From 6396261dbbfe7d64ddfa8b503680165ac0289f80 Mon Sep 17 00:00:00 2001
From: TellowKrinkle <tellowkrinkle@gmail.com>
Date: Sun, 2 Oct 2022 16:46:20 -0500
Subject: [PATCH 1/2] Don't heap-allocate block descriptors

---
 src/lib.rs | 35 +++++++++++++++++------------------
 1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index f05eee6..ab687bd 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -52,7 +52,6 @@ use std::marker::PhantomData;
 use std::mem;
 use std::ops::{Deref, DerefMut};
 use std::os::raw::{c_int, c_ulong, c_void};
-use std::ptr;
 
 enum Class { }
 
@@ -236,7 +235,7 @@ concrete_block_impl!(concrete_block_invoke_args12, a: A, b: B, c: C, d: D, e: E,
 #[repr(C)]
 pub struct ConcreteBlock<A, R, F> {
     base: BlockBase<A, R>,
-    descriptor: Box<BlockDescriptor<ConcreteBlock<A, R, F>>>,
+    descriptor: &'static BlockDescriptor,
     closure: F,
 }
 
@@ -264,7 +263,7 @@ impl<A, R, F> ConcreteBlock<A, R, F> {
                 _reserved: 0,
                 invoke: mem::transmute(invoke),
             },
-            descriptor: Box::new(BlockDescriptor::new()),
+            descriptor: &BlockDescriptorImpl::<ConcreteBlock<A, R, F>>::DESCRIPTOR,
             closure: closure,
         }
     }
@@ -308,32 +307,32 @@ impl<A, R, F> DerefMut for ConcreteBlock<A, R, F> {
     }
 }
 
-unsafe extern fn block_context_dispose<B>(block: &mut B) {
+unsafe extern fn block_context_dispose<B>(block: *mut c_void) {
     // Read the block onto the stack and let it drop
-    ptr::read(block);
+    block.cast::<B>().read();
 }
 
-unsafe extern fn block_context_copy<B>(_dst: &mut B, _src: &B) {
+unsafe extern fn block_context_copy(_dst: *mut c_void, _src: *const c_void) {
     // The runtime memmoves the src block into the dst block, nothing to do
 }
 
 #[repr(C)]
-struct BlockDescriptor<B> {
+struct BlockDescriptor {
     _reserved: c_ulong,
     block_size: c_ulong,
-    copy_helper: unsafe extern fn(&mut B, &B),
-    dispose_helper: unsafe extern fn(&mut B),
+    copy_helper: unsafe extern fn(*mut c_void, *const c_void),
+    dispose_helper: unsafe extern fn(*mut c_void),
 }
 
-impl<B> BlockDescriptor<B> {
-    fn new() -> BlockDescriptor<B> {
-        BlockDescriptor {
-            _reserved: 0,
-            block_size: mem::size_of::<B>() as c_ulong,
-            copy_helper: block_context_copy::<B>,
-            dispose_helper: block_context_dispose::<B>,
-        }
-    }
+struct BlockDescriptorImpl<B>(PhantomData<B>);
+
+impl<B> BlockDescriptorImpl<B> {
+    const DESCRIPTOR: BlockDescriptor = BlockDescriptor {
+        _reserved: 0,
+        block_size: mem::size_of::<B>() as c_ulong,
+        copy_helper: block_context_copy,
+        dispose_helper: block_context_dispose::<B>,
+    };
 }
 
 #[cfg(test)]

From 3d715d246cbfb974303f48e978d35f82e85b8694 Mon Sep 17 00:00:00 2001
From: TellowKrinkle <tellowkrinkle@gmail.com>
Date: Sun, 2 Oct 2022 17:52:12 -0500
Subject: [PATCH 2/2] Only add BLOCK_HAS_COPY_DISPOSE when needed

---
 src/lib.rs | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index ab687bd..eca877b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -255,15 +255,16 @@ impl<A, R, F> ConcreteBlock<A, R, F> {
     /// correct arguments.
     unsafe fn with_invoke(invoke: unsafe extern fn(*mut Self, ...) -> R,
             closure: F) -> Self {
+        let descriptor = &BlockDescriptorImpl::<ConcreteBlock<A, R, F>>::DESCRIPTOR;
+        const BLOCK_HAS_COPY_DISPOSE: c_int = 1 << 25;
         ConcreteBlock {
             base: BlockBase {
                 isa: &_NSConcreteStackBlock,
-                // 1 << 25 = BLOCK_HAS_COPY_DISPOSE
-                flags: 1 << 25,
+                flags: if descriptor.dispose_helper.is_some() { BLOCK_HAS_COPY_DISPOSE } else { 0 },
                 _reserved: 0,
                 invoke: mem::transmute(invoke),
             },
-            descriptor: &BlockDescriptorImpl::<ConcreteBlock<A, R, F>>::DESCRIPTOR,
+            descriptor: descriptor,
             closure: closure,
         }
     }
@@ -320,8 +321,8 @@ unsafe extern fn block_context_copy(_dst: *mut c_void, _src: *const c_void) {
 struct BlockDescriptor {
     _reserved: c_ulong,
     block_size: c_ulong,
-    copy_helper: unsafe extern fn(*mut c_void, *const c_void),
-    dispose_helper: unsafe extern fn(*mut c_void),
+    copy_helper: Option<unsafe extern fn(*mut c_void, *const c_void)>,
+    dispose_helper: Option<unsafe extern fn(*mut c_void)>,
 }
 
 struct BlockDescriptorImpl<B>(PhantomData<B>);
@@ -330,8 +331,8 @@ impl<B> BlockDescriptorImpl<B> {
     const DESCRIPTOR: BlockDescriptor = BlockDescriptor {
         _reserved: 0,
         block_size: mem::size_of::<B>() as c_ulong,
-        copy_helper: block_context_copy,
-        dispose_helper: block_context_dispose::<B>,
+        copy_helper: if std::mem::needs_drop::<B>() { Some(block_context_copy) } else { None },
+        dispose_helper: if std::mem::needs_drop::<B>() { Some(block_context_dispose::<B>) } else { None },
     };
 }