Override size_hint for all Iterators and add ExactSizeIterator where applicable (#1734)

After #1697 I looked at all other Iterators from Bevy and added overrides for `size_hint` where it wasn't done.
Also implemented `ExactSizeIterator` where applicable.

crates/bevy_reflect/src/list.rs

@@ -149,7 +149,14 @@ impl<'a> Iterator for ListIter<'a> {
         self.index += 1;
         value
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.list.len();
+        (size, Some(size))
     }
+}
+
+impl<'a> ExactSizeIterator for ListIter<'a> {}
 
 #[inline]
 pub fn list_apply<L: List>(a: &mut L, b: &dyn Reflect) {

crates/bevy_reflect/src/map.rs

@@ -167,7 +167,14 @@ impl<'a> Iterator for MapIter<'a> {
         self.index += 1;
         value
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.map.len();
+        (size, Some(size))
     }
+}
+
+impl<'a> ExactSizeIterator for MapIter<'a> {}
 
 #[inline]
 pub fn map_partial_eq<M: Map>(a: &M, b: &dyn Reflect) -> Option<bool> {

crates/bevy_reflect/src/struct_trait.rs

@@ -37,7 +37,14 @@ impl<'a> Iterator for FieldIter<'a> {
         self.index += 1;
         value
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.struct_val.field_len();
+        (size, Some(size))
     }
+}
+
+impl<'a> ExactSizeIterator for FieldIter<'a> {}
 
 pub trait GetField {
     fn get_field<T: Reflect>(&self, name: &str) -> Option<&T>;

crates/bevy_reflect/src/tuple.rs

@@ -32,7 +32,14 @@ impl<'a> Iterator for TupleFieldIter<'a> {
         self.index += 1;
         value
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.tuple.field_len();
+        (size, Some(size))
     }
+}
+
+impl<'a> ExactSizeIterator for TupleFieldIter<'a> {}
 
 pub trait GetTupleField {
     fn get_field<T: Reflect>(&self, index: usize) -> Option<&T>;

crates/bevy_reflect/src/tuple_struct.rs

@@ -32,7 +32,14 @@ impl<'a> Iterator for TupleStructFieldIter<'a> {
         self.index += 1;
         value
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.tuple_struct.field_len();
+        (size, Some(size))
     }
+}
+
+impl<'a> ExactSizeIterator for TupleStructFieldIter<'a> {}
 
 pub trait GetTupleStructField {
     fn get_field<T: Reflect>(&self, index: usize) -> Option<&T>;

crates/bevy_render/src/renderer/render_resource/render_resource.rs

@@ -118,7 +118,14 @@ impl<'a> Iterator for RenderResourceIterator<'a> {
             Some(render_resource)
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.render_resources.render_resources_len();
+        (size, Some(size))
     }
+}
+
+impl<'a> ExactSizeIterator for RenderResourceIterator<'a> {}
 
 #[macro_export]
 macro_rules! impl_render_resource_bytes {

crates/bevy_render/src/shader/shader.rs

@@ -228,7 +228,16 @@ impl<'a> Iterator for ShaderStagesIterator<'a> {
         self.state += 1;
         ret
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.shader_stages.fragment.is_some() {
+            return (2, Some(2));
         }
+        (1, Some(1))
+    }
+}
+
+impl<'a> ExactSizeIterator for ShaderStagesIterator<'a> {}
 
 impl ShaderStages {
     pub fn new(vertex_shader: Handle<Shader>) -> Self {

crates/bevy_render/src/shader/shader_defs.rs

@@ -49,6 +49,10 @@ impl<'a> Iterator for ShaderDefIterator<'a> {
             }
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(self.shader_defs.shader_defs_len()))
+    }
 }
 
 impl ShaderDef for bool {