Add soft keywords

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.

Author: gvanrossum

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

@@ -121,6 +121,7 @@ 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

@@ -110,6 +110,16 @@ 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:
@@ -147,7 +157,10 @@ 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]