Share inline(never) generics across crates

This reduces code sizes and better respects programmer intent when
marking inline(never). Previously such a marking was essentially ignored
for generic functions, as we'd still inline them in remote crates.

Author: Mark-Simulacrum

alloc/src/raw_vec.rs

@@ -757,7 +757,9 @@ impl<A: Allocator> RawVecInner<A> {
     }
 }
 
-#[inline(never)]
+// not marked inline(never) since we want optimizers to be able to observe the specifics of this
+// function, see tests/codegen/vec-reserve-extend.rs.
+#[cold]
 fn finish_grow<A>(
     new_layout: Layout,
     current_memory: Option<(NonNull<u8>, Layout)>,

std/src/lib.rs

@@ -362,6 +362,7 @@
 #![feature(strict_provenance_atomic_ptr)]
 #![feature(sync_unsafe_cell)]
 #![feature(ub_checks)]
+#![feature(used_with_arg)]
 // tidy-alphabetical-end
 //
 // Library features (alloc):

std/src/panicking.rs

@@ -27,6 +27,22 @@ use crate::sys::backtrace;
 use crate::sys::stdio::panic_output;
 use crate::{fmt, intrinsics, process, thread};
 
+// This forces codegen of the function called by panic!() inside the std crate, rather than in
+// downstream crates. Primarily this is useful for rustc's codegen tests, which rely on noticing
+// complete removal of panic from generated IR. Since begin_panic is inline(never), it's only
+// codegen'd once per crate-graph so this pushes that to std rather than our codegen test crates.
+//
+// (See https://github.com/rust-lang/rust/pull/123244 for more info on why).
+//
+// If this is causing problems we can also modify those codegen tests to use a crate type like
+// cdylib which doesn't export "Rust" symbols to downstream linkage units.
+#[unstable(feature = "libstd_sys_internals", reason = "used by the panic! macro", issue = "none")]
+#[doc(hidden)]
+#[allow(dead_code)]
+#[used(compiler)]
+pub static EMPTY_PANIC: fn(&'static str) -> ! =
+    begin_panic::<&'static str> as fn(&'static str) -> !;
+
 // Binary interface to the panic runtime that the standard library depends on.
 //
 // The standard library is tagged with `#![needs_panic_runtime]` (introduced in