Auto merge of rust-lang#121114 - Nilstrieb:no-inline!, r=<try>

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

should help with rust-lang#121110 and also with rust-lang#120848

I am not entirely sure whether this is the correct solution and I haven't validated it, I just quickly threw it together before going to sleep.

r? `@saethlin`

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:

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;
 
@@ -413,6 +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");
         }

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,

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 $(,)?) => {
         {
-            #[inline(never)]
+            #[cfg_attr(not(bootstrap), rustc_no_mir_inline, inline)]
             #[rustc_nounwind]
             fn precondition_check($($name:$ty),*) {
                 if !$e {