Allow panic!("{}", computed_str) in const fn.

Author: nbdd0121

core/src/panic.rs

@@ -27,9 +27,14 @@ pub macro panic_2015 {
     ($msg:literal $(,)?) => (
         $crate::panicking::panic($msg)
     ),
+    // Use `panic_str` instead of `panic_display::<&str>` for non_fmt_panic lint.
     ($msg:expr $(,)?) => (
         $crate::panicking::panic_str($msg)
     ),
+    // Special-case the single-argument case for const_panic.
+    ("{}", $arg:expr $(,)?) => (
+        $crate::panicking::panic_display(&$arg)
+    ),
     ($fmt:expr, $($arg:tt)+) => (
         $crate::panicking::panic_fmt($crate::const_format_args!($fmt, $($arg)+))
     ),
@@ -44,6 +49,10 @@ pub macro panic_2021 {
     () => (
         $crate::panicking::panic("explicit panic")
     ),
+    // Special-case the single-argument case for const_panic.
+    ("{}", $arg:expr $(,)?) => (
+        $crate::panicking::panic_display(&$arg)
+    ),
     ($($t:tt)+) => (
         $crate::panicking::panic_fmt($crate::const_format_args!($($t)+))
     ),

core/src/panicking.rs

@@ -60,6 +60,13 @@ pub fn panic_str(expr: &str) -> ! {
     panic_fmt(format_args!("{}", expr));
 }
 
+#[inline]
+#[track_caller]
+#[cfg_attr(not(bootstrap), lang = "panic_display")] // needed for const-evaluated panics
+pub fn panic_display<T: fmt::Display>(x: &T) -> ! {
+    panic_fmt(format_args!("{}", *x));
+}
+
 #[cold]
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[track_caller]

std/src/lib.rs

@@ -258,6 +258,7 @@
 #![feature(const_trait_impl)]
 #![feature(container_error_extra)]
 #![feature(core_intrinsics)]
+#![feature(core_panic)]
 #![feature(custom_test_frameworks)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]

std/src/panic.rs

@@ -10,7 +10,7 @@ use crate::thread::Result;
 
 #[doc(hidden)]
 #[unstable(feature = "edition_panic", issue = "none", reason = "use panic!() instead")]
-#[allow_internal_unstable(libstd_sys_internals, const_format_args)]
+#[allow_internal_unstable(libstd_sys_internals, const_format_args, core_panic)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "std_panic_2015_macro")]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro panic_2015 {
@@ -20,6 +20,10 @@ pub macro panic_2015 {
     ($msg:expr $(,)?) => ({
         $crate::rt::begin_panic($msg)
     }),
+    // Special-case the single-argument case for const_panic.
+    ("{}", $arg:expr $(,)?) => ({
+        $crate::rt::panic_display(&$arg)
+    }),
     ($fmt:expr, $($arg:tt)+) => ({
         $crate::rt::begin_panic_fmt(&$crate::const_format_args!($fmt, $($arg)+))
     }),

std/src/rt.rs

@@ -16,6 +16,7 @@
 
 // Re-export some of our utilities which are expected by other crates.
 pub use crate::panicking::{begin_panic, begin_panic_fmt, panic_count};
+pub use core::panicking::panic_display;
 
 // To reduce the generated code of the new `lang_start`, this function is doing
 // the real work.