Draw each primitive with its own pipeline, registered rather than declared

The `primitives!` macro, `PrimitiveData`, `PrimitiveVec`,
`PrimitiveBuffers`, the `Primitive` trait and the shader's dispatch
switch are gone. A primitive is now a registration: its WGSL and,
implicitly, the size of the entry that WGSL reads. Everything else --
its instance list, its free list, its buffers, its bind group and its
pipeline -- follows from that, so adding one is a `register` call and a
shader file, with nothing per-type to remember and no cross-type
dispatch to extend.

Nothing dispatches dynamically. Push, free, renumber, upload and draw
are identical for every primitive; what differs is the entry size and
the pipeline, which are data. So `InstanceList` carries a runtime
stride and its instances' data as bytes, and one concrete type serves
every primitive and the textures. Measured against a typed list it
costs 0.2ns per write, where a trait object costs 1.6ns.

Because each type has its own list, an instance's index is also its
data index: `@builtin(instance_index)` replaces the `idx` field, the
`binding` field goes with the switch, and `PrimitiveInstance` drops from
28 bytes to 20. `PrimitiveVec`'s free list merges into the instance
list's, so an instance and its data are freed by one `swap_remove`
rather than two arenas kept in step.

`shader.wgsl` becomes `shader/prelude.wgsl` plus one file per primitive.
The prelude carries the window, masks, sampled texture, vertex shader
and `masked()`, and is compiled ahead of each primitive's own source --
which is also what a caller's own primitive would be. Masks move back
into group 0 beside the window uniform, since every pipeline shares one
layout.

Within a layer, types now draw in registration order: a rect under a
glyph under a texture. Order within a layer was never meaningful --
freeing an instance swaps another into its place -- so this replaces an
accident with a defined order, and backgrounds land under their content.

Verified with a headless run per case: text over its own rect and an
image over its own rect in one layer, a masked stack clipping, the
text-layout tab, and adding three images and deleting one.

core/src/primitive/layer.rs

@@ -2,7 +2,7 @@ use std::ops::{Index, IndexMut};
 
 use crate::{
     UiRegion, WidgetId,
-    render::{LayerDraws, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst},
+    render::{LayerDraws, MaskIdx, PrimitiveHandle, PrimitiveKind},
     util::to_mut,
 };
 
@@ -121,12 +121,16 @@ impl<T: Default> Layers<T> {
 }
 
 impl DrawLayers {
-    pub fn write<P: Primitive>(
+    pub fn write<P: bytemuck::Pod>(
         &mut self,
         layer: LayerId,
-        info: PrimitiveInst<P>,
+        kind: PrimitiveKind<P>,
+        id: WidgetId,
+        primitive: P,
+        region: UiRegion,
+        mask_idx: MaskIdx,
     ) -> PrimitiveHandle {
-        self[layer].write(layer, info)
+        self[layer].write(layer, kind, id, primitive, region, mask_idx)
     }
 
     pub fn free(&mut self, h: &PrimitiveHandle) -> MaskIdx {

core/src/render/data.rs

@@ -12,20 +12,16 @@ pub struct WindowUniform {
 #[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct PrimitiveInstance {
     pub region: UiRegion,
-    pub binding: u32,
-    pub idx: u32,
     pub mask_idx: MaskIdx,
 }
 
 impl PrimitiveInstance {
-    const ATTRIBS: [VertexAttribute; 7] = vertex_attr_array![
+    const ATTRIBS: [VertexAttribute; 5] = vertex_attr_array![
         0 => Float32x2,
         1 => Float32x2,
         2 => Float32x2,
         3 => Float32x2,
         4 => Uint32,
-        5 => Uint32,
-        6 => Uint32,
     ];
 
     pub fn desc() -> VertexBufferLayout<'static> {

core/src/render/mod.rs

@@ -25,16 +25,22 @@ pub use atlas::*;
 pub use data::{Mask, MaskIdx};
 pub use primitive::*;
 
-const SHAPE_SHADER: &str = include_str!("./shader.wgsl");
+const PRELUDE: &str = include_str!("./shader/prelude.wgsl");
+const TEXTURE_SHADER: &str = include_str!("./shader/texture.wgsl");
 
 pub struct UiRenderNode {
-    uniform_group: BindGroup,
-    primitive_layout: BindGroupLayout,
+    device: Device,
+    shared_layout: BindGroupLayout,
+    shared_group: BindGroup,
+    data_layout: BindGroupLayout,
     texture_layout: BindGroupLayout,
-    mask_layout: BindGroupLayout,
-    mask_group: BindGroup,
+    pipeline_layout: PipelineLayout,
+    format: TextureFormat,
 
-    pipeline: RenderPipeline,
+    /// One per registered primitive, in id order. The texture pipeline is
+    /// apart because a texture binds group 2 per instance, not per draw.
+    pipelines: Vec<RenderPipeline>,
+    texture_pipeline: RenderPipeline,
 
     layers: HashMap<usize, RenderLayer>,
     active: Vec<usize>,
@@ -46,30 +52,41 @@ pub struct UiRenderNode {
 }
 
 struct RenderLayer {
-    instance: ArrBuf<PrimitiveInstance>,
-    primitives: PrimitiveBuffers,
-    primitive_group: BindGroup,
-    texture_instance: ArrBuf<PrimitiveInstance>,
-    /// Which texture each entry of `texture_instance` samples, in the same
-    /// order. Not in the vertex buffer because it names a bind group.
+    /// One per registered primitive, in id order, matching `LayerDraws`.
+    primitives: Vec<ListBuffers>,
+    textures: ListBuffers,
+    /// Which texture each entry of `textures` samples, in the same order.
     texture_slots: Vec<u32>,
 }
 
