPTOR};
+
 /// Types that may be used as the arguments to an Objective-C block.
 pub trait BlockArguments: Sized {
     /// Calls the given `Block` with self as the arguments.

tests/src/lib.rs

@@ -37,7 +37,19 @@ pub fn invoke_add_block(block: &Block<(i32,), i32>, a: i32) -> i32 {
 mod tests {
     use super::*;
     use alloc::string::ToString;
-    use block2::{ConcreteBlock, RcBlock};
+    use block2::{global_block, ConcreteBlock, RcBlock};
+
+    global_block! {
+        /// Test `global_block` in an external crate
+        static MY_BLOCK = || -> i32 {
+            42
+        }
+    }
+
+    #[test]
+    fn test_global_block() {
+        assert_eq!(invoke_int_block(&MY_BLOCK), 42);
+    }
 
     #[test]
     fn test_call_block() {