[naga] Let constant evaluation of As preserve Splat expressions.

When asked to evaluate an `Expression::As` cast applied to a `Splat`
expression, change `ConstantEvaluator::cast` to preserve the `Splat`,
rather than expanding it out to a `Compose` expression.

naga/src/proc/constant_evaluator.rs

@@ -863,6 +863,22 @@ impl<'a> ConstantEvaluator<'a> {
         }
     }
 
+    /// Lower [`ZeroValue`] expressions to [`Literal`] and [`Compose`] expressions.
+    ///
+    /// [`ZeroValue`]: Expression::ZeroValue
+    /// [`Literal`]: Expression::Literal
+    /// [`Compose`]: Expression::Compose
+    fn eval_zero_value(
+        &mut self,
+        expr: Handle<Expression>,
+        span: Span,
+    ) -> Result<Handle<Expression>, ConstantEvaluatorError> {
+        match self.expressions[expr] {
+            Expression::ZeroValue(ty) => self.eval_zero_value_impl(ty, span),
+            _ => Ok(expr),
+        }
+    }
+
     /// Lower [`ZeroValue`] expressions to [`Literal`] and [`Compose`] expressions.
     ///
     /// [`ZeroValue`]: Expression::ZeroValue
@@ -953,7 +969,7 @@ impl<'a> ConstantEvaluator<'a> {
     ) -> Result<Handle<Expression>, ConstantEvaluatorError> {
         use crate::Scalar as Sc;
 
-        let expr = self.eval_zero_value_and_splat(expr, span)?;
+        let expr = self.eval_zero_value(expr, span)?;
 
         let expr = match self.expressions[expr] {
             Expression::Literal(literal) => {
@@ -1022,6 +1038,14 @@ impl<'a> ConstantEvaluator<'a> {
 
                 Expression::Compose { ty, components }
             }
+            Expression::Splat { size, value } => {
+                let value_span = self.expressions.get_span(value);
+                let cast_value = self.cast(value, target, value_span)?;
+                Expression::Splat {
+                    size,
+                    value: cast_value,
+                }
+            }
             _ => return Err(ConstantEvaluatorError::InvalidCastArg),
         };
 

naga/tests/out/wgsl/900-implicit-conversions.frag.wgsl

@@ -57,7 +57,7 @@ fn implicit_dims_3(v_6: vec4<f32>) {
 fn main_1() {
     exact_1(1);
     implicit(1.0);
-    implicit_dims_2(vec3<f32>(1.0, 1.0, 1.0));
+    implicit_dims_2(vec3(1.0));
     return;
 }