Add const_panic_extra feature gate for allowing panicking with format

Author: nbdd0121

library/core/src/fmt/mod.rs

@@ -312,7 +312,8 @@ macro_rules! arg_new {
     ($f: ident, $t: ident) => {
         #[doc(hidden)]
         #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-        pub fn $f<'b, T: $t>(x: &'b T) -> ArgumentV1<'_> {
+        #[rustc_const_unstable(feature = "const_panic_extra", issue = "none")]
+        pub const fn $f<'b, T: $t>(x: &'b T) -> ArgumentV1<'_> {
             Self::new(x, $t::fmt)
         }
     };
@@ -321,7 +322,8 @@ macro_rules! arg_new {
 impl<'a> ArgumentV1<'a> {
     #[doc(hidden)]
     #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
-    pub fn new<'b, T>(x: &'b T, f: fn(&T, &mut Formatter<'_>) -> Result) -> ArgumentV1<'b> {
+    #[rustc_const_unstable(feature = "const_panic_extra", issue = "none")]
+    pub const fn new<'b, T>(x: &'b T, f: fn(&T, &mut Formatter<'_>) -> Result) -> ArgumentV1<'b> {
         // SAFETY: `mem::transmute(x)` is safe because
         //     1. `&'b T` keeps the lifetime it originated with `'b`
         //              (so as to not have an unbounded lifetime)

library/core/src/lib.rs

@@ -120,6 +120,7 @@
 #![feature(const_num_from_num)]
 #![feature(const_ops)]
 #![feature(const_option)]
+#![feature(const_panic_extra)]
 #![feature(const_pin)]
 #![feature(const_replace)]
 #![feature(const_ptr_offset)]

library/core/src/panicking.rs

@@ -112,10 +112,7 @@ pub const fn const_panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
     if let Some(msg) = fmt.as_str() {
         panic_str(msg);
     } else {
-        // SAFETY: This is only evaluated at compile time, which reliably
-        // handles this UB (in case this branch turns out to be reachable
-        // somehow).
-        unsafe { crate::hint::unreachable_unchecked() };
+        panic_str("<failed to format Arguments>");
     }
 }