Put checks that detect UB under their own flag below debug_assertions

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_interface/src/tests.rs

@@ -847,6 +847,7 @@ 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_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/options.rs

@@ -1994,6 +1994,9 @@ written to standard error output)"),
         "in diagnostics, use heuristics to shorten paths referring to items"),
     tune_cpu: Option<String> = (None, parse_opt_string, [TRACKED],
         "select processor to schedule for (`rustc --print target-cpus` for details)"),
+    #[rustc_lint_opt_deny_field_access("use `Session::ub_checks` instead of this field")]
+    ub_checks: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "emit runtime checks for Undefined Behavior (default: -Cdebug-assertions)"),
     ui_testing: bool = (false, parse_bool, [UNTRACKED],
         "emit compiler diagnostics in a form suitable for UI testing (default: no)"),
     uninit_const_chunk_threshold: usize = (16, parse_number, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -735,6 +735,10 @@ impl Session {
         self.opts.cg.overflow_checks.unwrap_or(self.opts.debug_assertions)
     }
 
+    pub fn ub_checks(&self) -> bool {
+        self.opts.unstable_opts.ub_checks.unwrap_or(self.opts.debug_assertions)
+    }
+
     pub fn relocation_model(&self) -> RelocModel {
         self.opts.cg.relocation_model.unwrap_or(self.target.relocation_model)
     }

src/tools/miri/src/lib.rs

@@ -144,7 +144,7 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zmir-emit-retag",
     "-Zmir-keep-place-mention",
     "-Zmir-opt-level=0",
-    "-Zmir-enable-passes=-CheckAlignment",
+    "-Zub-checks=no",
     // Deduplicating diagnostics means we miss events when tracking what happens during an
     // execution. Let's not do that.
     "-Zdeduplicate-diagnostics=no",

src/tools/miri/tests/fail/unaligned_pointers/alignment.rs

@@ -1,5 +1,4 @@
 //@normalize-stderr-test: "\| +\^+" -> "| ^"
-//@compile-flags: -Cdebug-assertions=no
 
 fn main() {
     // No retry needed, this fails reliably.

src/tools/miri/tests/fail/unaligned_pointers/atomic_unaligned.rs

@@ -1,4 +1,4 @@
-//@compile-flags: -Zmiri-symbolic-alignment-check -Cdebug-assertions=no
+//@compile-flags: -Zmiri-symbolic-alignment-check
 #![feature(core_intrinsics)]
 
 fn main() {

src/tools/miri/tests/fail/unaligned_pointers/drop_in_place.rs

@@ -1,5 +1,3 @@
-//@compile-flags: -Cdebug-assertions=no
-
 #[repr(transparent)]
 struct HasDrop(u8);
 

src/tools/miri/tests/fail/unaligned_pointers/dyn_alignment.rs

@@ -1,5 +1,5 @@
 // should find the bug even without, but gets masked by optimizations
-//@compile-flags: -Zmiri-disable-stacked-borrows -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-stacked-borrows
 //@normalize-stderr-test: "but found [0-9]+" -> "but found $$ALIGN"
 
 #[repr(align(256))]

src/tools/miri/tests/fail/unaligned_pointers/intptrcast_alignment_check.rs

@@ -1,4 +1,4 @@
-//@compile-flags: -Zmiri-symbolic-alignment-check -Zmiri-permissive-provenance -Cdebug-assertions=no
+//@compile-flags: -Zmiri-symbolic-alignment-check -Zmiri-permissive-provenance
 // With the symbolic alignment check, even with intptrcast and without
 // validation, we want to be *sure* to catch bugs that arise from pointers being
 // insufficiently aligned. The only way to achieve that is not to let programs

src/tools/miri/tests/fail/unaligned_pointers/reference_to_packed.rs

@@ -1,5 +1,5 @@
 // This should fail even without SB
-//@compile-flags: -Zmiri-disable-stacked-borrows -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-stacked-borrows
 
