fix(naga): Forbid negative indexing in const expressions (#7155)

Signed-off-by: sagudev <[email protected]>

naga/src/valid/expression.rs

@@ -280,25 +280,26 @@ impl super::Validator {
                     }
                 }
 
-                // If we know both the length and the index, we can do the
-                // bounds check now.
-                if let crate::proc::IndexableLength::Known(known_length) =
-                    base_type.indexable_length(module)?
+                // If index is const we can do check for non-negative index
+                match module
+                    .to_ctx()
+                    .eval_expr_to_u32_from(index, &function.expressions)
                 {
-                    match module
-                        .to_ctx()
-                        .eval_expr_to_u32_from(index, &function.expressions)
-                    {
-                        Ok(value) => {
+                    Ok(value) => {
+                        // If we know both the length and the index, we can do the
+                        // bounds check now.
+                        if let crate::proc::IndexableLength::Known(known_length) =
+                            base_type.indexable_length(module)?
+                        {
                             if value >= known_length {
                                 return Err(ExpressionError::IndexOutOfBounds(base, value));
                             }
                         }
-                        Err(crate::proc::U32EvalError::Negative) => {
-                            return Err(ExpressionError::NegativeIndex(base))
-                        }
-                        Err(crate::proc::U32EvalError::NonConst) => {}
                     }
+                    Err(crate::proc::U32EvalError::Negative) => {
+                        return Err(ExpressionError::NegativeIndex(base))
+                    }
+                    Err(crate::proc::U32EvalError::NonConst) => {}
                 }
 
                 ShaderStages::all()