Put checks that detect UB under their own flag below debug_assertions

42 files changed, 206 insertions, 64 deletions

diff --git a/compiler/rustc_codegen_cranelift/src/base.rs b/compiler/rustc_codegen_cranelift/src/base.rs
index 0aa2bae8f78..1307a62a60d 100644
--- a/compiler/rustc_codegen_cranelift/src/base.rs
+++ b/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),
diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
index 4d746c89f1f..d62f560f11f 100644
--- a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
+++ b/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)
                     }
                 };
diff --git a/compiler/rustc_const_eval/src/interpret/step.rs b/compiler/rustc_const_eval/src/interpret/step.rs
index 9114ffff6fd..db6c2833b9d 100644
--- a/compiler/rustc_const_eval/src/interpret/step.rs
+++ b/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)?;
             }
diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index 6a8a1722bcb..db94e0b08bc 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/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,
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 36db377f7e0..6fe51c62936 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/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.
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index b9025917d13..d2fb65b5d4f 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/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));
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index ad166620bcc..7ecac0c0e78 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/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))
diff --git a/compiler/rustc_mir_transform/src/check_alignment.rs b/compiler/rustc_mir_transform/src/check_alignment.rs
index b71c5894ff7..0af88729887 100644
--- a/compiler/rustc_mir_transform/src/check_alignment.rs
+++ b/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>) {
diff --git a/compiler/rustc_mir_transform/src/instsimplify.rs b/compiler/rustc_mir_transform/src/instsimplify.rs
index 1b38eeccfad..ff786d44d6a 100644
--- a/compiler/rustc_mir_transform/src/instsimplify.rs
+++ b/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)));
         }
diff --git a/compiler/rustc_session/src/config/cfg.rs b/compiler/rustc_session/src/config/cfg.rs
index 337019f720b..34dcd0cf598 100644
--- a/compiler/rustc_session/src/config/cfg.rs
+++ b/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);
     }
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index a76eb6b06aa..5e7c2465097 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1992,6 +1992,9 @@ 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],
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 55fff4421ae..22ca8a3cf3e 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/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)
     }
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index ea0f7adf6f9..7a5647e979a 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -514,6 +514,7 @@ symbols! {
         cfg_target_has_atomic_equal_alignment,
         cfg_target_thread_local,
         cfg_target_vendor,
+        cfg_ub_checks,
         cfg_version,
         cfi,
         cfi_encoding,
diff --git a/compiler/stable_mir/src/mir/body.rs b/compiler/stable_mir/src/mir/body.rs
index 8f77a19fc0e..593b1868f26 100644
--- a/compiler/stable_mir/src/mir/body.rs
+++ b/compiler/stable_mir/src/mir/body.rs
@@ -993,7 +993,7 @@ pub enum NullOp {
     AlignOf,
     /// Returns the offset of a field.
     OffsetOf(Vec<(VariantIdx, FieldIdx)>),
-    /// cfg!(debug_assertions), but at codegen time
+    /// cfg!(ub_checks), but at codegen time
     UbChecks,
 }
 
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index 0b22f3dc374..9406efd7ab2 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -2704,17 +2704,17 @@ pub const unsafe fn typed_swap<T>(x: *mut T, y: *mut T) {
 }
 
