Add validate_scene function

Cargo.toml

@@ -51,6 +51,7 @@ tree_arena = { version = "0.2.0", path = "tree_arena" }
 
 anymore = "1.0.0"
 vello = { version = "0.6.0", default-features = false, features = ["wgpu"] }
+vello_encoding = { version = "0.6.0", default-features = false }
 kurbo = "0.12.0"
 parley = { version = "0.7.0", features = ["accesskit"] }
 # TODO: Use no_std correctly in Xilem Web.

masonry/src/tests/paint.rs

@@ -5,8 +5,10 @@ use assert_matches::assert_matches;
 use masonry_core::core::{NewWidget, WidgetTag};
 use masonry_core::palette::css::{BLUE, GREEN, RED};
 use masonry_core::util::{fill, stroke};
-use masonry_testing::{ModularWidget, Record, TestHarness, TestWidgetExt, assert_render_snapshot};
-use vello::kurbo::{Affine, Circle, Dashes, Point, Size, Stroke, Vec2};
+use masonry_testing::{
+    ModularWidget, Record, TestHarness, TestWidgetExt, assert_debug_panics, assert_render_snapshot,
+};
+use vello::kurbo::{Affine, Circle, Dashes, Point, Rect, Size, Stroke, Vec2};
 use vello::peniko::Color;
 
 use crate::properties::Background;
@@ -155,3 +157,36 @@ fn paint_clipping() {
     // The dashed circle shouldn't.
     assert_render_snapshot!(harness, "paint_clipping");
 }
+
+#[test]
+fn scene_validation_nan() {
+    let widget = NewWidget::new(
+        ModularWidget::new(())
+            .layout_fn(|_, _, _, _| Size::new(10., 10.))
+            .paint_fn(move |_, _, _, scene| {
+                let nan_rect = Rect::new(0., 1., f64::NAN, 3.);
+                fill(scene, &nan_rect, Color::WHITE);
+            }),
+    );
+
+    assert_debug_panics!(
+        TestHarness::create(test_property_set(), widget),
+        "HasNanValues"
+    );
+}
+
+#[test]
+fn scene_validation_push_layer() {
+    let widget = NewWidget::new(
+        ModularWidget::new(())
+            .layout_fn(|_, _, _, _| Size::new(10., 10.))
+            .paint_fn(move |_, _, _, scene| {
+                scene.push_clip_layer(Affine::IDENTITY, &Rect::ZERO);
+            }),
+    );
+
+    assert_debug_panics!(
+        TestHarness::create(test_property_set(), widget),
+        "UnbalancedPushLayer"
+    );
+}

masonry_core/Cargo.toml

@@ -37,6 +37,7 @@ tracing-tracy = { version = "0.11.4", optional = true }
 tree_arena.workspace = true
 ui-events.workspace = true
 vello.workspace = true
+vello_encoding.workspace = true
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 console_error_panic_hook.workspace = true

masonry_core/src/passes/paint.rs

@@ -12,7 +12,7 @@ use vello::peniko::{Color, Fill};
 use crate::app::{RenderRoot, RenderRootState};
 use crate::core::{DefaultProperties, PaintCtx, PropertiesRef, WidgetArenaNode, WidgetId};
 use crate::passes::{enter_span_if, recurse_on_children};
-use crate::util::{get_debug_color, stroke};
+use crate::util::{get_debug_color, stroke, validate_scene};
 
 // --- MARK: PAINT WIDGET
 fn paint_widget(
@@ -65,6 +65,11 @@ fn paint_widget(
         if ctx.widget_state.request_post_paint {
             widget.post_paint(&mut ctx, &props, postfix_scene);
         }
+
+        if cfg!(debug_assertions) {
+            validate_scene(&scene).unwrap();
+            validate_scene(&postfix_scene).unwrap();
+        }
     }
 
     state.request_paint = false;

masonry_core/src/util.rs

@@ -4,10 +4,12 @@
 //! Miscellaneous utility functions.
 
 use std::any::Any;
+use std::fmt::Display;
 
 use vello::Scene;
 use vello::kurbo::{Affine, Join, Shape, Stroke};
 use vello::peniko::{BrushRef, Color, Fill};
+use vello_encoding::DrawTag;
 
 /// Panic in debug and `tracing::error` in release mode.
 ///
@@ -81,6 +83,94 @@ pub fn fill_color(scene: &mut Scene, path: &impl Shape, color: Color) {
 
 // ---
 
+/// Error type returned by [`validate_scene()`].
+#[derive(Debug, Clone, Copy)]
+#[non_exhaustive]
+pub enum ValidationError {
+    /// Scene was constructed with NaN values in its path data.
+    HasNanValues,
+    /// Scene had a `push_layer` command that was never popped.
+    UnbalancedPushLayer,
+    /// Scene had a `pop_layer` command with no layer pushed.
+    /// This is currently "unreachable" because Vello silently swallows these cases.
+    #[doc(hidden)]
+    UnbalancedPopLayer,
+}
+
+impl Display for ValidationError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ValidationError::HasNanValues => {
+                write!(f, "Scene was constructed with NaN values in its path data")
+            }
+            ValidationError::UnbalancedPushLayer => {
+                write!(f, "Scene had a `push_layer` command that was never popped")
+            }
+            ValidationError::UnbalancedPopLayer => {
+                write!(f, "Scene had a `pop_layer` command with no layer pushed")
+            }
+        }
+    }
+}
+
+/// Take a scene and return an error if the scene is invalid.
+///
+/// A scene is invalid if:
+///
+/// - It was constructed with NaN values in its path data.
+/// - It had a `push_layer` command that was never popped.
+///
+/// ## Missing checks
+///
+/// This function may have some false negative in some cases, because Vello can
+/// sometimes silently remove NaN values from the paths given to it.
+///
+/// We'd like to catch `pop_layer` commands with no layer pushed, but Vello
+/// currently also swallows them silently.
+pub fn validate_scene(scene: &Scene) -> Result<(), ValidationError> {
+    // This assumes that `vello_encoding::Encoding::path_data` only ever stores
+    // the float values of its paths.
+    // While in theory it can store other things, in practice it never does when created
+    // using a Vello Scene, and this will not change until vello is replaced with the sparse
+    // strips API, at which point this function will likely be discarded.
+    for path_data_elem in &scene.encoding().path_data {
+        if f32::from_bits(*path_data_elem).is_nan() {
+            return Err(ValidationError::HasNanValues);
+        }
+    }
+
+    for transform in &scene.encoding().transforms {
+        for value in &transform.matrix {
+            if value.is_nan() {
+                return Err(ValidationError::HasNanValues);
+            }
+        }
+    }
+
+    let mut layer_count = 0;
+    for tag in &scene.encoding().draw_tags {
+        match *tag {
+            DrawTag::BEGIN_CLIP => {
+                layer_count += 1;
+            }
+            DrawTag::END_CLIP => {
+                if layer_count == 0 {
+                    return Err(ValidationError::UnbalancedPopLayer);
+                }
+                layer_count -= 1;
+            }
+            _ => {}
+        }
+    }
+    if layer_count > 0 {
+        return Err(ValidationError::UnbalancedPushLayer);
+    }
+
+    Ok(())
+}
+
+// ---
+
 /// Convert a 2d rectangle from Parley to one used for drawing in Vello and other maths.
 pub fn bounding_box_to_rect(bb: parley::BoundingBox) -> vello::kurbo::Rect {
     vello::kurbo::Rect {