Improve error message for the 'in' operator.

In particular, display the type that was passed as argument.

--
PiperOrigin-RevId: 148995695
MOS_MIGRATED_REVID=148995695

1 files changed, 9 insertions, 2 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
index 0da5e5c4fb..e6d37905c5 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
@@ -77,11 +77,18 @@ public final class BinaryOperatorExpression extends Expression {
         return ((String) rval).contains((String) lval);
       } else {
         throw new EvalException(
-            location, "in operator only works on strings if the left operand is also a string");
+            location,
+            "'in <string>' requires string as left operand, not '"
+                + EvalUtils.getDataTypeName(lval)
+                + "'");
       }
     } else {
       throw new EvalException(
-          location, "in operator only works on lists, tuples, sets, dicts and strings");
+          location,
+          "argument of type '"
+              + EvalUtils.getDataTypeName(rval)
+              + "' is not iterable. "
+              + "in operator only works on lists, tuples, dicts and strings.");
     }
   }