Add soft keywords (GH-20370)

These are like keywords but they only work in context; they are not reserved except when there is an exact match.

This would enable things like match statements without reserving `match` (which would be bad for the `re.match()` function and probably lots of other places).

Automerge-Triggered-By: @gvanrossum

Lib/test/test_peg_generator/test_c_parser.py

@@ -402,3 +402,33 @@ def test_error_in_rules(self) -> None:
             parse.parse_string("a", mode=0)
         """
         self.run_test(grammar_source, test_source)
+
+    def test_no_soft_keywords(self) -> None:
+        grammar_source = """
+        start: expr+ NEWLINE? ENDMARKER
+        expr: 'foo'
+        """
+        grammar = parse_string(grammar_source, GrammarParser)
+        parser_source = generate_c_parser_source(grammar)
+        assert "expect_soft_keyword" not in parser_source
+
+    def test_soft_keywords(self) -> None:
+        grammar_source = """
+        start: expr+ NEWLINE? ENDMARKER
+        expr: "foo"
+        """
+        grammar = parse_string(grammar_source, GrammarParser)
+        parser_source = generate_c_parser_source(grammar)
+        assert "expect_soft_keyword" in parser_source
+
+    def test_soft_keywords_parse(self) -> None:
+        grammar_source = """
+        start: "if" expr '+' expr NEWLINE
+        expr: NAME
+        """
+        test_source = """
+        valid_cases = ["if if + if"]
+        invalid_cases = ["if if"]
+        self.check_input_strings_for_grammar(valid_cases, invalid_cases)
+        """
+        self.run_test(grammar_source, test_source)

Parser/pegen/pegen.c

@@ -753,6 +753,30 @@ _PyPegen_expect_token(Parser *p, int type)
     return t;
 }
 
+expr_ty
+_PyPegen_expect_soft_keyword(Parser *p, const char *keyword)
+{
+    if (p->mark == p->fill) {
+        if (_PyPegen_fill_token(p) < 0) {
+            p->error_indicator = 1;
+            return NULL;
+        }
+    }
+    Token *t = p->tokens[p->mark];
+    if (t->type != NAME) {
+        return NULL;
+    }
+    char* s = PyBytes_AsString(t->bytes);
+    if (!s) {
+        return NULL;
+    }
+    if (strcmp(s, keyword) != 0) {
+        return NULL;
+    }
+    expr_ty res = _PyPegen_name_token(p);
+    return res;
+}
+
 Token *
 _PyPegen_get_last_nonnwhitespace_token(Parser *p)
 {

Parser/pegen/pegen.h

@@ -122,6 +122,7 @@ int _PyPegen_lookahead_with_int(int, Token *(func)(Parser *, int), Parser *, int
 int _PyPegen_lookahead(int, void *(func)(Parser *), Parser *);
 
 Token *_PyPegen_expect_token(Parser *p, int type);
+expr_ty _PyPegen_expect_soft_keyword(Parser *p, const char *keyword);
 Token *_PyPegen_get_last_nonnwhitespace_token(Parser *);
 int _PyPegen_fill_token(Parser *p);
 expr_ty _PyPegen_name_token(Parser *p);

Tools/peg_generator/pegen/c_generator.py

@@ -117,6 +117,16 @@ def keyword_helper(self, keyword: str) -> FunctionCall:
             comment=f"token='{keyword}'",
         )
 
+    def soft_keyword_helper(self, value: str) -> FunctionCall:
+        return FunctionCall(
+            assigned_variable="_keyword",
+            function="_PyPegen_expect_soft_keyword",
+            arguments=["p", value],
+            return_type="expr_ty",
+            nodetype=NodeTypes.NAME_TOKEN,
+            comment=f"soft_keyword='{value}'",
+        )
+
     def visit_NameLeaf(self, node: NameLeaf) -> FunctionCall:
         name = node.value
         if name in self.non_exact_tokens:
@@ -154,7 +164,10 @@ def visit_NameLeaf(self, node: NameLeaf) -> FunctionCall:
     def visit_StringLeaf(self, node: StringLeaf) -> FunctionCall:
         val = ast.literal_eval(node.value)
         if re.match(r"[a-zA-Z_]\w*\Z", val):  # This is a keyword
-            return self.keyword_helper(val)
+            if node.value.endswith("'"):
+                return self.keyword_helper(val)
+            else:
+                return self.soft_keyword_helper(node.value)
         else:
             assert val in self.exact_tokens, f"{node.value} is not a known literal"
             type = self.exact_tokens[val]
@@ -656,8 +669,9 @@ def handle_alt_normal(self, node: Alt, is_gather: bool, rulename: Optional[str])
         self.print("{")
         # We have parsed successfully all the conditions for the option.
         with self.indent():
+            node_str = str(node).replace('"', '\\"')
             self.print(
-                f'D(fprintf(stderr, "%*c+ {rulename}[%d-%d]: %s succeeded!\\n", p->level, \' \', _mark, p->mark, "{node}"));'
+                f'D(fprintf(stderr, "%*c+ {rulename}[%d-%d]: %s succeeded!\\n", p->level, \' \', _mark, p->mark, "{node_str}"));'
             )
             # Prepare to emmit the rule action and do so
             if node.action and "EXTRA" in node.action:
@@ -710,8 +724,9 @@ def visit_Alt(
         self.print(f"{{ // {node}")
         with self.indent():
             self._check_for_errors()
+            node_str = str(node).replace('"', '\\"')
             self.print(
-                f'D(fprintf(stderr, "%*c> {rulename}[%d-%d]: %s\\n", p->level, \' \', _mark, p->mark, "{node}"));'
+                f'D(fprintf(stderr, "%*c> {rulename}[%d-%d]: %s\\n", p->level, \' \', _mark, p->mark, "{node_str}"));'
             )
             # Prepare variable declarations for the alternative
             vars = self.collect_vars(node)
@@ -733,9 +748,10 @@ def visit_Alt(
                     self.handle_alt_normal(node, is_gather, rulename)
 
             self.print("p->mark = _mark;")
+            node_str = str(node).replace('"', '\\"')
             self.print(
                 f"D(fprintf(stderr, \"%*c%s {rulename}[%d-%d]: %s failed!\\n\", p->level, ' ',\n"
-                f'                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "{node}"));'
+                f'                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "{node_str}"));'
             )
             if "_cut_var" in vars:
                 self.print("if (_cut_var) {")