 /// Returns whether we should perform some UB-checking at runtime. This eventually evaluates to
-/// `cfg!(debug_assertions)`, but behaves different from `cfg!` when mixing crates built with different
-/// flags: if the crate has debug assertions enabled or carries the `#[rustc_preserve_ub_checks]`
+/// `cfg!(ub_checks)`, but behaves different from `cfg!` when mixing crates built with different
+/// flags: if the crate has UB checks enabled or carries the `#[rustc_preserve_ub_checks]`
 /// attribute, evaluation is delayed until monomorphization (or until the call gets inlined into
 /// a crate that does not delay evaluation further); otherwise it can happen any time.
 ///
-/// The common case here is a user program built with debug_assertions linked against the distributed
-/// sysroot which is built without debug_assertions but with `#[rustc_preserve_ub_checks]`.
+/// The common case here is a user program built with ub_checks linked against the distributed
+/// sysroot which is built without ub_checks but with `#[rustc_preserve_ub_checks]`.
 /// For code that gets monomorphized in the user crate (i.e., generic functions and functions with
-/// `#[inline]`), gating assertions on `ub_checks()` rather than `cfg!(debug_assertions)` means that
-/// assertions are enabled whenever the *user crate* has debug assertions enabled. However if the
-/// user has debug assertions disabled, the checks will still get optimized out. This intrinsic is
+/// `#[inline]`), gating assertions on `ub_checks()` rather than `cfg!(ub_checks)` means that
+/// assertions are enabled whenever the *user crate* has UB checks enabled. However if the
+/// user has UB checks disabled, the checks will still get optimized out. This intrinsic is
 /// primarily used by [`ub_checks::assert_unsafe_precondition`].
 #[rustc_const_unstable(feature = "const_ub_checks", issue = "none")]
 #[unstable(feature = "core_intrinsics", issue = "none")]
diff --git a/src/doc/unstable-book/src/compiler-flags/check-cfg.md b/src/doc/unstable-book/src/compiler-flags/check-cfg.md
index 13027eeaf4f..90a006b0a1e 100644
--- a/src/doc/unstable-book/src/compiler-flags/check-cfg.md
+++ b/src/doc/unstable-book/src/compiler-flags/check-cfg.md
@@ -77,7 +77,7 @@ Those well known names and values follows the same stability as what they refer
 Well known names and values checking is always enabled as long as at least one
 `--check-cfg` argument is present.
 
-As of `2024-02-15T`, the list of known names is as follows:
+As of `2024-04-06T`, the list of known names is as follows:
 
 <!--- See CheckCfg::fill_well_known in compiler/rustc_session/src/config.rs -->
 
@@ -107,6 +107,7 @@ As of `2024-02-15T`, the list of known names is as follows:
  - `target_thread_local`
  - `target_vendor`
  - `test`
+ - `ub_checks`
  - `unix`
  - `windows`
 
diff --git a/src/doc/unstable-book/src/compiler-flags/ub-checks.md b/src/doc/unstable-book/src/compiler-flags/ub-checks.md
new file mode 100644
index 00000000000..528c868d7ad
--- /dev/null
+++ b/src/doc/unstable-book/src/compiler-flags/ub-checks.md
@@ -0,0 +1,17 @@
+# `ub-checks`
+
+The tracking issue for this feature is: [#123499](https://github.com/rust-lang/rust/issues/123499).
+
+--------------------
+
+The `-Zub-checks` compiler flag enables additional runtime checks that detect some causes of Undefined Behavior at runtime.
+By default, `-Zub-checks` flag inherits the value of `-Cdebug-assertions`.
+
+All checks are generated on a best-effort basis; even if we have a check implemented for some cause of Undefined Behavior, it may be possible for the check to not fire.
+If a dependency is compiled with `-Zub-checks=no` but the final binary or library is compiled with `-Zub-checks=yes`, UB checks reached by the dependency are likely to be optimized out.
+
+When `-Zub-checks` detects UB, a non-unwinding panic is produced.
+That means that we will not unwind the stack and will not call any `Drop` impls, but we will execute the configured panic hook.
+We expect that unsafe code has been written which relies on code not unwinding which may have UB checks inserted.
+Ergo, an unwinding panic could easily turn works-as-intended UB into a much bigger problem.
+Calling the panic hook theoretically has the same implications, but we expect that the standard library panic hook will be stateless enough to be always called, and that if a user has configured a panic hook that the hook may be very helpful to debugging the detected UB.
diff --git a/tests/codegen/ub-checks.rs b/tests/codegen/ub-checks.rs
new file mode 100644
index 00000000000..de48d74e652
--- /dev/null
+++ b/tests/codegen/ub-checks.rs
@@ -0,0 +1,28 @@
+// 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.
+
+//@ revisions: DEBUG NOCHECKS
+//@ [DEBUG] compile-flags:
+//@ [NOCHECKS] compile-flags: -Zub-checks=no
+//@ compile-flags: -O -Cdebug-assertions=yes
+
+#![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: icmp ult
+    // NOCHECKS: tail call void @llvm.assume
+    //    DEBUG: br i1
+    //    DEBUG: call core::panicking::panic_nounwind
+    //    DEBUG: unreachable
+    //    CHECK: getelementptr inbounds
+    //    CHECK: ret ptr
+    x.get_unchecked(i)
+}
diff --git a/tests/ui/check-cfg/allow-same-level.stderr b/tests/ui/check-cfg/allow-same-level.stderr
index 011ef6e101d..493132d462a 100644
--- a/tests/ui/check-cfg/allow-same-level.stderr
+++ b/tests/ui/check-cfg/allow-same-level.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `FALSE`
 LL | #[cfg(FALSE)]
    |       ^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(FALSE)`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/cargo-feature.none.stderr b/tests/ui/check-cfg/cargo-feature.none.stderr
index d6c377e21ad..09a1c950267 100644
--- a/tests/ui/check-cfg/cargo-feature.none.stderr
+++ b/tests/ui/check-cfg/cargo-feature.none.stderr
@@ -25,7 +25,7 @@ warning: unexpected `cfg` condition name: `tokio_unstable`
 LL | #[cfg(tokio_unstable)]
    |       ^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: consider using a Cargo feature instead or adding `println!("cargo:rustc-check-cfg=cfg(tokio_unstable)");` to the top of a `build.rs`
    = note: see <https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg> for more information about checking conditional configuration
 
