Disable numbered access to positional args on tuples

tests/test_display.rs

@@ -131,7 +131,7 @@ fn test_nested() {
 #[test]
 fn test_match() {
     #[derive(Error, Debug)]
-    #[error("{}: {0}", match .1 {
+    #[error("{intro}: {0}", intro = match .1 {
         Some(n) => format!("error occurred with {}", n),
         None => "there was an empty error".to_owned(),
     })]

tests/ui/numbered-positional-tuple.rs

@@ -0,0 +1,7 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
+pub struct Error(u32);
+
+fn main() {}

tests/ui/numbered-positional-tuple.stderr

@@ -0,0 +1,5 @@
+error: ambiguous reference to positional arguments by number in a tuple struct; change this to a named argument
+ --> tests/ui/numbered-positional-tuple.rs:4:61
+  |
+4 | #[error("invalid rdo_lookahead_frames {0} (expected < {})", i32::MAX)]
+  |                                                             ^^^^^^^^