Treat line continuation at EOF as a SyntaxError

This makes the parser consistent with the tokenize module (already the case
in `pypy`).

sample
------

```python
x = 5\
```

before
------

```console
$ python3 t.py
$ python3 -mtokenize t.py
t.py:2:0: error: EOF in multi-line statement
```

after
-----

```console
$ ./python t.py
  File "t.py", line 3
    x = 5\

         ^
SyntaxError: unexpected EOF while parsing
$ ./python -m tokenize t.py
t.py:2:0: error: EOF in multi-line statement
```

Author: asottile

Lib/test/test_eof.py

@@ -24,5 +24,11 @@ def test_EOFS(self):
         else:
             raise support.TestFailed
 
+    def test_line_continuation_EOF(self):
+        expect = 'unexpected EOF while parsing (<string>, line 1)'
+        with self.assertRaises(SyntaxError) as excinfo:
+            exec('x = 5\\')
+        self.assertEqual(str(excinfo.exception), expect)
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2019-05-17-18-34-30.bpo-2180.aBqHeW.rst

@@ -0,0 +1 @@
+Treat line continuation at EOF as a ``SyntaxError`` by Anthony Sottile.

Parser/tokenizer.c

@@ -983,7 +983,8 @@ tok_nextc(struct tok_state *tok)
                         return EOF;
                     /* Last line does not end in \n,
                        fake one */
-                    strcpy(tok->inp, "\n");
+                    if (tok->inp[-1] != '\n')
+                        strcpy(tok->inp, "\n");
                 }
                 tok->inp = strchr(tok->inp, '\0');
                 done = tok->inp[-1] == '\n';
@@ -1674,6 +1675,14 @@ tok_get(struct tok_state *tok, char **p_start, char **p_end)
             tok->cur = tok->inp;
             return ERRORTOKEN;
         }
+        c = tok_nextc(tok);
+        if (c == EOF) {
+            tok->done = E_EOF;
+            tok->cur = tok->inp;
+            return ERRORTOKEN;
+        } else {
+            tok_backup(tok, c);
+        }
         tok->cont_line = 1;
         goto again; /* Read next line */
     }