naga-agal: Fix single-component write mask reading wrong source component

render/naga-agal/src/builder.rs

@@ -1090,27 +1090,13 @@ impl<'a> NagaBuilder<'a> {
                 value: expr,
             });
         } else {
-            // A scalar write occurs when we have exactly one component in the dest write mask.
-            let scalar_write = [Mask::X, Mask::Y, Mask::Z, Mask::W]
-                .into_iter()
-                .filter(|mask| dest.write_mask.contains(*mask))
-                .count()
-                == 1;
-
             for (i, mask) in [(0, Mask::X), (1, Mask::Y), (2, Mask::Z), (3, Mask::W)] {
                 if dest.write_mask.contains(mask) {
-                    let source_component = if scalar_write || source_is_scalar {
-                        if source_is_scalar {
-                            expr
-                        } else {
-                            // Grab the first component of the source expression - all of them should be
-                            // the same when doing a scalar write.
-                            self.evaluate_expr(Expression::AccessIndex {
-                                base: expr,
-                                index: 0,
-                            })
-                        }
+                    let source_component = if source_is_scalar {
+                        expr
                     } else {
+                        // AGAL swizzles are positional: dest component `i` takes component `i`
+                        // of the (already swizzled) source.
                         self.evaluate_expr(Expression::AccessIndex {
                             base: expr,
                             index: i,

render/naga-agal/tests/snapshots/wgsl__complex_raytrace.snap

@@ -1,6 +1,5 @@
 ---
 source: render/naga-agal/tests/wgsl.rs
-assertion_line: 90
 expression: output
 ---
 struct VertexOutput {
@@ -22,7 +21,7 @@ fn main(@location(0) param: vec4<f32>) -> VertexOutput {
     temporary0_.y = _e3.y;
     temporary0_.z = _e3.z;
     let _e13: vec4<f32> = constant_registers[4u];
-    temporary0_.w = _e13.yyyy.x;
+    temporary0_.w = _e13.yyyy.w;
     let _e19: vec4<f32> = constant_registers[0u];
     let _e22: vec4<f32> = constant_registers[1u];
     let _e25: vec4<f32> = constant_registers[2u];