Add #[rustc_no_ubchecks]

Author: saethlin

compiler/rustc_feature/src/builtin_attrs.rs

@@ -676,6 +676,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_nounwind, Normal, template!(Word), WarnFollowing,
         EncodeCrossCrate::No, IMPL_DETAIL
     ),
+    rustc_attr!(
+        rustc_no_ubchecks, Normal, template!(Word), WarnFollowing,
+        EncodeCrossCrate::No, IMPL_DETAIL
+    ),
     rustc_attr!(
         rustc_reallocator, Normal, template!(Word), WarnFollowing,
         EncodeCrossCrate::No, IMPL_DETAIL

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -37,19 +37,27 @@ impl<'tcx> MirPass<'tcx> for InstSimplify {
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        let def_id = body.source.def_id();
         let ctx = InstSimplifyContext {
             tcx,
             local_decls: &body.local_decls,
-            param_env: tcx.param_env_reveal_all_normalized(body.source.def_id()),
+            param_env: tcx.param_env_reveal_all_normalized(def_id),
         };
         let preserve_ub_checks =
             attr::contains_name(tcx.hir().krate_attrs(), sym::rustc_preserve_ub_checks);
+        let remove_ub_checks = tcx.has_attr(def_id, sym::rustc_no_ubchecks);
         for block in body.basic_blocks.as_mut() {
             for statement in block.statements.iter_mut() {
                 match statement.kind {
                     StatementKind::Assign(box (_place, ref mut rvalue)) => {
-                        if !preserve_ub_checks {
-                            ctx.simplify_ub_check(&statement.source_info, rvalue);
+                        if remove_ub_checks {
+                            ctx.simplify_ub_check(&statement.source_info, rvalue, false);
+                        } else if !preserve_ub_checks {
+                            ctx.simplify_ub_check(
+                                &statement.source_info,
+                                rvalue,
+                                tcx.sess.ub_checks(),
+                            );
                         }
                         ctx.simplify_bool_cmp(&statement.source_info, rvalue);
                         ctx.simplify_ref_deref(&statement.source_info, rvalue);
@@ -199,9 +207,14 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
         }
     }
 
-    fn simplify_ub_check(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
+    fn simplify_ub_check(
+        &self,
+        source_info: &SourceInfo,
+        rvalue: &mut Rvalue<'tcx>,
+        ub_checks: bool,
+    ) {
         if let Rvalue::NullaryOp(NullOp::UbChecks, _) = *rvalue {
-            let const_ = Const::from_bool(self.tcx, self.tcx.sess.ub_checks());
+            let const_ = Const::from_bool(self.tcx, ub_checks);
             let constant = ConstOperand { span: source_info.span, const_, user_ty: None };
             *rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
         }

compiler/rustc_span/src/symbol.rs

@@ -1655,6 +1655,7 @@ symbols! {
         rustc_never_returns_null_ptr,
         rustc_never_type_options,
         rustc_no_mir_inline,
+        rustc_no_ubchecks,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,
         rustc_object_lifetime_default,

library/core/src/mem/mod.rs

@@ -859,6 +859,7 @@ pub fn take<T: Default>(dest: &mut T) -> T {
 #[must_use = "if you don't need the old value, you can just assign the new value directly"]
 #[rustc_const_unstable(feature = "const_replace", issue = "83164")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_replace")]
+#[cfg_attr(not(bootstrap), rustc_no_ubchecks)]
 pub const fn replace<T>(dest: &mut T, src: T) -> T {
     // It may be tempting to use `swap` to avoid `unsafe` here. Don't!
     // The compiler optimizes the implementation below to two `memcpy`s