[wgsl-in] Avoid splatting all binary operator expressions (#2440)

* [wgsl-in] Avoid splatting all binary operator expressions

Fixes #2439.

* [wgsl-in] Expand binary_op_splat function comment

src/front/wgsl/lower/mod.rs

@@ -504,13 +504,25 @@ impl<'source, 'temp, 'out> ExpressionContext<'source, 'temp, 'out> {
     }
 
     /// Insert splats, if needed by the non-'*' operations.
+    ///
+    /// See the "Binary arithmetic expressions with mixed scalar and vector operands"
+    /// table in the WebGPU Shading Language specification for relevant operators.
+    ///
+    /// Multiply is not handled here as backends are expected to handle vec*scalar
+    /// operations, so inserting splats into the IR increases size needlessly.
     fn binary_op_splat(
         &mut self,
         op: crate::BinaryOperator,
         left: &mut Handle<crate::Expression>,
         right: &mut Handle<crate::Expression>,
     ) -> Result<(), Error<'source>> {
-        if op != crate::BinaryOperator::Multiply {
+        if matches!(
+            op,
+            crate::BinaryOperator::Add
+                | crate::BinaryOperator::Subtract
+                | crate::BinaryOperator::Divide
+                | crate::BinaryOperator::Modulo
+        ) {
             self.grow_types(*left)?.grow_types(*right)?;
 
             let left_size = match *self.resolved_inner(*left) {

src/front/wgsl/tests.rs

@@ -387,6 +387,54 @@ fn parse_expressions() {
     }").unwrap();
 }
 
+#[test]
+fn binary_expression_mixed_scalar_and_vector_operands() {
+    for (operand, expect_splat) in [
+        ('<', false),
+        ('>', false),
+        ('&', false),
+        ('|', false),
+        ('+', true),
+        ('-', true),
+        ('*', false),
+        ('/', true),
+        ('%', true),
+    ] {
+        let module = parse_str(&format!(
+            "
+            const some_vec = vec3<f32>(1.0, 1.0, 1.0);
+            @fragment
+            fn main() -> @location(0) vec4<f32> {{
+                if (all(1.0 {operand} some_vec)) {{
+                    return vec4(0.0);
+                }}
+                return vec4(1.0);
+            }}
+            "
+        ))
+        .unwrap();
+
+        let expressions = &&module.entry_points[0].function.expressions;
+
+        let found_expressions = expressions
+            .iter()
+            .filter(|&(_, e)| {
+                if let crate::Expression::Binary { left, .. } = *e {
+                    matches!(
+                        (expect_splat, &expressions[left]),
+                        (false, &crate::Expression::Literal(crate::Literal::F32(..)))
+                            | (true, &crate::Expression::Splat { .. })
+                    )
+                } else {
+                    false
+                }
+            })
+            .count();
+
+        assert_eq!(found_expressions, 1);
+    }
+}
+
 #[test]
 fn parse_pointers() {
     parse_str(