compiler: do not permit str(...). (#878)

artiq/compiler/transforms/inferencer.py

@@ -682,6 +682,11 @@ class Inferencer(algorithm.Visitor):
                 pass
             else:
                 diagnose(valid_forms())
+        elif types.is_builtin(typ, "str"):
+            diag = diagnostic.Diagnostic("error",
+                "strings currently cannot be constructed", {},
+                node.loc)
+            self.engine.process(diag)
         elif types.is_builtin(typ, "list") or types.is_builtin(typ, "array"):
             if types.is_builtin(typ, "list"):
                 valid_forms = lambda: [

artiq/test/lit/inferencer/error_builtin_calls.py

@@ -9,3 +9,6 @@ list(1)
 
 # CHECK-L: ${LINE:+1}: error: the arguments of min() must be of a numeric type
 min([1], [1])
+
+# CHECK-L: ${LINE:+1}: error: strings currently cannot be constructed
+str(1)