Merge #2183

2183: Heap-less descriptor sets in Metal r=grovesNL a=kvark

Fixes the heap allocations in descriptor sets, to help #2161.
Edit: the improvement is there, but it's not the one we were looking for.

Adds real value semantics for the `DescriptorPool`, which now owns the allocation and has the actual descriptor sets pointing to it.
PR checklist:
- [x] `make` succeeds (on *nix)
- [x] `make reftests` succeeds
- [x] tested examples with the following backends: metal

~~Note: it doesn't work correctly yet, but I'd be happy to get the review comments and notes. Perhaps, you can spot the mistake? ;)
Sadly, my CTS is non-operational atm, so it's not easy to figure out a test case.
Note2: the individual commits are not build-able. Will squash once finished.~~

Co-authored-by: Dzmitry Malyshau <[email protected]>

src/backend/auxil/range_alloc.rs

@@ -19,11 +19,12 @@ where
     pub fn new(range: Range<T>) -> Self {
         RangeAllocator {
             initial_range: range.clone(),
-            free_ranges: vec![range.clone()],
+            free_ranges: vec![range],
         }
     }
 
     pub fn allocate_range(&mut self, length: T) -> Option<Range<T>> {
+        assert_ne!(length + length, length);
         let mut best_fit: Option<(usize, Range<T>)> = None;
         for (index, range) in self.free_ranges.iter().cloned().enumerate() {
             let range_length = range.end - range.start;

src/backend/dx11/src/lib.rs

@@ -1525,7 +1525,10 @@ impl hal::DescriptorPool<Backend> for DescriptorPool {
         Ok(DescriptorSet::new())
     }
 
-    fn free_sets(&mut self, descriptor_sets: &[DescriptorSet]) {
+    fn free_sets<I>(&mut self, _descriptor_sets: I)
+    where
+        I: IntoIterator<Item = DescriptorSet>
+    {
         unimplemented!()
     }
 

src/backend/dx12/src/native.rs

@@ -494,7 +494,10 @@ impl HalDescriptorPool<Backend> for DescriptorPool {
         })
     }
 
-    fn free_sets(&mut self, descriptor_sets: &[DescriptorSet]) {
+    fn free_sets<I>(&mut self, descriptor_sets: I)
+    where
+        I: IntoIterator<Item = DescriptorSet>
+    {
         for descriptor_set in descriptor_sets {
             for binding_info in &descriptor_set.binding_infos {
                 if let Some(ref view_range) = binding_info.view_range {

src/backend/empty/src/lib.rs

@@ -755,7 +755,10 @@ impl command::RawCommandBuffer<Backend> for RawCommandBuffer {
 #[derive(Debug)]
 pub struct DescriptorPool;
 impl pso::DescriptorPool<Backend> for DescriptorPool {
-    fn free_sets(&mut self, _descriptor_sets: &[()]) {
+    fn free_sets<I>(&mut self, _descriptor_sets: I)
+    where
+        I: IntoIterator<Item = ()>
+    {
         unimplemented!()
     }
 

src/backend/gl/src/native.rs

@@ -185,7 +185,10 @@ impl pso::DescriptorPool<Backend> for DescriptorPool {
         })).collect()
     }
 
-    fn free_sets(&mut self, _descriptor_sets: &[DescriptorSet]) {
+    fn free_sets<I>(&mut self, _descriptor_sets: I)
+    where
+        I: IntoIterator<Item = DescriptorSet>
+    {
         // Poof!  Does nothing, because OpenGL doesn't have a meaningful concept of a `DescriptorSet`.
     }