in which ! is suggested for erroneous identifier not

Impressing confused Python users with magical diagnostics is perhaps
worth this not-grossly-unreasonable (only 40ish lines) extra complexity
in the parser?

Thanks to Vadim Petrochenkov for guidance.

This resolves #46836.

src/libsyntax/parse/token.rs

@@ -91,7 +91,7 @@ impl Lit {
     }
 }
 
-fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
+pub(crate) fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
     let ident_token: Token = Ident(ident, is_raw);
 
     !ident_token.is_reserved_ident() ||
@@ -348,6 +348,15 @@ impl Token {
         self.lifetime().is_some()
     }
 
+    /// Returns `true` if the token is a identifier whose name is the given
+    /// string slice.
+    pub fn is_ident_named(&self, name: &str) -> bool {
+        match self.ident() {
+            Some((ident, _)) => ident.name.as_str() == name,
+            None => false
+        }
+    }
+
     /// Returns `true` if the token is a documentation comment.
     pub fn is_doc_comment(&self) -> bool {
         match *self {