diff --git a/tests/ui/check-cfg/cargo-feature.some.stderr b/tests/ui/check-cfg/cargo-feature.some.stderr
index 11dfd4fa4b6..4db9c66fc86 100644
--- a/tests/ui/check-cfg/cargo-feature.some.stderr
+++ b/tests/ui/check-cfg/cargo-feature.some.stderr
@@ -25,7 +25,7 @@ warning: unexpected `cfg` condition name: `tokio_unstable`
 LL | #[cfg(tokio_unstable)]
    |       ^^^^^^^^^^^^^^
    |
-   = help: expected names are: `CONFIG_NVME`, `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `CONFIG_NVME`, `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: consider using a Cargo feature instead or adding `println!("cargo:rustc-check-cfg=cfg(tokio_unstable)");` to the top of a `build.rs`
    = note: see <https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#check-cfg> for more information about checking conditional configuration
 
diff --git a/tests/ui/check-cfg/cfg-value-for-cfg-name-duplicate.stderr b/tests/ui/check-cfg/cfg-value-for-cfg-name-duplicate.stderr
index 7d374fc81d3..9c190117e74 100644
--- a/tests/ui/check-cfg/cfg-value-for-cfg-name-duplicate.stderr
+++ b/tests/ui/check-cfg/cfg-value-for-cfg-name-duplicate.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `value`
 LL | #[cfg(value)]
    |       ^^^^^
    |
-   = help: expected names are: `bar`, `bee`, `clippy`, `cow`, `debug_assertions`, `doc`, `doctest`, `foo`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `bar`, `bee`, `clippy`, `cow`, `debug_assertions`, `doc`, `doctest`, `foo`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(value)`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/cfg-value-for-cfg-name-multiple.stderr b/tests/ui/check-cfg/cfg-value-for-cfg-name-multiple.stderr
index 13e6891c353..7113790b83a 100644
--- a/tests/ui/check-cfg/cfg-value-for-cfg-name-multiple.stderr
+++ b/tests/ui/check-cfg/cfg-value-for-cfg-name-multiple.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `my_value`
 LL | #[cfg(my_value)]
    |       ^^^^^^^^
    |
-   = help: expected names are: `bar`, `clippy`, `debug_assertions`, `doc`, `doctest`, `foo`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `bar`, `clippy`, `debug_assertions`, `doc`, `doctest`, `foo`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(my_value)`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/cfg-value-for-cfg-name.stderr b/tests/ui/check-cfg/cfg-value-for-cfg-name.stderr
index a2f9ccfec05..ba9f5f4acbd 100644
--- a/tests/ui/check-cfg/cfg-value-for-cfg-name.stderr
+++ b/tests/ui/check-cfg/cfg-value-for-cfg-name.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `linux`
 LL | #[cfg(linux)]
    |       ^^^^^ help: found config with similar value: `target_os = "linux"`
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(linux)`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/compact-names.stderr b/tests/ui/check-cfg/compact-names.stderr
index 37637e3b153..446b8f408e3 100644
--- a/tests/ui/check-cfg/compact-names.stderr
+++ b/tests/ui/check-cfg/compact-names.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `target_architecture`
 LL | #[cfg(target(os = "linux", architecture = "arm"))]
    |                            ^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(target_architecture, values("arm"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/exhaustive-names-values.empty_cfg.stderr b/tests/ui/check-cfg/exhaustive-names-values.empty_cfg.stderr
index 4f770e91c58..d2870263342 100644
--- a/tests/ui/check-cfg/exhaustive-names-values.empty_cfg.stderr
