bpo-40334: Always show the caret on SyntaxErrors (pythonGH-20050)

This commit fixes SyntaxError locations when the caret is not displayed,
by doing the following:

- `col_number` always gets set to the location of the offending
  node/expr. When no caret is to be displayed, this gets achieved
  by setting the object holding the error line to None.

- Introduce a new function `_PyPegen_raise_error_known_location`,
  which can be called, when an arbitrary `lineno`/`col_offset`
  needs to be passed. This function then gets used in the grammar
  (through some new macros and inline functions) so that SyntaxError
  locations of the new parser match that of the old.

Grammar/python.gram

@@ -623,26 +623,31 @@ t_atom[expr_ty]:
 # From here on, there are rules for invalid syntax with specialised error messages
 incorrect_arguments:
     | args ',' '*' { RAISE_SYNTAX_ERROR("iterable argument unpacking follows keyword argument unpacking") }
-    | expression for_if_clauses ',' [args | expression for_if_clauses] {
-        RAISE_SYNTAX_ERROR("Generator expression must be parenthesized") }
+    | a=expression for_if_clauses ',' [args | expression for_if_clauses] {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "Generator expression must be parenthesized") }
     | a=args ',' args { _PyPegen_arguments_parsing_error(p, a) }
 invalid_kwarg:
-    | expression '=' { RAISE_SYNTAX_ERROR("expression cannot contain assignment, perhaps you meant \"==\"?") }
+    | a=expression '=' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a, "expression cannot contain assignment, perhaps you meant \"==\"?") }
 invalid_named_expression:
     | a=expression ':=' expression {
-        RAISE_SYNTAX_ERROR("cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a, "cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }
 invalid_assignment:
-    | list ':' { RAISE_SYNTAX_ERROR("only single target (not list) can be annotated") }
-    | tuple ':' { RAISE_SYNTAX_ERROR("only single target (not tuple) can be annotated") }
-    | expression ':' expression ['=' annotated_rhs] {
-        RAISE_SYNTAX_ERROR("illegal target for annotation") }
+    | a=list ':' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not list) can be annotated") }
+    | a=tuple ':' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not tuple) can be annotated") }
+    | a=star_named_expression ',' star_named_expressions* ':' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not tuple) can be annotated") }
+    | a=expression ':' expression ['=' annotated_rhs] {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "illegal target for annotation") }
     | a=expression ('=' | augassign) (yield_expr | star_expressions) {
-        RAISE_SYNTAX_ERROR_NO_COL_OFFSET("cannot assign to %s", _PyPegen_get_expr_name(a)) }
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot assign to %s", _PyPegen_get_expr_name(a)) }
 invalid_block:
     | NEWLINE !INDENT { RAISE_INDENTATION_ERROR("expected an indented block") }
 invalid_comprehension:
-    | ('[' | '(' | '{') '*' expression for_if_clauses {
-        RAISE_SYNTAX_ERROR("iterable unpacking cannot be used in comprehension") }
+    | ('[' | '(' | '{') a=starred_expression for_if_clauses {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "iterable unpacking cannot be used in comprehension") }
 invalid_parameters:
     | param_no_default* (slash_with_default | param_with_default+) param_no_default {
         RAISE_SYNTAX_ERROR("non-default argument follows default argument") }
@@ -655,4 +660,4 @@ invalid_double_type_comments:
         RAISE_SYNTAX_ERROR("Cannot have two type comments on def") }
 invalid_del_target:
     | a=star_expression &del_target_end {
-        RAISE_SYNTAX_ERROR("cannot delete %s", _PyPegen_get_expr_name(a)) }
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot delete %s", _PyPegen_get_expr_name(a)) }

Lib/test/test_exceptions.py

@@ -242,16 +242,13 @@ def baz():
         check('from __future__ import doesnt_exist', 1, 1)
         check('from __future__ import braces', 1, 1)
         check('x=1\nfrom __future__ import division', 2, 1)
-        check('(yield i) = 2', 1, 1)
+        check('foo(1=2)', 1, 5)
+        check('def f():\n  x, y: int', 2, 3)
+        check('[*x for x in xs]', 1, 2)
+        check('foo(x for x in range(10), 100)', 1, 5)
+        check('(yield i) = 2', 1, 1 if support.use_old_parser() else 2)
         check('def f(*):\n  pass', 1, 7 if support.use_old_parser() else 8)
-        check('foo(1=2)', 1, 5 if support.use_old_parser() else 6)
-
-    @support.skip_if_new_parser("Pegen column offsets might be different")
-    def testSyntaxErrorOffsetCustom(self):
-        self.check('for 1 in []: pass', 1, 5)
-        self.check('[*x for x in xs]', 1, 2)
-        self.check('def f():\n  x, y: int', 2, 3)
-        self.check('foo(x for x in range(10), 100)', 1, 5)
+        check('for 1 in []: pass', 1, 5 if support.use_old_parser() else 7)
 
     @cpython_only
     def testSettingException(self):