Tweak some attributes to improve panic_immediate_abort

Author: saethlin

library/core/src/panic/location.rs

@@ -195,6 +195,7 @@ impl<'a> Location<'a> {
 
 #[stable(feature = "panic_hook_display", since = "1.26.0")]
 impl fmt::Display for Location<'_> {
+    #[inline]
     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(formatter, "{}:{}:{}", self.file, self.line, self.col)
     }

library/core/src/slice/mod.rs

@@ -3667,8 +3667,8 @@ impl<T> [T] {
     {
         // The panic code path was put into a cold function to not bloat the
         // call site.
-        #[inline(never)]
-        #[cold]
+        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+        #[cfg_attr(feature = "panic_immediate_abort", inline)]
         #[track_caller]
         fn len_mismatch_fail(dst_len: usize, src_len: usize) -> ! {
             panic!(

library/core/src/slice/sort/shared/smallsort.rs

@@ -840,7 +840,8 @@ unsafe fn bidirectional_merge<T: FreezeMarker, F: FnMut(&T, &T) -> bool>(
     }
 }
 
-#[inline(never)]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 fn panic_on_ord_violation() -> ! {
     // This is indicative of a logic bug in the user-provided comparison function or Ord
     // implementation. They are expected to implement a total order as explained in the Ord