Auto merge of rust-lang#121309 - Nilstrieb:inline-all-the-fallbacks, r=oli-obk

Make intrinsic fallback bodies cross-crate inlineable

This change was prompted by the stage1 compiler spending 4% of its time when compiling the polymorphic-recursion MIR opt test in `unlikely`.

Intrinsic fallback bodies like `unlikely` should always be inlined, it's very silly if they are not. To do this, we enable the fallback bodies to be cross-crate inlineable. Not that this matters for our workloads since the compiler never actually _uses_ the "fallback bodies", it just uses whatever was cfg(bootstrap)ped, so I've also added `#[inline]` to those.

See the comments for more information.

r? oli-obk

Author: bors

core/src/intrinsics.rs

@@ -953,6 +953,7 @@ extern "rust-intrinsic" {
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
+#[cfg_attr(bootstrap, inline)]
 pub const unsafe fn assume(b: bool) {
     if !b {
         // SAFETY: the caller must guarantee the argument is never `false`
@@ -975,6 +976,7 @@ pub const unsafe fn assume(b: bool) {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
 #[rustc_nounwind]
+#[cfg_attr(bootstrap, inline)]
 pub const fn likely(b: bool) -> bool {
     b
 }
@@ -994,6 +996,7 @@ pub const fn likely(b: bool) -> bool {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
 #[rustc_nounwind]
+#[cfg_attr(bootstrap, inline)]
 pub const fn unlikely(b: bool) -> bool {
     b
 }
@@ -2596,6 +2599,7 @@ extern "rust-intrinsic" {
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
+#[cfg_attr(bootstrap, inline)]
 pub const fn is_val_statically_known<T: Copy>(_arg: T) -> bool {
     false
 }
@@ -2633,6 +2637,7 @@ pub(crate) const fn debug_assertions() -> bool {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_nounwind]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
+#[cfg_attr(bootstrap, inline)]
 pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {
     // const eval overrides this function, but runtime code should always just return null pointers.
     crate::ptr::null_mut()
@@ -2652,6 +2657,7 @@ pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_nounwind]
 #[cfg_attr(not(bootstrap), rustc_intrinsic)]
+#[cfg_attr(bootstrap, inline)]
 pub const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
 
 // Some functions are defined here because they accidentally got made