Reject integer suffix when tuple indexing

src/libsyntax/parse/parser.rs

@@ -3199,11 +3199,20 @@ impl<'a> Parser<'a> {
                     token::Ident(..) => {
                         e = self.parse_dot_suffix(e, lo)?;
                     }
-                  token::Literal(token::Integer(name), _) => {
+                    token::Literal(token::Integer(name), suffix) => {
                         let span = self.span;
                         self.bump();
                         let field = ExprKind::Field(e, Ident::new(name, span));
                         e = self.mk_expr(lo.to(span), field, ThinVec::new());
+
+                        if let Some(suffix) = suffix {
+                            let mut err = self.diagnostic().struct_span_err(
+                                span,
+                                "tuple index with a suffix is invalid",
+                            );
+                            err.span_label(span, format!("invalid suffix `{}`", suffix));
+                            err.emit();
+                        }
                     }
                     token::Literal(token::Float(n), _suf) => {
                       self.bump();

src/test/ui/parser/issue-59418.rs

@@ -0,0 +1,9 @@
+struct X(i32,i32,i32);
+
+fn main() {
+    let a = X(1, 2, 3);
+    let b = a.1suffix;
+    //~^ ERROR tuple index with a suffix is invalid
+    println!("{}", b);
+}
+

src/test/ui/parser/issue-59418.stderr

@@ -0,0 +1,8 @@
+error: tuple index with a suffix is invalid
+  --> $DIR/issue-59418.rs:5:15
+   |
+LL |     let b = a.1suffix;
+   |               ^^^^^^^ invalid suffix `suffix`
+
+error: aborting due to previous error
+