+/// One list's vertex buffer and the data its shader reads at group 1.
+struct ListBuffers {
+    instance: ArrBuf<PrimitiveInstance>,
+    data: ArrBuf<u8>,
+    group: Option<BindGroup>,
+}
+
 impl UiRenderNode {
     pub fn draw<'a>(&'a self, pass: &mut RenderPass<'a>) {
-        pass.set_pipeline(&self.pipeline);
-        pass.set_bind_group(0, &self.uniform_group, &[]);
-        pass.set_bind_group(3, &self.mask_group, &[]);
+        pass.set_bind_group(0, &self.shared_group, &[]);
         for i in &self.active {
             let layer = &self.layers[i];
-            pass.set_bind_group(1, &layer.primitive_group, &[]);
-            if layer.instance.len() > 0 {
+            // Types run in registration order, so a rect is under a glyph is
+            // under a texture. Ordering beyond that is what `Layers` is for --
+            // freeing an instance swaps another into its place.
             pass.set_bind_group(2, self.pages.group(), &[]);
-                pass.set_vertex_buffer(0, layer.instance.buffer.slice(..));
-                pass.draw(0..4, 0..layer.instance.len() as u32);
+            for (id, list) in layer.primitives.iter().enumerate() {
+                let (Some(group), true) = (&list.group, list.instance.len() > 0) else {
+                    continue;
+                };
+                pass.set_pipeline(&self.pipelines[id]);
+                pass.set_bind_group(1, group, &[]);
+                pass.set_vertex_buffer(0, list.instance.buffer.slice(..));
+                pass.draw(0..4, 0..list.instance.len() as u32);
             }
             if !layer.texture_slots.is_empty() {
-                pass.set_vertex_buffer(0, layer.texture_instance.buffer.slice(..));
+                pass.set_pipeline(&self.texture_pipeline);
+                pass.set_vertex_buffer(0, layer.textures.instance.buffer.slice(..));
                 for (i, &slot) in layer.texture_slots.iter().enumerate() {
                     let Some(group) = self.textures.group(slot) else {
                         continue;
@@ -101,52 +118,40 @@ impl UiRenderNode {
                     }
                 }
             }
-            let rlayer = self.layers.entry(i).or_insert_with(|| {
-                let primitives = PrimitiveBuffers::new(device);
-                let primitive_group =
-                    Self::primitive_group(device, &self.primitive_layout, primitives.buffers());
-                RenderLayer {
-                    instance: ArrBuf::new(
-                        device,
-                        BufferUsages::VERTEX | BufferUsages::COPY_DST,
-                        "instance",
-                    ),
-                    primitives,
-                    primitive_group,
-                    texture_instance: ArrBuf::new(
-                        device,
-                        BufferUsages::VERTEX | BufferUsages::COPY_DST,
-                        "texture instance",
-                    ),
-                    texture_slots: Vec::new(),
-                }
-            });
+            let rlayer = self
+                .layers
+                .entry(i)
+                .or_insert_with(|| RenderLayer::new(device));
             if draws.updated {
-                rlayer
-                    .instance
-                    .update(device, queue, draws.primitives.instances());
                 rlayer
                     .primitives
-                    .update(device, queue, draws.primitives.data());
-                rlayer.primitive_group = Self::primitive_group(
-                    device,
-                    &self.primitive_layout,
-                    rlayer.primitives.buffers(),
-                );
+                    .resize_with(draws.primitives().len(), || ListBuffers::new(device));
+                for (list, draws) in rlayer.primitives.iter_mut().zip(draws.primitives()) {
+                    list.update(device, queue, &self.data_layout, draws);
+                }
                 rlayer
-                    .texture_instance
-                    .update(device, queue, draws.textures.instances());
+                    .textures
+                    .update(device, queue, &self.data_layout, &draws.textures);
                 rlayer.texture_slots.clear();
+                // Read rather than cast: the payload is a byte vec, so it
+                // carries no alignment a `u32` slice could borrow.
+                let slots = draws.textures.data().as_chunks::<4>().0;
                 rlayer
                     .texture_slots
-                    .extend(draws.textures.instances().iter().map(|inst| inst.idx));
+                    .extend(slots.iter().copied().map(u32::from_ne_bytes));
                 draws.updated = false;
             }
         }
+        self.build_pipelines(&ui.primitives);
         if ui.masks.changed {
             ui.masks.changed = false;
             if self.masks.update(device, queue, &ui.masks[..]) {
-                self.mask_group = Self::mask_group(device, &self.mask_layout, &self.masks);
+                self.shared_group = Self::shared_group(
+                    device,
+                    &self.shared_layout,
+                    &self.window_buffer,
+                    &self.masks,
+                );
             }
         }
         self.pages
@@ -165,11 +170,6 @@ impl UiRenderNode {
     }
 
     pub fn new(device: &Device, queue: &Queue, config: &SurfaceConfiguration) -> Self {
-        let shader = device.create_shader_module(ShaderModuleDescriptor {
-            label: Some("UI Shape Shader"),
-            source: ShaderSource::Wgsl(SHAPE_SHADER.into()),
-        });
-
         let window_uniform = WindowUniform {
             width: config.width as f32,
             height: config.height as f32,
@@ -180,74 +180,95 @@ impl UiRenderNode {
             usage: BufferUsages::UNIFORM | BufferUsages::COPY_DST,
         });
 
-        let uniform_layout = device.create_bind_group_layout(&BindGroupLayoutDescriptor {
-            entries: &[BindGroupLayoutEntry {
-                binding: 0,
-                visibility: ShaderStages::VERTEX | ShaderStages::FRAGMENT,
-                ty: BindingType::Buffer {
-                    ty: BufferBindingType::Uniform,
-                    has_dynamic_offset: false,
-                    min_binding_size: None,
-                },
-                count: None,
-            }],
-            label: Some("window"),
-        });
-
-        let uniform_group = Self::bind_group_0(device, &uniform_layout, &window_buffer);
-
-        let primitive_layout = device.create_bind_group_layout(&BindGroupLayoutDescriptor {
-            entries: &PrimitiveBuffers::BINDINGS.map(|binding| BindGroupLayoutEntry {
-                binding,
-                visibility: ShaderStages::FRAGMENT,
-                ty: BindingType::Buffer {
-                    ty: BufferBindingType::Storage { read_only: true },
-                    has_dynamic_offset: false,
-                    min_binding_size: None,
-                },
-                count: None,
-            }),
-            label: Some("primitive"),
-        });
+        let shared_layout = Self::shared_layout(device);
+        let data_layout = Self::data_layout(device);
+        let texture_layout = Self::texture_layout(device);
 
         let masks = ArrBuf::new(
             device,
             BufferUsages::STORAGE | BufferUsages::COPY_DST,
             "ui masks",
         );
-
-        let texture_layout = Self::texture_layout(device);
-        let mask_layout = Self::mask_layout(device);
-        let mask_group = Self::mask_group(device, &mask_layout, &masks);
+        let shared_group = Self::shared_group(device, &shared_layout, &window_buffer, &masks);
 
         let sampler = default_sampler(device);
         let pages = GpuPages::new(device, queue, &texture_layout, &sampler);
         let textures = GpuTextures::new(device, queue);
 
