New flag --incompatible_string_is_not_iterable to forbid iteration over strings.

RELNOTES: None.
PiperOrigin-RevId: 161706309

2 files changed, 23 insertions, 2 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
index 7ef2eb3641..e6e9089f43 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
@@ -348,7 +348,7 @@ public final class EvalUtils {
     if (o instanceof String) {
       // This is not as efficient as special casing String in for and dict and list comprehension
       // statements. However this is a more unified way.
-      return split((String) o);
+      return split((String) o, loc, env);
     } else if (o instanceof SkylarkNestedSet) {
       return nestedSetToCollection((SkylarkNestedSet) o, loc, env);
     } else if (o instanceof Iterable) {
@@ -401,7 +401,15 @@ public final class EvalUtils {
     }
   }
 
-  private static ImmutableList<String> split(String value) {
+  private static ImmutableList<String> split(String value, Location loc, @Nullable Environment env)
+      throws EvalException {
+    if (env != null && env.getSemantics().incompatibleStringIsNotIterable) {
+      throw new EvalException(
+          loc,
+          "type 'string' is not iterable. You may still use `len` and string indexing. Use "
+              + "--incompatible_string_is_not_iterable=false to temporarily disable this check.");
+    }
+
     ImmutableList.Builder<String> builder = new ImmutableList.Builder<>();
     for (char c : value.toCharArray()) {
       builder.add(String.valueOf(c));
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
index b73b0a4cea..2bfb51b22d 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkSemanticsOptions.java
@@ -161,6 +161,19 @@ public class SkylarkSemanticsOptions extends OptionsBase implements Serializable
   public boolean incompatibleDepsetIsNotIterable;
 
   @Option(
+    name = "incompatible_string_is_not_iterable",
+    defaultValue = "false",
+    category = "incompatible changes",
+    documentationCategory = OptionDocumentationCategory.UNCATEGORIZED,
+    effectTags = {OptionEffectTag.UNKNOWN},
+    metadataTags = {OptionMetadataTag.INCOMPATIBLE_CHANGE},
+    help =
+        "If set to true, iterating over a string will throw an error. String indexing and `len` "
+            + "are still allowed."
+  )
+  public boolean incompatibleStringIsNotIterable;
+
+  @Option(
     name = "incompatible_dict_literal_has_no_duplicates",
     defaultValue = "false",
     category = "incompatible changes",