Emit more informative errors for direct use of program counter

Resolves #43

hera/typechecker.py

@@ -169,10 +169,18 @@ def check_types(name, expected, got, symtab):
 
     ordinals = ["first", "second", "third"]
     for ordinal, pattern, arg in zip(ordinals, expected, got):
-        prefix = "{} arg to {} ".format(ordinal, name)
-        error = check_one_type(pattern, arg, symtab)
-        if error:
-            emit_error(prefix + error, line=arg.line, column=arg.column)
+        if isinstance(arg, Token) and arg.type == "REGISTER" and arg.lower() == "pc":
+            # TODO: This error-handling logic is a little messy.
+            emit_error(
+                "program counter cannot be accessed or changed directly",
+                line=arg.line,
+                column=arg.column,
+            )
+        else:
+            prefix = "{} arg to {} ".format(ordinal, name)
+            error = check_one_type(pattern, arg, symtab)
+            if error:
+                emit_error(prefix + error, line=arg.line, column=arg.column)
 
 
 def check_one_type(pattern, arg, symtab):

hera/utils.py

@@ -57,7 +57,7 @@ def to_u32(n):
         return n
 
 
-NAMED_REGISTERS = {"rt": 11, "fp": 14, "sp": 15, "pc_ret": 13, "fp_alt": 12}
+NAMED_REGISTERS = {"rt": 11, "fp": 14, "sp": 15, "pc_ret": 13, "fp_alt": 12, "pc": -1}
 
 
 def register_to_index(rname):

test/test_typechecker.py

@@ -57,6 +57,16 @@ def test_check_types_with_wrong_type():
         assert "not an integer" in mock_emit_error.call_args[0][0]
 
 
+def test_check_types_with_program_counter():
+    with patch("hera.utils._emit_msg") as mock_emit_error:
+        check_types(SYM(), [REGISTER], [R("PC")], {})
+        assert mock_emit_error.call_count == 1
+        assert (
+            "program counter cannot be accessed or changed directly"
+            in mock_emit_error.call_args[0][0]
+        )
+
+
 def test_check_one_type_with_u16_out_of_range():
     assert check_one_type(U16, IntToken(65536), {}) == "out of range"
 

test/test_utils.py

@@ -144,6 +144,8 @@ def test_register_to_index_with_named_registers():
     assert register_to_index("pc_ret") == 13
     assert register_to_index("FP_alt") == 12
     assert register_to_index("fp_alt") == 12
+    # Make sure program counter does not correspond to actual register.
+    assert not (0 <= register_to_index("pc") <= 15)
 
 
 def test_register_to_index_with_invalid_register():