+        // One layout for every pipeline, so a primitive that does not sample
+        // or read a buffer binds what is there instead of needing its own.
         let pipeline_layout = device.create_pipeline_layout(&PipelineLayoutDescriptor {
-            label: Some("UI Shape Pipeline Layout"),
-            bind_group_layouts: &[
-                &uniform_layout,
-                &primitive_layout,
-                &texture_layout,
-                &mask_layout,
-            ],
+            label: Some("ui"),
+            bind_group_layouts: &[&shared_layout, &data_layout, &texture_layout],
             immediate_size: 0,
         });
-        let pipeline = device.create_render_pipeline(&RenderPipelineDescriptor {
-            label: Some("UI Shape Pipeline"),
-            layout: Some(&pipeline_layout),
+        let texture_pipeline = Self::pipeline(
+            device,
+            &pipeline_layout,
+            config.format,
+            TEXTURE_SHADER,
+            "texture",
+        );
+
+        Self {
+            device: device.clone(),
+            shared_layout,
+            shared_group,
+            data_layout,
+            texture_layout,
+            pipeline_layout,
+            format: config.format,
+            pipelines: Vec::new(),
+            texture_pipeline,
+            window_buffer,
+            layers: HashMap::default(),
+            active: Vec::new(),
+            sampler,
+            pages,
+            textures,
+            masks,
+        }
+    }
+
+    /// Compiles a pipeline for every primitive registered since the last call.
+    /// Sources only ever arrive at the end, so an id keeps its pipeline.
+    fn build_pipelines(&mut self, registry: &PrimitiveRegistry) {
+        for source in &registry.sources()[self.pipelines.len()..] {
+            self.pipelines.push(Self::pipeline(
+                &self.device,
+                &self.pipeline_layout,
+                self.format,
+                source.wgsl,
+                source.label,
+            ));
+        }
+    }
+
+    fn pipeline(
+        device: &Device,
+        layout: &PipelineLayout,
+        format: TextureFormat,
+        wgsl: &str,
+        label: &str,
+    ) -> RenderPipeline {
+        let module = device.create_shader_module(ShaderModuleDescriptor {
+            label: Some(label),
+            source: ShaderSource::Wgsl(format!("{PRELUDE}\n{wgsl}").into()),
+        });
+        device.create_render_pipeline(&RenderPipelineDescriptor {
+            label: Some(label),
+            layout: Some(layout),
             vertex: VertexState {
-                module: &shader,
+                module: &module,
                 entry_point: Some("vs_main"),
                 buffers: &[PrimitiveInstance::desc()],
                 compilation_options: Default::default(),
             },
             fragment: Some(FragmentState {
-                module: &shader,
+                module: &module,
                 entry_point: Some("fs_main"),
                 targets: &[Some(ColorTargetState {
-                    format: config.format,
+                    format,
                     blend: Some(BlendState::ALPHA_BLENDING),
                     write_mask: ColorWrites::ALL,
                 })],
@@ -270,52 +291,74 @@ impl UiRenderNode {
             },
             multiview_mask: None,
             cache: None,
-        });
-
-        Self {
-            uniform_group,
-            primitive_layout,
-            texture_layout,
-            mask_layout,
-            mask_group,
-            pipeline,
-            window_buffer,
-            layers: HashMap::default(),
-            active: Vec::new(),
-            sampler,
-            pages,
-            textures,
-            masks,
-        }
-    }
-
-    fn bind_group_0(
-        device: &Device,
-        layout: &BindGroupLayout,
-        window_buffer: &Buffer,
-    ) -> BindGroup {
-        device.create_bind_group(&BindGroupDescriptor {
-            layout,
-            entries: &[BindGroupEntry {
-                binding: 0,
-                resource: window_buffer.as_entire_binding(),
-            }],
-            label: Some("ui window"),
         })
     }
 
-    fn primitive_group(
+    /// Group 0: what every draw in the ui shares.
+    fn shared_layout(device: &Device) -> BindGroupLayout {
+        device.create_bind_group_layout(&BindGroupLayoutDescriptor {
+            entries: &[
+                BindGroupLayoutEntry {
+                    binding: 0,
+                    visibility: ShaderStages::VERTEX | ShaderStages::FRAGMENT,
+                    ty: BindingType::Buffer {
+                        ty: BufferBindingType::Uniform,
+                        has_dynamic_offset: false,
+                        min_binding_size: None,
+                    },
+                    count: None,
+                },
+                BindGroupLayoutEntry {
+                    binding: 1,
+                    visibility: ShaderStages::FRAGMENT,
+                    ty: BindingType::Buffer {
+                        ty: BufferBindingType::Storage { read_only: true },
+                        has_dynamic_offset: false,
+                        min_binding_size: None,
+                    },
+                    count: None,
+                },
+            ],
+            label: Some("ui shared"),
+        })
+    }
+
+    fn shared_group(
         device: &Device,
         layout: &BindGroupLayout,
-        buffers: [(u32, &Buffer); PrimitiveBuffers::LEN],
+        window: &Buffer,
+        masks: &ArrBuf<Mask>,
     ) -> BindGroup {
         device.create_bind_group(&BindGroupDescriptor {
             layout,
-            entries: &buffers.map(|(binding, buf)| BindGroupEntry {
-                binding,
-                resource: buf.as_entire_binding(),
-            }),
-            label: Some("ui primitives"),
+            entries: &[
+                BindGroupEntry {
+                    binding: 0,
+                    resource: window.as_entire_binding(),
+                },
+                BindGroupEntry {
+                    binding: 1,
+                    resource: masks.buffer.as_entire_binding(),
+                },
+            ],
+            label: Some("ui shared"),
+        })
+    }
+
+    /// Group 1: one list's per-instance data, whatever its shader reads it as.
+    fn data_layout(device: &Device) -> BindGroupLayout {
+        device.create_bind_group_layout(&BindGroupLayoutDescriptor {
+            entries: &[BindGroupLayoutEntry {
+                binding: 0,
+                visibility: ShaderStages::FRAGMENT,
+                ty: BindingType::Buffer {
+                    ty: BufferBindingType::Storage { read_only: true },
+                    has_dynamic_offset: false,
+                    min_binding_size: None,
+                },
+                count: None,
+            }],
+            label: Some("ui primitive data"),
         })
     }
 
@@ -345,36 +388,55 @@ impl UiRenderNode {
         })
     }
 
-    /// Group 3, apart from the textures so that resizing the masks buffer
-    /// leaves every texture group intact.
-    fn mask_layout(device: &Device) -> BindGroupLayout {
-        device.create_bind_group_layout(&BindGroupLayoutDescriptor {
-            entries: &[BindGroupLayoutEntry {
-                binding: 0,
-                visibility: ShaderStages::FRAGMENT,
-                ty: BindingType::Buffer {
-                    ty: BufferBindingType::Storage { read_only: true },
-                    has_dynamic_offset: false,
-                    min_binding_size: None,
-                },
-                count: None,
-            }],
-            label: Some("ui masks"),
-        })
-    }
-
-    fn mask_group(device: &Device, layout: &BindGroupLayout, masks: &ArrBuf<Mask>) -> BindGroup {
-        device.create_bind_group(&BindGroupDescriptor {
-            layout,
-            entries: &[BindGroupEntry {
-                binding: 0,
-                resource: masks.buffer.as_entire_binding(),
-            }],
-            label: Some("ui masks"),
-        })
-    }
-
     pub fn texture_count(&self) -> usize {
         self.textures.count()
     }
 }
+
+impl RenderLayer {
+    fn new(device: &Device) -> Self {
+        Self {
+            primitives: Vec::new(),
+            textures: ListBuffers::new(device),
+            texture_slots: Vec::new(),
+        }
+    }
+}
+
+impl ListBuffers {
+    fn new(device: &Device) -> Self {
+        Self {
+            instance: ArrBuf::new(
+                device,
+                BufferUsages::VERTEX | BufferUsages::COPY_DST,
+                "instance",
+            ),
+            data: ArrBuf::new(
+                device,
+                BufferUsages::STORAGE | BufferUsages::COPY_DST,
+                "primitive data",
+            ),
+            group: None,
+        }
+    }
+
+    fn update(
+        &mut self,
+        device: &Device,
+        queue: &Queue,
+        layout: &BindGroupLayout,
+        list: &InstanceList,
+    ) {
+        self.instance.update(device, queue, list.instances());
+        if self.data.update(device, queue, list.data()) || self.group.is_none() {
+            self.group = Some(device.create_bind_group(&BindGroupDescriptor {
+                layout,
+                entries: &[BindGroupEntry {
+                    binding: 0,
+                    resource: self.data.buffer.as_entire_binding(),
+                }],
+                label: Some("ui primitive data"),
+            }));
+        }
+    }
+}

core/src/render/primitive.rs

@@ -1,68 +1,143 @@
-use std::ops::{Deref, DerefMut};
+use std::marker::PhantomData;
 
 use crate::{
     Color, UiRegion, WidgetId,
-    render::{
-        ArrBuf,
-        data::{MaskIdx, PrimitiveInstance},
-    },
+    render::data::{MaskIdx, PrimitiveInstance},
     util::Vec2,
 };
 use bytemuck::Pod;
-use wgpu::*;
 
