bpo-42997: Improve error message for missing : before suites (GH-24292)

* Add to the peg generator a new directive ('&&') that allows to expect
  a token and hard fail the parsing if the token is not found. This
  allows to quickly emmit syntax errors for missing tokens.

* Use the new grammar element to hard-fail if the ':' is missing before
  suites.

1 files changed, 22 insertions, 1 deletions

diff --git a/Parser/pegen.c b/Parser/pegen.c
index 2554273..68f0e32 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -782,7 +782,6 @@ _PyPegen_is_memoized(Parser *p, int type, void *pres)
     return 0;
 }
 
-
 int
 _PyPegen_lookahead_with_name(int positive, expr_ty (func)(Parser *), Parser *p)
 {
@@ -836,6 +835,28 @@ _PyPegen_expect_token(Parser *p, int type)
     return t;
 }
 
+Token *
+_PyPegen_expect_forced_token(Parser *p, int type, const char* expected) {
+
+    if (p->error_indicator == 1) {
+        return NULL;
+    }
+
+    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 != type) {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(t, "expected '%s'", expected);
+        return NULL;
+    }
+    p->mark += 1;
+    return t;
+}
+
 expr_ty
 _PyPegen_expect_soft_keyword(Parser *p, const char *keyword)
 {