[naga wgsl-in] Drop spanless labels from front-end error messages.

When a label in a WGSL front end error has an undefined span, omit the
label from the error message. This is not great, but because of the
way Naga IR represents local variable references it is hard to get the
right span, and omitting the label better than panicking in `unwrap`,
since the error message has a general message anyway.

naga/src/front/wgsl/error.rs

@@ -34,8 +34,9 @@ impl ParseError {
             .with_labels(
                 self.labels
                     .iter()
-                    .map(|label| {
-                        Label::primary((), label.0.to_range().unwrap())
+                    .filter_map(|label| label.0.to_range().map(|range| (label, range)))
+                    .map(|(label, range)| {
+                        Label::primary((), range)
                             .with_message(label.1.to_string())
                     })
                     .collect(),

naga/tests/wgsl_errors.rs

@@ -1985,6 +1985,25 @@ fn function_param_redefinition_as_local() {
     )
 }
 
+#[test]
+fn constructor_type_error_span() {
+    check(
+        "
+        fn unfortunate() {
+            var i: i32;
+            var a: array<f32, 1> = array<f32, 1>(i);
+        }
+    ",
+        r###"error: automatic conversions cannot convert `i32` to `f32`
+  ┌─ wgsl:4:36
+  │
+4 │             var a: array<f32, 1> = array<f32, 1>(i);
+  │                                    ^^^^^^^^^^^^^^^^ a value of type f32 is required here
+
+"###,
+    )
+}
+
 #[test]
 fn binding_array_local() {
     check_validation! {