-/// Everything one layer draws. A texture binds its own group 2, so it draws on
-/// its own and cannot join the instanced draw the primitives share.
-pub struct LayerDraws {
-    pub primitives: Primitives,
-    pub textures: InstanceList,
-    pub updated: bool,
+/// Which registered primitive an instance is, and so which list it lives in
+/// and which pipeline draws it. The type ties a `write` to what its shader reads.
+pub struct PrimitiveKind<P> {
+    id: u32,
+    _p: PhantomData<fn(P)>,
 }
 
-impl Default for LayerDraws {
-    fn default() -> Self {
+impl<P> PrimitiveKind<P> {
+    const fn new(id: u32) -> Self {
         Self {
-            primitives: Default::default(),
-            textures: Default::default(),
-            updated: true,
-        }
+            id,
+            _p: PhantomData,
         }
     }
 
-/// Which of a layer's two lists an instance is in. They index independently,
-/// so a handle or a renumbering naming only a position would be ambiguous.
+    pub fn id(&self) -> u32 {
+        self.id
+    }
+}
+
+impl<P> Clone for PrimitiveKind<P> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<P> Copy for PrimitiveKind<P> {}
+
+pub const RECT: PrimitiveKind<RectPrimitive> = PrimitiveKind::new(0);
+pub const GLYPH: PrimitiveKind<GlyphPrimitive> = PrimitiveKind::new(1);
+
+/// Every primitive a ui can draw, in id order. Registering one is all the
+/// wiring it needs: its list, buffers, free list and pipeline follow, and
+/// nothing here knows its type.
+///
+/// A source is compiled after `prelude.wgsl` and supplies its data at
+/// `@group(1) @binding(0)` and an `fs_main` shading one instance.
+pub struct PrimitiveRegistry {
+    kinds: Vec<PrimitiveSource>,
+}
+
+pub struct PrimitiveSource {
+    pub wgsl: &'static str,
+    pub label: &'static str,
+}
+
+impl Default for PrimitiveRegistry {
+    fn default() -> Self {
+        let mut kinds = Self { kinds: Vec::new() };
+        assert_eq!(
+            kinds
+                .register::<RectPrimitive>(include_str!("shader/rect.wgsl"), "rect")
+                .id(),
+            RECT.id()
+        );
+        assert_eq!(
+            kinds
+                .register::<GlyphPrimitive>(include_str!("shader/glyph.wgsl"), "glyph")
+                .id(),
+            GLYPH.id()
+        );
+        kinds
+    }
+}
+
+impl PrimitiveRegistry {
+    pub fn register<P: Pod>(
+        &mut self,
+        wgsl: &'static str,
+        label: &'static str,
+    ) -> PrimitiveKind<P> {
+        self.kinds.push(PrimitiveSource { wgsl, label });
+        PrimitiveKind::new(self.kinds.len() as u32 - 1)
+    }
+
+    pub fn sources(&self) -> &[PrimitiveSource] {
+        &self.kinds
+    }
+}
+
+/// Which of a layer's lists an instance is in. They index independently, so a
+/// handle or a renumbering naming only a position would be ambiguous.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum InstanceKind {
-    Primitive,
+    Primitive(u32),
     Texture,
 }
 
-/// A texture instance's `idx` names the texture to bind rather than a group-1
-/// entry, so it is not in `primitives!` and has no buffer of its own.
-pub const TEXTURE_BINDING: u32 = 2;
-
-/// Instances, the widget each belongs to, and the slots waiting to be reused.
+/// Instances, the widget each belongs to, the slots waiting to be reused, and
+/// `stride` bytes of data per instance at the same index.
+///
+/// That data is the struct a primitive's shader reads, or the slot a texture
+/// binds. Keeping both here is what holds them in step through a
+/// `swap_remove`.
 #[derive(Default)]
 pub struct InstanceList {
     instances: Vec<PrimitiveInstance>,
     assoc: Vec<WidgetId>,
     free: Vec<usize>,
+    data: Vec<u8>,
+    stride: usize,
 }
 
 impl InstanceList {
-    pub fn instances(&self) -> &Vec<PrimitiveInstance> {
+    pub fn instances(&self) -> &[PrimitiveInstance] {
         &self.instances
     }
 
-    fn push(&mut self, id: WidgetId, inst: PrimitiveInstance) -> usize {
+    pub fn data(&self) -> &[u8] {
+        &self.data
+    }
+
+    fn push(&mut self, id: WidgetId, inst: PrimitiveInstance, data: &[u8]) -> usize {
+        if self.instances.is_empty() {
+            self.stride = data.len();
+        }
+        debug_assert_eq!(
+            data.len(),
+            self.stride,
+            "primitive written to the wrong list"
+        );
         if let Some(i) = self.free.pop() {
             self.instances[i] = inst;
             self.assoc[i] = id;
+            self.data[i * self.stride..][..self.stride].copy_from_slice(data);
             i
         } else {
             let i = self.instances.len();
             self.instances.push(inst);
             self.assoc.push(id);
+            self.data.extend_from_slice(data);
             i
         }
     }
@@ -76,156 +151,75 @@ impl InstanceList {
         self.free.sort_by(|a, b| b.cmp(a));
         let instances = &mut self.instances;
         let assoc = &mut self.assoc;
+        let data = &mut self.data;
+        let stride = self.stride;
         self.free.drain(..).filter_map(move |i| {
             instances.swap_remove(i);
             assoc.swap_remove(i);
-            if i == instances.len() {
+            let last = instances.len();
+            data.copy_within(last * stride..(last + 1) * stride, i * stride);
+            data.truncate(last * stride);
+            if i == last {
                 return None;
             }
             let id = assoc[i];
-            let old = instances.len();
             Some(PrimitiveChange {
                 id,
                 kind,
-                old,
+                old: last,
                 new: i,
             })
         })
     }
 }
 
-pub trait Primitive: Pod {
-    const BINDING: u32;
-    fn vec(data: &mut PrimitiveData) -> &mut PrimitiveVec<Self>;
+/// Everything one layer draws. A texture binds its own group 2, so it draws on
+/// its own and cannot join the instanced draw a primitive gets.
+pub struct LayerDraws {
+    primitives: Vec<InstanceList>,
+    pub textures: InstanceList,
+    pub updated: bool,
 }
 
