feat: restrict result tag sizes to 256 bytes (#596)

Closes #595

guppylang/prelude/_internal/checker.py

@@ -233,6 +233,10 @@ def check(self, args: list[ast.expr], ty: Type) -> tuple[ast.expr, Subst]:
                 raise InternalGuppyError(f"Invalid array type args: {type_args}")
 
 
+#: Maximum length of a tag in the `result` function.
+TAG_MAX_LEN = 200
+
+
 class ResultChecker(CustomCallChecker):
     """Call checker for the `result` function."""
 
@@ -241,6 +245,10 @@ def synthesize(self, args: list[ast.expr]) -> tuple[ast.expr, Type]:
         [tag, value] = args
         if not isinstance(tag, ast.Constant) or not isinstance(tag.value, str):
             raise GuppyTypeError("Expected a string literal", tag)
+        if len(tag.value.encode("utf-8")) > TAG_MAX_LEN:
+            raise GuppyTypeError(
+                f"Tag is too long, limited to {TAG_MAX_LEN} bytes", tag
+            )
         value, ty = ExprSynthesizer(self.ctx).synthesize(value)
         # We only allow numeric values or vectors of numeric values
         err = (

tests/error/misc_errors/result_tag_too_big.err

@@ -0,0 +1,7 @@
+Guppy compilation failed. Error in file $FILE:11
+
+9:     def foo(y: bool) -> None:
+10:        # each tick or cross is 3 bytes. The cross sends the tag over the limit.
+11:        result("✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅❌", y)
+                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+GuppyTypeError: Tag is too long, limited to 200 bytes

tests/error/misc_errors/result_tag_too_big.py

@@ -0,0 +1,11 @@
+from guppylang.prelude.builtins import result, py
+from tests.util import compile_guppy
+
+# TODO use this once https://github.com/CQCL/guppylang/issues/599 lands
+# from guppylang.prelude._internal.checker import TAG_MAX_LEN
+# BIG_TAG = "a" * (TAG_MAX_LEN + 1)
+
+@compile_guppy
+def foo(y: bool) -> None:
+    # each tick or cross is 3 bytes. The cross sends the tag over the limit.
+    result("✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅✅❌", y)