compiler: do not permit iterating str values.

This currently breaks badly on UTF-8, and doesn't even return
a value of a correct type.

artiq/compiler/transforms/inferencer.py

@@ -183,7 +183,7 @@ class Inferencer(algorithm.Visitor):
         if builtins.is_bytes(collection.type) or builtins.is_bytearray(collection.type):
             self._unify(element.type, builtins.get_iterable_elt(collection.type),
                         element.loc, None)
-        elif builtins.is_iterable(collection.type):
+        elif builtins.is_iterable(collection.type) and not builtins.is_str(collection.type):
             rhs_type = collection.type.find()
             rhs_wrapped_lhs_type = types.TMono(rhs_type.name, {"elt": element.type})
             self._unify(rhs_wrapped_lhs_type, rhs_type,

artiq/test/lit/inferencer/error_str_iter.py

@@ -0,0 +1,5 @@
+# RUN: %python -m artiq.compiler.testbench.inferencer +diag %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+# CHECK-L: ${LINE:+1}: error: type str is not iterable
+("a")[0]