 #![allow(dead_code, unused_variables)]
 

src/tools/miri/tests/fail/unaligned_pointers/unaligned_ptr1.rs

@@ -1,5 +1,5 @@
 // This should fail even without validation or Stacked Borrows.
-//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
 
 fn main() {
     // Try many times as this might work by chance.

src/tools/miri/tests/fail/unaligned_pointers/unaligned_ptr2.rs

@@ -1,5 +1,5 @@
 // This should fail even without validation or Stacked Borrows.
-//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
 
 fn main() {
     // No retry needed, this fails reliably.

src/tools/miri/tests/fail/unaligned_pointers/unaligned_ptr3.rs

@@ -1,5 +1,5 @@
 // This should fail even without validation or Stacked Borrows.
-//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
 
 fn main() {
     // Try many times as this might work by chance.

src/tools/miri/tests/fail/unaligned_pointers/unaligned_ptr4.rs

@@ -1,5 +1,5 @@
 // This should fail even without validation or Stacked Borrows.
-//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
 
 fn main() {
     // Make sure we notice when a u16 is loaded at offset 1 into a u8 allocation.

src/tools/miri/tests/fail/unaligned_pointers/unaligned_ptr_zst.rs

@@ -1,5 +1,5 @@
 // This should fail even without validation
-//@compile-flags: -Zmiri-disable-validation -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-validation
 
 fn main() {
     // Try many times as this might work by chance.

src/tools/miri/tests/fail/unaligned_pointers/unaligned_ref_addr_of.rs

@@ -1,5 +1,5 @@
 // This should fail even without Stacked Borrows.
-//@compile-flags: -Zmiri-disable-stacked-borrows -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-stacked-borrows
 
 #![allow(invalid_reference_casting)] // for u16 -> u32
 

src/tools/miri/tests/pass/disable-alignment-check.rs

@@ -1,6 +1,6 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
-//@compile-flags: -Zmiri-disable-alignment-check -Cdebug-assertions=no
+//@compile-flags: -Zmiri-disable-alignment-check
 
 fn main() {
     let mut x = [0u8; 20];

tests/codegen/ub-checks.rs

@@ -0,0 +1,23 @@
+// With -Zub-checks=yes (enabled by default by -Cdebug-assertions=yes) we will produce a runtime
+// check that the index to slice::get_unchecked is in-bounds of the slice. That is tested for by
+// tests/ui/precondition-checks/out-of-bounds-get-unchecked.rs
+//
+// This test ensures that such a runtime check is *not* emitted when debug-assertions are enabled,
+// but ub-checks are explicitly disabled.
+
+//@ compile-flags: -O -Cdebug-assertions=yes -Zub-checks=no
+
+#![crate_type = "lib"]
+
+use std::ops::Range;
+
+// CHECK-LABEL: @slice_get_unchecked(
+#[no_mangle]
+pub unsafe fn slice_get_unchecked(x: &[i32], i: usize) -> &i32 {
+    // CHECK-NEXT: start:
+    // CHECK-NEXT: icmp ult
+    // CHECK-NEXT: tail call void @llvm.assume
+    // CHECK-NEXT: getelementptr inbounds
+    // CHECK-NEXT: ret ptr
+    x.get_unchecked(i)
+}

tests/ui/precondition-checks/misaligned-slice.rs

@@ -1,5 +1,5 @@
 //@ run-fail
-//@ compile-flags: -Copt-level=3 -Cdebug-assertions=yes
+//@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: slice::from_raw_parts
 //@ ignore-debug
 

tests/ui/precondition-checks/null-slice.rs

@@ -1,5 +1,5 @@
 //@ run-fail
-//@ compile-flags: -Copt-level=3 -Cdebug-assertions=yes
+//@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: slice::from_raw_parts
 //@ ignore-debug
 

tests/ui/precondition-checks/out-of-bounds-get-unchecked.rs

@@ -1,5 +1,5 @@
 //@ run-fail
-//@ compile-flags: -Copt-level=3 -Cdebug-assertions=yes
+//@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: slice::get_unchecked requires
 //@ ignore-debug