-macro_rules! primitives {
-    ($($name:ident: $ty:ty => $binding:expr,)*) => {
-        #[derive(Default)]
-        pub struct PrimitiveData {
-            $(pub(crate) $name: PrimitiveVec<$ty>,)*
-        }
-
-        pub struct PrimitiveBuffers {
-            $($name: ArrBuf<$ty>,)*
-        }
-
-        impl PrimitiveBuffers {
-            pub fn update(&mut self, device: &Device, queue: &Queue, data: &PrimitiveData) {
-                $(self.$name.update(device, queue, &data.$name);)*
-            }
-        }
-
-        impl PrimitiveBuffers {
-            pub const LEN: usize = primitives!(@count $($name)*);
-            pub const BINDINGS: [u32; Self::LEN] = [$(<$ty>::BINDING,)*];
-            pub fn buffers(&self) -> [(u32, &Buffer); Self::LEN] {
-                [
-                    $((<$ty>::BINDING, &self.$name.buffer),)*
-                ]
-            }
-            pub fn new(device: &Device) -> Self {
+impl Default for LayerDraws {
+    fn default() -> Self {
         Self {
-                    $($name: ArrBuf::new(
-                        device,
-                        BufferUsages::STORAGE | BufferUsages::COPY_DST,
-                        stringify!($name),
-                    ),)*
+            primitives: Vec::new(),
+            textures: InstanceList::default(),
+            updated: true,
         }
     }
 }
 
-        impl PrimitiveData {
-            pub fn clear(&mut self) {
-                $(self.$name.clear();)*
-            }
-            pub fn free(&mut self, binding: u32, idx: usize) {
-                match binding {
-                    $(<$ty>::BINDING => self.$name.free(idx),)*
-                    _ => unreachable!()
-                }
-            }
-        }
-
-        $(
-            unsafe impl bytemuck::Pod for $ty {}
-            unsafe impl bytemuck::Zeroable for $ty {}
-            impl Primitive for $ty {
-                const BINDING: u32 = $binding;
-                fn vec(data: &mut PrimitiveData) -> &mut PrimitiveVec<Self> {
-                    &mut data.$name
-                }
-            }
-        )*
-    };
-    (@count $t1:tt $($t:tt)+) => { 1 + primitives!(@count $($t)+) };
-    (@count $t:tt) => { 1 };
-}
-
-pub struct PrimitiveInst<P> {
-    pub id: WidgetId,
-    pub primitive: P,
-    pub region: UiRegion,
-    pub mask_idx: MaskIdx,
-}
-
-/// The instanced half of a layer: a list, plus the group-1 data it reads.
-#[derive(Default)]
-pub struct Primitives {
-    list: InstanceList,
-    data: PrimitiveData,
-}
-
-impl Primitives {
-    fn write<P: Primitive>(
-        &mut self,
-        PrimitiveInst {
-            id,
-            primitive,
-            region,
-            mask_idx,
-        }: PrimitiveInst<P>,
-    ) -> usize {
-        let idx = P::vec(&mut self.data).add(primitive) as u32;
-        self.list.push(
-            id,
-            PrimitiveInstance {
-                region,
-                idx,
-                mask_idx,
-                binding: P::BINDING,
-            },
-        )
-    }
-
-    fn free(&mut self, i: usize) -> MaskIdx {
-        // The instance says where its group-1 entry is, so the handle need not.
-        let inst = self.list.instances[i];
-        self.data.free(inst.binding, inst.idx as usize);
-        self.list.free(i)
-    }
-
-    pub fn data(&self) -> &PrimitiveData {
-        &self.data
-    }
-
-    pub fn instances(&self) -> &Vec<PrimitiveInstance> {
-        &self.list.instances
-    }
-}
-
 impl LayerDraws {
-    pub fn write<P: Primitive>(&mut self, layer: usize, inst: PrimitiveInst<P>) -> PrimitiveHandle {
+    pub fn write<P: Pod>(
+        &mut self,
+        layer: usize,
+        kind: PrimitiveKind<P>,
+        id: WidgetId,
+        primitive: P,
+        region: UiRegion,
+        mask_idx: MaskIdx,
+    ) -> PrimitiveHandle {
         self.updated = true;
+        if self.primitives.len() <= kind.id as usize {
+            self.primitives
+                .resize_with(kind.id as usize + 1, Default::default);
+        }
+        let inst_idx = self.primitives[kind.id as usize].push(
+            id,
+            PrimitiveInstance { region, mask_idx },
+            bytemuck::bytes_of(&primitive),
+        );
         PrimitiveHandle {
             layer,
-            kind: InstanceKind::Primitive,
-            inst_idx: self.primitives.write(inst),
+            kind: InstanceKind::Primitive(kind.id),
+            inst_idx,
         }
     }
 
-    /// Writes an instance that samples the texture in slot `texture` instead of
-    /// reading a group-1 buffer.
+    /// Writes an instance that samples the texture in slot `texture`, which is
+    /// bound for it alone rather than read from a buffer.
     pub fn write_texture(
         &mut self,
         layer: usize,
@@ -240,16 +234,16 @@ impl LayerDraws {
             kind: InstanceKind::Texture,
             inst_idx: self.textures.push(
                 id,
-                PrimitiveInstance {
-                    region,
-                    idx: texture,
-                    mask_idx,
-                    binding: TEXTURE_BINDING,
-                },
+                PrimitiveInstance { region, mask_idx },
+                bytemuck::bytes_of(&texture),
             ),
         }
     }
 
+    pub fn primitives(&self) -> &[InstanceList] {
+        &self.primitives
+    }
+
     pub fn apply_free(&mut self) -> impl Iterator<Item = PrimitiveChange> {
         let Self {
             primitives,
@@ -257,17 +251,15 @@ impl LayerDraws {
             ..
         } = self;
         primitives
-            .list
-            .apply_free(InstanceKind::Primitive)
+            .iter_mut()
+            .enumerate()
+            .flat_map(|(i, list)| list.apply_free(InstanceKind::Primitive(i as u32)))
             .chain(textures.apply_free(InstanceKind::Texture))
     }
 
     pub fn free(&mut self, h: &PrimitiveHandle) -> MaskIdx {
         self.updated = true;
-        match h.kind {
-            InstanceKind::Primitive => self.primitives.free(h.inst_idx),
-            InstanceKind::Texture => self.textures.free(h.inst_idx),
-        }
+        self.list_mut(h.kind).free(h.inst_idx)
     }
 
     pub fn region_mut(&mut self, h: &PrimitiveHandle) -> &mut UiRegion {
@@ -277,7 +269,7 @@ impl LayerDraws {
 
     fn list_mut(&mut self, kind: InstanceKind) -> &mut InstanceList {
         match kind {
-            InstanceKind::Primitive => &mut self.primitives.list,
+            InstanceKind::Primitive(i) => &mut self.primitives[i as usize],
             InstanceKind::Texture => &mut self.textures,
         }
     }
@@ -297,11 +289,6 @@ pub struct PrimitiveHandle {
     pub inst_idx: usize,
 }
 
