Rollup merge of rust-lang#78069 - fusion-engineering-forks:core-const-panic-str, r=RalfJung

Fix const core::panic!(non_literal_str).

Invocations of `core::panic!(x)` where `x` is not a string literal expand to `panic!("{}", x)`, which is not understood by the const panic logic right now. This adds `panic_str` as a lang item, and modifies the const eval implementation to hook into this item as well.

This fixes the issue mentioned here: rust-lang#51999 (comment)

r? `@RalfJung`

`@rustbot` modify labels: +A-const-eval

Author: jonas-schievink

core/src/macros/mod.rs

@@ -10,7 +10,7 @@ macro_rules! panic {
         $crate::panicking::panic($msg)
     );
     ($msg:expr) => (
-        $crate::panic!("{}", $crate::convert::identity::<&str>($msg))
+        $crate::panicking::panic_str($msg)
     );
     ($msg:expr,) => (
         $crate::panic!($msg)

core/src/panicking.rs

@@ -50,6 +50,13 @@ pub fn panic(expr: &'static str) -> ! {
     panic_fmt(fmt::Arguments::new_v1(&[expr], &[]));
 }
 
+#[inline]
+#[track_caller]
+#[cfg_attr(not(bootstrap), lang = "panic_str")] // needed for const-evaluated panics
+pub fn panic_str(expr: &str) -> ! {
+    panic_fmt(format_args!("{}", expr));
+}
+
 #[cold]
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[track_caller]