Simplify to invoking Trait on ident

library/alloc/src/macros.rs

@@ -64,15 +64,6 @@ macro_rules! vec {
     ($($x:expr,)*) => (vec![$($x),*])
 }
 
-// HACK(jubilee): Shim for specializing format! It is possible to manually
-// implement ToString, bypassing Display. "{}" normally formats in terms of
-// Display. NeedsDisplay enforces equally strict type boundarie.
-#[unstable(feature = "display_type_guard", issue = "none")]
-#[allow(dead_code)]
-struct NeedsDisplay<T: crate::fmt::Display> {
-    inner: T,
-}
-
 /// Creates a `String` using interpolation of runtime expressions.
 ///
 /// The first argument `format!` receives is a format string. This must be a string
@@ -108,15 +99,12 @@ struct NeedsDisplay<T: crate::fmt::Display> {
 /// format!("hello {}", "world!");
 /// format!("x = {}, y = {y}", 10, y = 30);
 /// ```
-#[allow_internal_unstable(display_type_guard)]
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! format {
     // A faster path for simple format! calls.
-    ("{}", $($arg:tt,?)+) => {{
-        let fast = |t: NeedsDisplay<_> | $crate::string::ToString::to_string(t.inner);
-        // This TokenTree must resolve to a Displayable value to be valid.
-        fast(NeedsDisplay { inner: &{$($arg)*} })
+    ("{}", $arg:ident) => {{
+        $crate::fmt::Display::to_string(&$arg)
     }};
     ($($arg:tt)*) => {{
         let res = $crate::fmt::format($crate::__export::format_args!($($arg)*));