-primitives!(
-    rects: RectPrimitive => 0,
-    glyphs: GlyphPrimitive => 1,
-);
-
 #[repr(C)]
 #[derive(Copy, Clone)]
 pub struct RectPrimitive {
@@ -311,6 +298,9 @@ pub struct RectPrimitive {
     pub inner_radius: f32,
 }
 
+unsafe impl bytemuck::Pod for RectPrimitive {}
+unsafe impl bytemuck::Zeroable for RectPrimitive {}
+
 impl RectPrimitive {
     pub fn color(color: Color<u8>) -> Self {
         Self {
@@ -335,53 +325,5 @@ pub struct GlyphPrimitive {
     pub flags: u32,
 }
 
-pub struct PrimitiveVec<T> {
-    vec: Vec<T>,
-    free: Vec<usize>,
-}
-
-impl<T> PrimitiveVec<T> {
-    pub fn new() -> Self {
-        Self {
-            vec: Vec::new(),
-            free: Vec::new(),
-        }
-    }
-    pub fn add(&mut self, t: T) -> usize {
-        if let Some(i) = self.free.pop() {
-            self.vec[i] = t;
-            i
-        } else {
-            let i = self.vec.len();
-            self.vec.push(t);
-            i
-        }
-    }
-    pub fn free(&mut self, i: usize) {
-        self.free.push(i);
-    }
-    pub fn clear(&mut self) {
-        self.free.clear();
-        self.vec.clear();
-    }
-}
-
-impl<T> Default for PrimitiveVec<T> {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
-impl<T> Deref for PrimitiveVec<T> {
-    type Target = Vec<T>;
-
-    fn deref(&self) -> &Self::Target {
-        &self.vec
-    }
-}
-
-impl<T> DerefMut for PrimitiveVec<T> {
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        &mut self.vec
-    }
-}
+unsafe impl bytemuck::Pod for GlyphPrimitive {}
+unsafe impl bytemuck::Zeroable for GlyphPrimitive {}

core/src/render/shader.wgsl

@@ -1,198 +0,0 @@
-const RECT: u32 = 0u;
-const GLYPH: u32 = 1u;
-// No group 1 entry: a texture instance's idx names the texture bound in group 2.
-const TEXTURE: u32 = 2u;
-
-@group(0) @binding(0)
-var<uniform> window: WindowUniform;
-@group(1) @binding(RECT)
-var<storage> rects: array<Rect>;
-@group(1) @binding(GLYPH)
-var<storage> glyphs: array<GlyphInfo>;
-
-struct Rect {
-    color: u32,
-    radius: f32,
-    thickness: f32,
-    inner_radius: f32,
-}
-
-struct GlyphInfo {
-    uv_min: vec2<f32>,
-    uv_max: vec2<f32>,
-    layer: u32,
-    color: u32,
-    flags: u32,
-}
-
-struct Mask {
-    x: UiSpan,
-    y: UiSpan,
-}
-
-struct UiSpan {
-    start: UiScalar,
-    end: UiScalar,
-}
-
-struct UiScalar {
-    rel: f32,
-    abs: f32,
-}
-
-struct UiVec2 {
-    rel: vec2<f32>,
-    abs: vec2<f32>,
-}
-
-// What this draw samples: the glyph atlas, whose layers are its pages, or one
-// standalone image as an array of one.
-@group(2) @binding(0)
-var tex: texture_2d_array<f32>;
-@group(2) @binding(1)
-var samp: sampler;
-@group(3) @binding(0)
-var<storage> masks: array<Mask>;
-
-struct WindowUniform {
-    dim: vec2<f32>,
-};
-
-struct InstanceInput {
-    @location(0) x_start: vec2<f32>,
-    @location(1) x_end: vec2<f32>,
-    @location(2) y_start: vec2<f32>,
-    @location(3) y_end: vec2<f32>,
-    @location(4) binding: u32,
-    @location(5) idx: u32,
-    @location(6) mask_idx: u32,
-}
-
-struct VertexOutput {
-    @location(0) top_left: vec2<f32>,
-    @location(1) bot_right: vec2<f32>,
-    @location(2) uv: vec2<f32>,
-    @location(3) binding: u32,
-    @location(4) idx: u32,
-    @location(5) mask_idx: u32,
-    @builtin(position) clip_position: vec4<f32>,
-};
-
-struct Region {
-    pos: vec2<f32>,
-    uv: vec2<f32>,
-    top_left: vec2<f32>,
-    bot_right: vec2<f32>,
-}
-
-@vertex
-fn vs_main(
-    @builtin(vertex_index) vi: u32,
-    in: InstanceInput,
-) -> VertexOutput {
-    var out: VertexOutput;
-
-    let top_left_rel = vec2(in.x_start.x, in.y_start.x);
-    let top_left_abs = vec2(in.x_start.y, in.y_start.y);
-    let bot_right_rel = vec2(in.x_end.x, in.y_end.x);
-    let bot_right_abs = vec2(in.x_end.y, in.y_end.y);
-
-    let top_left = floor(top_left_rel * window.dim) + floor(top_left_abs);
-    let bot_right = floor(bot_right_rel * window.dim) + floor(bot_right_abs);
-    let size = bot_right - top_left;
-
-    let uv = vec2<f32>(
-        f32(vi % 2u),
-        f32(vi / 2u)
-    );
-    let pos = (top_left + uv * size) / window.dim * 2.0 - 1.0;
-    out.clip_position = vec4<f32>(pos.x, -pos.y, 0.0, 1.0);
-    out.uv = uv;
-    out.binding = in.binding;
-    out.idx = in.idx;
-    out.top_left = top_left;
-    out.bot_right = bot_right;
-    out.mask_idx = in.mask_idx;
-
-    return out;
-}
-
-@fragment
-fn fs_main(
-    in: VertexOutput
-) -> @location(0) vec4<f32> {
-    let pos = in.clip_position.xy;
-    let region = Region(pos, in.uv, in.top_left, in.bot_right);
-    let i = in.idx;
-    var color: vec4<f32>;
-    switch in.binding {
-        case RECT: {
-            color = draw_rounded_rect(region, rects[i]);
-        }
-        case TEXTURE: {
-            color = draw_texture(region);
-        }
-        case GLYPH: {
-            color = draw_glyph(region, glyphs[i]);
-        }
-        default: {
-            color = vec4(1.0, 0.0, 1.0, 1.0);
-        }
-    }
-    if in.mask_idx != 4294967295u {
-        let mask = masks[in.mask_idx];
-        let tl = UiVec2(vec2(mask.x.start.rel, mask.y.start.rel), vec2(mask.x.start.abs, mask.y.start.abs));
-        let br = UiVec2(vec2(mask.x.end.rel, mask.y.end.rel), vec2(mask.x.end.abs, mask.y.end.abs));
-
-        let top_left = floor(tl.rel * window.dim) + floor(tl.abs);
-        let bot_right = floor(br.rel * window.dim) + floor(br.abs);
-        if pos.x < top_left.x || pos.x > bot_right.x || pos.y < top_left.y || pos.y > bot_right.y {
-            color *= 0.0;
-        }
-    }
-    return color;
-}
-
-fn draw_texture(region: Region) -> vec4<f32> {
-    return textureSample(tex, samp, region.uv, 0);
-}
-
-fn draw_glyph(region: Region, g: GlyphInfo) -> vec4<f32> {
-    let uv = mix(g.uv_min, g.uv_max, region.uv);
-    let texel = textureSample(tex, samp, uv, i32(g.layer));
-    if (g.flags & 1u) != 0u {
-        return texel;
-    }
-    var color = unpack4x8unorm(g.color);
-    color.a *= texel.a;
-    return color;
-}
-
-fn draw_rounded_rect(region: Region, rect: Rect) -> vec4<f32> {
-    var color = unpack4x8unorm(rect.color);
-
-    let edge = 0.5;
-
-    let size = region.bot_right - region.top_left;
-    let corner = size / 2.0;
-    let center = region.top_left + corner;
-
-    let dist = distance_from_rect(region.pos, center, corner, rect.radius);
-    color.a *= 1.0 - smoothstep(-min(edge, rect.radius), edge, dist);
-
-    if rect.thickness > 0.0 {
-        let dist2 = distance_from_rect(region.pos, center, corner - rect.thickness, rect.inner_radius);
-        color.a *= smoothstep(-min(edge, rect.inner_radius), edge, dist2);
-    }
-
-    return color;
-}
-
-fn distance_from_rect(pixel_pos: vec2<f32>, rect_center: vec2<f32>, rect_corner: vec2<f32>, radius: f32) -> f32 {
-    // vec from center to pixel
-    let p = pixel_pos - rect_center;
-    // vec from inner rect corner to pixel
-    let q = abs(p) - (rect_corner - radius);
-    return length(max(q, vec2(0.0))) - radius;
-}
-

