Rollup merge of #91737 - Manishearth:panic-immediate-stdlib, r=joshtriplett

Make certain panicky stdlib functions behave better under panic_immediate_abort

The stdlib has a `panic_immediate_abort` feature that turns panics into immediate aborts, without any formatting/display logic. This feature was [introduced](https://github.com/rust-lang/rust/pull/55011) primarily for codesize-constrained situations.

Unfortunately, this win doesn't quite propagate to `Result::expect()` and `Result::unwrap()`, while the formatting machinery is reduced, `expect()` and `unwrap()` both call `unwrap_failed("msg", &err)` which has a signature of `fn unwrap_failed(msg: &str, error: &dyn fmt::Debug)` and is `#[inline(never)]`. This means that `unwrap_failed` will unconditionally construct a `dyn Debug` trait object even though the object is never used in the function.

Constructing a trait object (even if you never call a method on it!) forces rust to include the vtable and any dependencies. This means that in `panic_immediate_abort` mode, calling expect/unwrap on a Result will pull in a whole bunch of formatting code for the error type even if it's completely unused.

This PR swaps out the function with one that won't require a trait object such that it won't force the inclusion of vtables in the code. It also gates off `#[inline(never)]` in a bunch of other places where allowing the inlining of an abort may be useful (this kind of thing is already done elsewhere in the stdlib).

I don't know how to write a test for this; we don't really seem to have any tests for `panic_immediate_abort` anyway so perhaps it's fine as is.

library/core/src/option.rs

@@ -1671,7 +1671,8 @@ pub const fn transpose(self) -> Result<Option<T>, E> {
 }
 
 // This is a separate function to reduce the code size of .expect() itself.
-#[inline(never)]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
 const fn expect_failed(msg: &str) -> ! {

library/core/src/result.rs

@@ -1653,6 +1653,7 @@ pub const fn into_ok_or_err(self) -> T {
 }
 
 // This is a separate function to reduce the code size of the methods
+#[cfg(not(feature = "panic_immediate_abort"))]
 #[inline(never)]
 #[cold]
 #[track_caller]
@@ -1660,6 +1661,18 @@ fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
     panic!("{}: {:?}", msg, error)
 }
 
+// This is a separate function to avoid constructing a `dyn Debug`
+// that gets immediately thrown away, since vtables don't get cleaned up
+// by dead code elimination if a trait object is constructed even if it goes
+// unused
+#[cfg(feature = "panic_immediate_abort")]
+#[inline]
+#[cold]
+#[track_caller]
+fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
+    panic!()
+}
+
 /////////////////////////////////////////////////////////////////////////////
 // Trait implementations
 /////////////////////////////////////////////////////////////////////////////

library/core/src/slice/index.rs

@@ -27,35 +27,40 @@ fn index_mut(&mut self, index: I) -> &mut I::Output {
     }
 }
 
-#[inline(never)]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
 fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
     panic!("range start index {} out of range for slice of length {}", index, len);
 }
 
-#[inline(never)]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
 fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
     panic!("range end index {} out of range for slice of length {}", index, len);
 }
 
-#[inline(never)]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
 fn slice_index_order_fail(index: usize, end: usize) -> ! {
     panic!("slice index starts at {} but ends at {}", index, end);
 }
 
-#[inline(never)]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
 fn slice_start_index_overflow_fail() -> ! {
     panic!("attempted to index slice from after maximum usize");
 }
 
-#[inline(never)]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
 fn slice_end_index_overflow_fail() -> ! {