Put checks that detect UB under their own flag below debug_assertions

Author: saethlin

compiler/rustc_codegen_cranelift/src/base.rs

@@ -789,7 +789,7 @@ fn codegen_stmt<'tcx>(
                             layout.offset_of_subfield(fx, fields.iter()).bytes()
                         }
                         NullOp::UbChecks => {
-                            let val = fx.tcx.sess.opts.debug_assertions;
+                            let val = fx.tcx.sess.ub_checks();
                             let val = CValue::by_val(
                                 fx.bcx.ins().iconst(types::I8, i64::try_from(val).unwrap()),
                                 fx.layout_of(fx.tcx.types.bool),

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -682,7 +682,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         bx.cx().const_usize(val)
                     }
                     mir::NullOp::UbChecks => {
-                        let val = bx.tcx().sess.opts.debug_assertions;
+                        let val = bx.tcx().sess.ub_checks();
                         bx.cx().const_bool(val)
                     }
                 };

compiler/rustc_const_eval/src/interpret/step.rs

@@ -258,7 +258,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         let val = layout.offset_of_subfield(self, fields.iter()).bytes();
                         Scalar::from_target_usize(val, self)
                     }
-                    mir::NullOp::UbChecks => Scalar::from_bool(self.tcx.sess.opts.debug_assertions),
+                    mir::NullOp::UbChecks => Scalar::from_bool(self.tcx.sess.ub_checks()),
                 };
                 self.write_scalar(val, &dest)?;
             }

compiler/rustc_feature/src/builtin_attrs.rs

@@ -25,6 +25,7 @@ pub type GatedCfg = (Symbol, Symbol, GateFn);
 const GATED_CFGS: &[GatedCfg] = &[
     // (name in cfg, feature, function to check if the feature is enabled)
     (sym::overflow_checks, sym::cfg_overflow_checks, cfg_fn!(cfg_overflow_checks)),
+    (sym::ub_checks, sym::cfg_ub_checks, cfg_fn!(cfg_ub_checks)),
     (sym::target_thread_local, sym::cfg_target_thread_local, cfg_fn!(cfg_target_thread_local)),
     (
         sym::target_has_atomic_equal_alignment,

compiler/rustc_feature/src/unstable.rs

@@ -381,6 +381,8 @@ declare_features! (
     (unstable, cfg_target_has_atomic_equal_alignment, "1.60.0", Some(93822)),
     /// Allows `cfg(target_thread_local)`.
     (unstable, cfg_target_thread_local, "1.7.0", Some(29594)),
+    /// Allows the use of `#[cfg(ub_checks)` to check if UB checks are enabled.
+    (unstable, cfg_ub_checks, "CURRENT_RUSTC_VERSION", Some(123499)),
     /// Allow conditional compilation depending on rust version
     (unstable, cfg_version, "1.45.0", Some(64796)),
     /// Allows to use the `#[cfi_encoding = ""]` attribute.

compiler/rustc_interface/src/tests.rs

@@ -846,6 +846,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(trap_unreachable, Some(false));
     tracked!(treat_err_as_bug, NonZero::new(1));
     tracked!(tune_cpu, Some(String::from("abc")));
+    tracked!(ub_checks, Some(false));
     tracked!(uninit_const_chunk_threshold, 123);
     tracked!(unleash_the_miri_inside_of_you, true);
     tracked!(use_ctors_section, Some(true));

compiler/rustc_middle/src/mir/mod.rs

@@ -777,8 +777,8 @@ impl<'tcx> Body<'tcx> {
         // _1 = const _
         // SwitchInt(_1)
         //
-        // And MIR for if intrinsics::debug_assertions() looks like this:
-        // _1 = cfg!(debug_assertions)
+        // And MIR for if intrinsics::ub_checks() looks like this:
+        // _1 = UbChecks()
         // SwitchInt(_1)
         //
         // So we're going to try to recognize this pattern.
@@ -799,9 +799,7 @@ impl<'tcx> Body<'tcx> {
         }
 
         match rvalue {
-            Rvalue::NullaryOp(NullOp::UbChecks, _) => {
-                Some((tcx.sess.opts.debug_assertions as u128, targets))
-            }
+            Rvalue::NullaryOp(NullOp::UbChecks, _) => Some((tcx.sess.ub_checks() as u128, targets)),
             Rvalue::Use(Operand::Constant(constant)) => {
                 let bits = eval_mono_const(constant);
                 Some((bits, targets))

compiler/rustc_mir_transform/src/check_alignment.rs

@@ -16,7 +16,7 @@ impl<'tcx> MirPass<'tcx> for CheckAlignment {
         if sess.target.llvm_target == "i686-pc-windows-msvc" {
             return false;
         }
-        sess.opts.debug_assertions
+        sess.ub_checks()
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -149,7 +149,7 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
 
     fn simplify_ub_check(&self, source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
         if let Rvalue::NullaryOp(NullOp::UbChecks, _) = *rvalue {
-            let const_ = Const::from_bool(self.tcx, self.tcx.sess.opts.debug_assertions);
+            let const_ = Const::from_bool(self.tcx, self.tcx.sess.ub_checks());
             let constant = ConstOperand { span: source_info.span, const_, user_ty: None };
             *rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
         }

compiler/rustc_session/src/config/cfg.rs

@@ -212,6 +212,10 @@ pub(crate) fn default_configuration(sess: &Session) -> Cfg {
         ins_none!(sym::test);
     }
 
+    if sess.ub_checks() {
+        ins_none!(sym::ub_checks);
+    }
+
     ret
 }
 
@@ -367,6 +371,8 @@ impl CheckCfg {
 
         ins!(sym::test, no_values);
 
+        ins!(sym::ub_checks, no_values);
+
         ins!(sym::unix, no_values);
         ins!(sym::windows, no_values);
     }