core/src/render/shader/glyph.wgsl

@@ -0,0 +1,24 @@
+struct GlyphInfo {
+    uv_min: vec2<f32>,
+    uv_max: vec2<f32>,
+    // Which layer of the atlas array this glyph's page is.
+    layer: u32,
+    color: u32,
+    flags: u32,
+}
+
+@group(1) @binding(0)
+var<storage> glyphs: array<GlyphInfo>;
+
+@fragment
+fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+    let g = glyphs[in.idx];
+    let uv = mix(g.uv_min, g.uv_max, in.uv);
+    let texel = textureSample(tex, samp, uv, i32(g.layer));
+    if (g.flags & 1u) != 0u {
+        return masked(in, texel);
+    }
+    var color = unpack4x8unorm(g.color);
+    color.a *= texel.a;
+    return masked(in, color);
+}

core/src/render/shader/prelude.wgsl

@@ -0,0 +1,102 @@
+// Prepended to every primitive's shader, which supplies only its own data
+// struct at group 1 and an `fs_main` that shades one instance.
+
+@group(0) @binding(0)
+var<uniform> window: WindowUniform;
+@group(0) @binding(1)
+var<storage> masks: array<Mask>;
+
+// The texture this draw samples: the glyph atlas, whose layers are its pages,
+// or one standalone image as an array of one.
+@group(2) @binding(0)
+var tex: texture_2d_array<f32>;
+@group(2) @binding(1)
+var samp: sampler;
+
+struct WindowUniform {
+    dim: vec2<f32>,
+};
+
+struct Mask {
+    x: UiSpan,
+    y: UiSpan,
+}
+
+struct UiSpan {
+    start: UiScalar,
+    end: UiScalar,
+}
+
+struct UiScalar {
+    rel: f32,
+    abs: f32,
+}
+
+struct InstanceInput {
+    @location(0) x_start: vec2<f32>,
+    @location(1) x_end: vec2<f32>,
+    @location(2) y_start: vec2<f32>,
+    @location(3) y_end: vec2<f32>,
+    @location(4) mask_idx: u32,
+}
+
+struct VertexOutput {
+    @location(0) top_left: vec2<f32>,
+    @location(1) bot_right: vec2<f32>,
+    @location(2) uv: vec2<f32>,
+    @location(3) mask_idx: u32,
+    // The instance's own index, which is also where its data sits in group 1.
+    @location(4) @interpolate(flat) idx: u32,
+    @builtin(position) clip_position: vec4<f32>,
+};
+
+@vertex
+fn vs_main(
+    @builtin(vertex_index) vi: u32,
+    @builtin(instance_index) ii: u32,
+    in: InstanceInput,
+) -> VertexOutput {
+    var out: VertexOutput;
+
+    let top_left_rel = vec2(in.x_start.x, in.y_start.x);
+    let top_left_abs = vec2(in.x_start.y, in.y_start.y);
+    let bot_right_rel = vec2(in.x_end.x, in.y_end.x);
+    let bot_right_abs = vec2(in.x_end.y, in.y_end.y);
+
+    let top_left = floor(top_left_rel * window.dim) + floor(top_left_abs);
+    let bot_right = floor(bot_right_rel * window.dim) + floor(bot_right_abs);
+    let size = bot_right - top_left;
+
+    let uv = vec2<f32>(
+        f32(vi % 2u),
+        f32(vi / 2u)
+    );
+    let pos = (top_left + uv * size) / window.dim * 2.0 - 1.0;
+    out.clip_position = vec4<f32>(pos.x, -pos.y, 0.0, 1.0);
+    out.uv = uv;
+    out.top_left = top_left;
+    out.bot_right = bot_right;
+    out.mask_idx = in.mask_idx;
+    out.idx = ii;
+
+    return out;
+}
+
+fn masked(in: VertexOutput, color: vec4<f32>) -> vec4<f32> {
+    if in.mask_idx == 4294967295u {
+        return color;
+    }
+    let mask = masks[in.mask_idx];
+    let tl = vec2(mask.x.start.rel, mask.y.start.rel);
+    let tl_abs = vec2(mask.x.start.abs, mask.y.start.abs);
+    let br = vec2(mask.x.end.rel, mask.y.end.rel);
+    let br_abs = vec2(mask.x.end.abs, mask.y.end.abs);
+
+    let top_left = floor(tl * window.dim) + floor(tl_abs);
+    let bot_right = floor(br * window.dim) + floor(br_abs);
+    let pos = in.clip_position.xy;
+    if pos.x < top_left.x || pos.x > bot_right.x || pos.y < top_left.y || pos.y > bot_right.y {
+        return color * 0.0;
+    }
+    return color;
+}

