Add `#[rustc_no_mir_inline]` for standard library UB checks

Co-authored-by: Ben Kimock <kimockb@gmail.com>

compiler/rustc_feature/src/builtin_attrs.rs

@@ -792,6 +792,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_intrinsic, Normal, template!(Word), ErrorFollowing,
         "the `#[rustc_intrinsic]` attribute is used to declare intrinsics with function bodies",
     ),
+    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:

compiler/rustc_mir_transform/src/inline.rs

@@ -421,6 +421,10 @@ 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");
         }

compiler/rustc_span/src/symbol.rs

@@ -1441,6 +1441,7 @@ 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,

library/core/src/intrinsics.rs

@@ -2706,13 +2706,25 @@ pub const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize)
 macro_rules! assert_unsafe_precondition {
     ($message:expr, ($($name:ident:$ty:ty = $arg:expr),*$(,)?) => $e:expr $(,)?) => {
         {
+            // #[cfg(bootstrap)] (this comment)
             // When the standard library is compiled with debug assertions, we want the check to inline for better performance.
             // This is important when working on the compiler, which is compiled with debug assertions locally.
             // When not compiled with debug assertions (so the precompiled std) we outline the check to minimize the compile
             // time impact when debug assertions are disabled.
-            // It is not clear whether that is the best solution, see #120848.
-            #[cfg_attr(debug_assertions, inline(always))]
-            #[cfg_attr(not(debug_assertions), inline(never))]
+            // The proper solution to this is the `#[rustc_no_mir_inline]` below, but we still want decent performance for cfg(bootstrap).
+            #[cfg_attr(all(debug_assertions, bootstrap), inline(always))]
+            #[cfg_attr(all(not(debug_assertions), bootstrap), inline(never))]
+
+            // This check is inlineable, but not by the MIR inliner.
+            // The reason for this is that the MIR inliner is in an exceptionally bad position
+            // to think about whether or not to inline this. In MIR, this call is gated behind `debug_assertions`,
+            // which will codegen to `false` in release builds. Inlining the check would be wasted work in that case and
+            // would be bad for compile times.
+            //
+            // LLVM on the other hand sees the constant branch, so if it's `false`, it can immediately delete it without
+            // inlining the check. If it's `true`, it can inline it and get significantly better performance.
+            #[cfg_attr(not(bootstrap), rustc_no_mir_inline)]
+            #[cfg_attr(not(bootstrap), inline)]
             #[rustc_nounwind]
             fn precondition_check($($name:$ty),*) {
                 if !$e {

tests/mir-opt/inline/rustc_no_mir_inline.caller.Inline.panic-abort.diff

@@ -0,0 +1,19 @@
+- // MIR for `caller` before Inline
++ // MIR for `caller` after Inline
+  
+  fn caller() -> () {
+      let mut _0: ();
+      let _1: ();
+  
+      bb0: {
+          StorageLive(_1);
+          _1 = callee() -> [return: bb1, unwind unreachable];
+      }
+  
+      bb1: {
+          StorageDead(_1);
+          _0 = const ();
+          return;
+      }
+  }
+  

tests/mir-opt/inline/rustc_no_mir_inline.caller.Inline.panic-unwind.diff

@@ -0,0 +1,19 @@
+- // MIR for `caller` before Inline
++ // MIR for `caller` after Inline
+  
+  fn caller() -> () {
+      let mut _0: ();
+      let _1: ();
+  
+      bb0: {
+          StorageLive(_1);
+          _1 = callee() -> [return: bb1, unwind continue];
+      }
+  
+      bb1: {
+          StorageDead(_1);
+          _0 = const ();
+          return;
+      }
+  }
+  

tests/mir-opt/inline/rustc_no_mir_inline.caller.PreCodegen.after.panic-abort.mir

@@ -0,0 +1,14 @@
+// MIR for `caller` after PreCodegen
+
+fn caller() -> () {
+    let mut _0: ();
+    let _1: ();
+
+    bb0: {
+        _1 = callee() -> [return: bb1, unwind unreachable];
+    }
+
+    bb1: {
+        return;
+    }
+}

tests/mir-opt/inline/rustc_no_mir_inline.caller.PreCodegen.after.panic-unwind.mir

@@ -0,0 +1,14 @@
+// MIR for `caller` after PreCodegen
+
+fn caller() -> () {
+    let mut _0: ();
+    let _1: ();
+
+    bb0: {
+        _1 = callee() -> [return: bb1, unwind continue];
+    }
+
+    bb1: {
+        return;
+    }
+}

tests/mir-opt/inline/rustc_no_mir_inline.rs

@@ -0,0 +1,17 @@
+// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+#![crate_type = "lib"]
+#![feature(rustc_attrs)]
+
+//@ compile-flags: -Zmir-opt-level=2 -Zinline-mir
+
+#[inline]
+#[rustc_no_mir_inline]
+pub fn callee() {}
+
+// EMIT_MIR rustc_no_mir_inline.caller.Inline.diff
+// EMIT_MIR rustc_no_mir_inline.caller.PreCodegen.after.mir
+pub fn caller() {
+    // CHECK-LABEL: fn caller(
+    // CHECK: callee()
+    callee();
+}