From 4aef1011e64d6be56f6223151aaf6e694e5a49b0 Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Wed, 14 Feb 2024 22:08:03 +0100
Subject: [PATCH 1/3] Add `#[rustc_no_mir_inline]` for standard library UB
 checks

---
 compiler/rustc_feature/src/builtin_attrs.rs | 4 ++++
 compiler/rustc_mir_transform/src/inline.rs  | 5 +++++
 compiler/rustc_span/src/symbol.rs           | 1 +
 library/core/src/intrinsics.rs              | 2 +-
 4 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index 6aedd2a5e3341..0d5aa8a5d5eb5 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/compiler/rustc_feature/src/builtin_attrs.rs
@@ -788,6 +788,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_safe_intrinsic, Normal, template!(Word), WarnFollowing,
         "the `#[rustc_safe_intrinsic]` attribute is used internally to mark intrinsics as safe"
     ),
+    rustc_attr!(
+        rustc_no_mir_inline, Normal, template!(Word), WarnFollowing,
+        "#[rustc_no_mir_inline] prevents the MIR inliner from inlining a function while not affecting codegen"
+    ),
 
     // ==========================================================================
     // Internal attributes, Testing:
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 956d855ab81a2..6521ba166b72b 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -13,6 +13,7 @@ use rustc_middle::ty::TypeVisitableExt;
 use rustc_middle::ty::{self, Instance, InstanceDef, ParamEnv, Ty, TyCtxt};
 use rustc_session::config::OptLevel;
 use rustc_span::source_map::Spanned;
+use rustc_span::sym;
 use rustc_target::abi::FieldIdx;
 use rustc_target::spec::abi::Abi;
 
@@ -279,6 +280,10 @@ impl<'tcx> Inliner<'tcx> {
             }
         }
 
+        if self.tcx.has_attr(callsite.callee.def_id(), sym::rustc_no_mir_inline) {
+            return Err("found #[rustc_no_mir_inline] on callee");
+        }
+
         let old_blocks = caller_body.basic_blocks.next_index();
         self.inline_call(caller_body, callsite, callee_body);
         let new_blocks = old_blocks..caller_body.basic_blocks.next_index();
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 134aa9efc01b6..c15f06c6214b0 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1435,6 +1435,7 @@ symbols! {
         rustc_mir,
         rustc_must_implement_one_of,
         rustc_never_returns_null_ptr,
+        rustc_no_mir_inline,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,
         rustc_object_lifetime_default,
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index c8259c0024c75..59e79d6e398e0 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -2640,7 +2640,7 @@ pub(crate) const fn debug_assertions() -> bool {
 macro_rules! assert_unsafe_precondition {
     ($message:expr, ($($name:ident:$ty:ty = $arg:expr),*$(,)?) => $e:expr $(,)?) => {
         {
-            #[inline(never)]
+            #[cfg_attr(not(bootstrap), rustc_no_mir_inline)]
             #[rustc_nounwind]
             fn precondition_check($($name:$ty),*) {
                 if !$e {

From ffcbf96336f914711c9546008b3c334d31b547c4 Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Wed, 14 Feb 2024 16:59:44 -0500
Subject: [PATCH 2/3] Check for rustc_no_mir_inline earlier

---
 compiler/rustc_mir_transform/src/inline.rs | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 6521ba166b72b..f96b92f1c3c94 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -280,10 +280,6 @@ impl<'tcx> Inliner<'tcx> {
             }
         }
 
-        if self.tcx.has_attr(callsite.callee.def_id(), sym::rustc_no_mir_inline) {
-            return Err("found #[rustc_no_mir_inline] on callee");
-        }
-
         let old_blocks = caller_body.basic_blocks.next_index();
         self.inline_call(caller_body, callsite, callee_body);
         let new_blocks = old_blocks..caller_body.basic_blocks.next_index();
@@ -418,6 +414,10 @@ impl<'tcx> Inliner<'tcx> {
         callee_attrs: &CodegenFnAttrs,
         cross_crate_inlinable: bool,
     ) -> Result<(), &'static str> {
+        if self.tcx.has_attr(callsite.callee.def_id(), sym::rustc_no_mir_inline) {
+            return Err("#[rustc_no_mir_inline]");
+        }
+
         if let InlineAttr::Never = callee_attrs.inline {
             return Err("never inline hint");
         }

From 749d7e25a760894e26d5a68f8e2a87aa06823e9a Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Wed, 14 Feb 2024 16:59:49 -0500
Subject: [PATCH 3/3] Enable backend inlining

---
 library/core/src/intrinsics.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index 59e79d6e398e0..51d488d898b3d 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -2640,7 +2640,7 @@ pub(crate) const fn debug_assertions() -> bool {
 macro_rules! assert_unsafe_precondition {
     ($message:expr, ($($name:ident:$ty:ty = $arg:expr),*$(,)?) => $e:expr $(,)?) => {
         {
-            #[cfg_attr(not(bootstrap), rustc_no_mir_inline)]
+            #[cfg_attr(not(bootstrap), rustc_no_mir_inline, inline)]
             #[rustc_nounwind]
             fn precondition_check($($name:$ty),*) {
                 if !$e {