core/src/render/shader/rect.wgsl

@@ -0,0 +1,39 @@
+struct Rect {
+    color: u32,
+    radius: f32,
+    thickness: f32,
+    inner_radius: f32,
+}
+
+@group(1) @binding(0)
+var<storage> rects: array<Rect>;
+
+@fragment
+fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+    let rect = rects[in.idx];
+    var color = unpack4x8unorm(rect.color);
+
+    let edge = 0.5;
+    let size = in.bot_right - in.top_left;
+    let corner = size / 2.0;
+    let center = in.top_left + corner;
+    let pos = in.clip_position.xy;
+
+    let dist = distance_from_rect(pos, center, corner, rect.radius);
+    color.a *= 1.0 - smoothstep(-min(edge, rect.radius), edge, dist);
+
+    if rect.thickness > 0.0 {
+        let dist2 = distance_from_rect(pos, center, corner - rect.thickness, rect.inner_radius);
+        color.a *= smoothstep(-min(edge, rect.inner_radius), edge, dist2);
+    }
+
+    return masked(in, color);
+}
+
+fn distance_from_rect(pixel_pos: vec2<f32>, rect_center: vec2<f32>, rect_corner: vec2<f32>, radius: f32) -> f32 {
+    // vec from center to pixel
+    let p = pixel_pos - rect_center;
+    // vec from inner rect corner to pixel
+    let q = abs(p) - (rect_corner - radius);
+    return length(max(q, vec2(0.0))) - radius;
+}

core/src/render/shader/texture.wgsl

@@ -0,0 +1,7 @@
+// No group 1: the texture is bound at group 2 for this instance alone, so
+// there is nothing per-instance left to look up.
+
+@fragment
+fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
+    return masked(in, textureSample(tex, samp, in.uv, 0));
+}

core/src/render/util/mod.rs

@@ -39,7 +39,11 @@ impl<T: Pod> ArrBuf<T> {
     fn init_buf(device: &Device, size: usize, usage: BufferUsages, label: &'static str) -> Buffer {
         let mut size = size as u64;
         if usage.contains(BufferUsages::STORAGE) {
-            size = size.max(std::mem::size_of::<T>() as u64);
+            // An empty storage buffer is still bound, and a binding has to be
+            // non-empty and a multiple of four however small `T` is.
+            size = size
+                .max(std::mem::size_of::<T>() as u64)
+                .next_multiple_of(4);
         }
         device.create_buffer(&BufferDescriptor {
             label: Some(label),

core/src/ui/mod.rs

@@ -1,4 +1,6 @@
-use crate::{Mask, TextData, Textures, WeakWidget, WidgetId, Widgets, util::TrackedArena};
+use crate::{
+    Mask, PrimitiveRegistry, TextData, Textures, WeakWidget, WidgetId, Widgets, util::TrackedArena,
+};
 
 mod active;
 mod cache;
@@ -14,6 +16,8 @@ pub use size::*;
 #[derive(Default)]
 pub struct UiData {
     pub widgets: Widgets,
+    /// Every primitive this ui can draw.
+    pub primitives: PrimitiveRegistry,
     pub textures: Textures,
     pub text: TextData,
     pub masks: TrackedArena<Mask, u32>,

core/src/ui/painter.rs

@@ -1,7 +1,9 @@
+use bytemuck::Pod;
+
 use crate::{
     Axis, Len, RenderedText, Size, SizeCtx, StrongWidget, TextAttrs, TextBuffer, TextData,
     TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, Widget, WidgetId,
-    render::{GlyphPrimitive, Mask, MaskIdx, Primitive, PrimitiveHandle, PrimitiveInst},
+    render::{GLYPH, GlyphPrimitive, Mask, MaskIdx, PrimitiveHandle, PrimitiveKind},
     util::Vec2,
 };
 
@@ -20,16 +22,11 @@ pub struct Painter<'a> {
 }
 
 impl<'a> Painter<'a> {
-    fn primitive_at<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
-        let h = self.state.layers.write(
-            self.layer,
-            PrimitiveInst {
-                id: self.id,
-                primitive,
-                region,
-                mask_idx: self.mask,
-            },
-        );
+    fn primitive_at<P: Pod>(&mut self, kind: PrimitiveKind<P>, primitive: P, region: UiRegion) {
+        let h = self
+            .state
+            .layers
+            .write(self.layer, kind, self.id, primitive, region, self.mask);
         self.push_primitive(h);
     }
 
@@ -42,12 +39,17 @@ impl<'a> Painter<'a> {
     }
 
     /// Writes a primitive to be rendered
-    pub fn primitive<P: Primitive>(&mut self, primitive: P) {
-        self.primitive_at(primitive, self.region)
+    pub fn primitive<P: Pod>(&mut self, kind: PrimitiveKind<P>, primitive: P) {
+        self.primitive_at(kind, primitive, self.region)
     }
 
-    pub fn primitive_within<P: Primitive>(&mut self, primitive: P, region: UiRegion) {
-        self.primitive_at(primitive, region.within(&self.region));
+    pub fn primitive_within<P: Pod>(
+        &mut self,
+        kind: PrimitiveKind<P>,
+        primitive: P,
+        region: UiRegion,
+    ) {
+        self.primitive_at(kind, primitive, region.within(&self.region));
     }
 
     pub fn set_mask(&mut self, region: UiRegion) {
@@ -115,6 +117,7 @@ impl<'a> Painter<'a> {
             region.x.end = region.x.start + UiScalar::abs(glyph.entry.width as f32);
             region.y.end = region.y.start + UiScalar::abs(glyph.entry.height as f32);
             self.primitive_at(
+                GLYPH,
                 GlyphPrimitive {
                     uv_min: glyph.entry.uv_min,
                     uv_max: glyph.entry.uv_max,

core/src/ui/render_state.rs

@@ -245,7 +245,7 @@ impl UiRenderState {
     pub fn debug_layers(&self) {
         for ((idx, depth), draws) in self.layers.iter_depth() {
             let indent = " ".repeat(depth * 2);
-            let primitives = draws.primitives.instances().len();
+            let primitives: usize = draws.primitives().iter().map(|l| l.instances().len()).sum();
             let textures = draws.textures.instances().len();
             println!("{indent}{idx}: {primitives} primitives, {textures} textures");
         }

src/widget/rect.rs

@@ -29,12 +29,15 @@ impl Rect {
 
 impl Widget for Rect {
     fn draw(&mut self, painter: &mut Painter) {
-        painter.primitive(RectPrimitive {
+        painter.primitive(
+            RECT,
+            RectPrimitive {
                 color: self.color,
                 radius: self.radius,
                 thickness: self.thickness,
                 inner_radius: self.inner_radius,
-        });
+            },
+        );
     }
 
     fn desired_width(&mut self, _: &mut SizeCtx) -> Len {

src/widget/text/edit.rs

@@ -73,6 +73,7 @@ impl Widget for TextEdit {
             let size = vec2(rect.width() as f32, rect.height() as f32);
             let top_left = vec2(rect.x0 as f32, rect.y0 as f32);
             painter.primitive_within(
+                RECT,
                 RectPrimitive::color(Color::SKY),
                 size.align(Align::TOP_LEFT).offset(top_left).within(&region),
             );
@@ -82,6 +83,7 @@ impl Widget for TextEdit {
         let size = vec2(caret.width() as f32, caret.height() as f32);
         let top_left = vec2(caret.x0 as f32, caret.y0 as f32);
         painter.primitive_within(
+            RECT,
             RectPrimitive::color(Color::WHITE),
             size.align(Align::TOP_LEFT).offset(top_left).within(&region),
         );