+++ b/tests/ui/check-cfg/exhaustive-names-values.empty_cfg.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `unknown_key`
 LL | #[cfg(unknown_key = "value")]
    |       ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/exhaustive-names-values.feature.stderr b/tests/ui/check-cfg/exhaustive-names-values.feature.stderr
index c7eaf435f75..b24b10bb615 100644
--- a/tests/ui/check-cfg/exhaustive-names-values.feature.stderr
+++ b/tests/ui/check-cfg/exhaustive-names-values.feature.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `unknown_key`
 LL | #[cfg(unknown_key = "value")]
    |       ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/exhaustive-names-values.full.stderr b/tests/ui/check-cfg/exhaustive-names-values.full.stderr
index c7eaf435f75..b24b10bb615 100644
--- a/tests/ui/check-cfg/exhaustive-names-values.full.stderr
+++ b/tests/ui/check-cfg/exhaustive-names-values.full.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `unknown_key`
 LL | #[cfg(unknown_key = "value")]
    |       ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/exhaustive-names.stderr b/tests/ui/check-cfg/exhaustive-names.stderr
index 3ed5c77e2a4..6ca7ed93625 100644
--- a/tests/ui/check-cfg/exhaustive-names.stderr
+++ b/tests/ui/check-cfg/exhaustive-names.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `unknown_key`
 LL | #[cfg(unknown_key = "value")]
    |       ^^^^^^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(unknown_key, values("value"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/mix.stderr b/tests/ui/check-cfg/mix.stderr
index 007f9de0331..87fabf8245f 100644
--- a/tests/ui/check-cfg/mix.stderr
+++ b/tests/ui/check-cfg/mix.stderr
@@ -44,7 +44,7 @@ warning: unexpected `cfg` condition name: `uu`
 LL | #[cfg_attr(uu, test)]
    |            ^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(uu)`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
diff --git a/tests/ui/check-cfg/stmt-no-ice.stderr b/tests/ui/check-cfg/stmt-no-ice.stderr
index f2660e4775b..1afdbe84d34 100644
--- a/tests/ui/check-cfg/stmt-no-ice.stderr
+++ b/tests/ui/check-cfg/stmt-no-ice.stderr
@@ -4,7 +4,7 @@ warning: unexpected `cfg` condition name: `crossbeam_loom`
 LL |     #[cfg(crossbeam_loom)]
    |           ^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(crossbeam_loom)`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
diff --git a/tests/ui/check-cfg/well-known-names.stderr b/tests/ui/check-cfg/well-known-names.stderr
index a0386403e25..467f9675f7f 100644
--- a/tests/ui/check-cfg/well-known-names.stderr
+++ b/tests/ui/check-cfg/well-known-names.stderr
@@ -18,7 +18,7 @@ warning: unexpected `cfg` condition name: `features`
 LL | #[cfg(features = "foo")]
    |       ^^^^^^^^^^^^^^^^
    |
-   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `unix`, `windows`
+   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `doctest`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
    = help: to expect this configuration use `--check-cfg=cfg(features, values("foo"))`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
diff --git a/tests/ui/check-cfg/well-known-values.rs b/tests/ui/check-cfg/well-known-values.rs
index fa062a3fe2e..2758a793538 100644
--- a/tests/ui/check-cfg/well-known-values.rs
+++ b/tests/ui/check-cfg/well-known-values.rs
@@ -14,6 +14,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(cfg_target_has_atomic_equal_alignment)]
 #![feature(cfg_target_thread_local)]
+#![feature(cfg_ub_checks)]
 
