Rollup merge of #88860 - nbdd0121:panic, r=m-ou-se

Deduplicate panic_fmt

std's begin_panic_fmt and core's panic_fmt are duplicates. Merge them to declutter code and remove a lang item.

library/core/src/panic/panic_info.rs

@@ -121,7 +121,7 @@ impl<'a> PanicInfo<'a> {
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn location(&self) -> Option<&Location<'_>> {
         // NOTE: If this is changed to sometimes return None,
-        // deal with that case in std::panicking::default_hook and std::panicking::begin_panic_fmt.
+        // deal with that case in std::panicking::default_hook and core::panicking::panic_fmt.
         Some(&self.location)
     }
 }

library/core/src/panicking.rs

@@ -76,8 +76,15 @@ fn panic_bounds_check(index: usize, len: usize) -> ! {
     panic!("index out of bounds: the len is {} but the index is {}", len, index)
 }
 
-/// The underlying implementation of libcore's `panic!` macro when formatting is used.
+/// The entry point for panicking with a formatted message.
+///
+/// This is designed to reduce the amount of code required at the call
+/// site as much as possible (so that `panic!()` has as low an impact
+/// on (e.g.) the inlining of other functions as possible), by moving
+/// the actual formatting into this shared place.
 #[cold]
+// If panic_immediate_abort, inline the abort call,
+// otherwise avoid inlining because of it is cold path.
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]