[Skylark] Support dictionaries in structs when serializing them using struct.to_json.

Dictionaries are frequently used for generic configuration descriptions especially
given that `struct` is not allowed in build files.

In order to be JSON-compatible, only dictionaries with string keys are allowed.
Technically Python also allows integers and booleans by automatically converting
them to strings, but this is confusing and not necessarily a good thing to do.

Fixes #5542

Closes #5543.

PiperOrigin-RevId: 206049754

1 files changed, 3 insertions, 2 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/StructApi.java b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/StructApi.java
index 5c0974975c..49d90e5ac7 100644
--- a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/StructApi.java
+++ b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/StructApi.java
@@ -60,8 +60,9 @@ public interface StructApi extends SkylarkValue {
       name = "to_json",
       doc =
           "Creates a JSON string from the struct parameter. This method only works if all "
-              + "struct elements (recursively) are strings, ints, booleans, other structs or a "
-              + "list of these types. Quotes and new lines in strings are escaped. "
+              + "struct elements (recursively) are strings, ints, booleans, other structs, a "
+              + "list of these types or a dictionary with string keys and values of these types. "
+              + "Quotes and new lines in strings are escaped. "
               + "Examples:<br><pre class=language-python>"
               + "struct(key=123).to_json()\n# {\"key\":123}\n\n"
               + "struct(key=True).to_json()\n# {\"key\":true}\n\n"