Don't allow `const` to begin a nonterminal

Thanks to Vadim Petrochenkov who [told me what the fix was][z]!

[z]: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/finding.20which.20macro.20rule.20to.20use/near/220240422

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -27,6 +27,8 @@ impl<'a> Parser<'a> {
                 token.can_begin_expr()
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Let)
+                // This exception is here for backwards compatibility.
+                && !token.is_keyword(kw::Const)
             }
             NonterminalKind::Ty => token.can_begin_type(),
             NonterminalKind::Ident => get_macro_ident(token).is_some(),

src/test/ui/inline-const/macro-with-const.rs

@@ -0,0 +1,20 @@
+// check-pass
+
+macro_rules! exp {
+    (const $n:expr) => {
+        $n
+    };
+}
+
+macro_rules! stmt {
+    (exp $e:expr) => {
+        $e
+    };
+    (exp $($t:tt)+) => {
+        exp!($($t)+)
+    };
+}
+
+fn main() {
+    stmt!(exp const 1);
+}