 // This part makes sure that none of the well known names are
 // unexpected.
@@ -71,6 +72,8 @@
     //~^ WARN unexpected `cfg` condition value
     test = "_UNEXPECTED_VALUE",
     //~^ WARN unexpected `cfg` condition value
+    ub_checks = "_UNEXPECTED_VALUE",
+    //~^ WARN unexpected `cfg` condition value
     unix = "_UNEXPECTED_VALUE",
     //~^ WARN unexpected `cfg` condition value
     windows = "_UNEXPECTED_VALUE",
diff --git a/tests/ui/check-cfg/well-known-values.stderr b/tests/ui/check-cfg/well-known-values.stderr
index 31553371101..729794150f6 100644
--- a/tests/ui/check-cfg/well-known-values.stderr
+++ b/tests/ui/check-cfg/well-known-values.stderr
@@ -1,5 +1,5 @@
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:26:5
+  --> $DIR/well-known-values.rs:27:5
    |
 LL |     clippy = "_UNEXPECTED_VALUE",
    |     ^^^^^^----------------------
@@ -11,7 +11,7 @@ LL |     clippy = "_UNEXPECTED_VALUE",
    = note: `#[warn(unexpected_cfgs)]` on by default
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:28:5
+  --> $DIR/well-known-values.rs:29:5
    |
 LL |     debug_assertions = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^----------------------
@@ -22,7 +22,7 @@ LL |     debug_assertions = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:30:5
+  --> $DIR/well-known-values.rs:31:5
    |
 LL |     doc = "_UNEXPECTED_VALUE",
    |     ^^^----------------------
@@ -33,7 +33,7 @@ LL |     doc = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:32:5
+  --> $DIR/well-known-values.rs:33:5
    |
 LL |     doctest = "_UNEXPECTED_VALUE",
    |     ^^^^^^^----------------------
@@ -44,7 +44,7 @@ LL |     doctest = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:34:5
+  --> $DIR/well-known-values.rs:35:5
    |
 LL |     miri = "_UNEXPECTED_VALUE",
    |     ^^^^----------------------
@@ -55,7 +55,7 @@ LL |     miri = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:36:5
+  --> $DIR/well-known-values.rs:37:5
    |
 LL |     overflow_checks = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^----------------------
@@ -66,7 +66,7 @@ LL |     overflow_checks = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:38:5
+  --> $DIR/well-known-values.rs:39:5
    |
 LL |     panic = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -75,7 +75,7 @@ LL |     panic = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:40:5
+  --> $DIR/well-known-values.rs:41:5
    |
 LL |     proc_macro = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^----------------------
@@ -86,7 +86,7 @@ LL |     proc_macro = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:42:5
+  --> $DIR/well-known-values.rs:43:5
    |
 LL |     relocation_model = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -95,7 +95,7 @@ LL |     relocation_model = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:44:5
+  --> $DIR/well-known-values.rs:45:5
    |
 LL |     sanitize = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -104,7 +104,7 @@ LL |     sanitize = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:46:5
+  --> $DIR/well-known-values.rs:47:5
    |
 LL |     target_abi = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -113,7 +113,7 @@ LL |     target_abi = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:48:5
+  --> $DIR/well-known-values.rs:49:5
    |
 LL |     target_arch = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -122,7 +122,7 @@ LL |     target_arch = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:50:5
+  --> $DIR/well-known-values.rs:51:5
    |
 LL |     target_endian = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -131,7 +131,7 @@ LL |     target_endian = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:52:5
+  --> $DIR/well-known-values.rs:53:5
    |
 LL |     target_env = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -140,7 +140,7 @@ LL |     target_env = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:54:5
+  --> $DIR/well-known-values.rs:55:5
    |
 LL |     target_family = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -149,7 +149,7 @@ LL |     target_family = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:56:5
+  --> $DIR/well-known-values.rs:57:5
    |
 LL |     target_feature = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -158,7 +158,7 @@ LL |     target_feature = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:58:5
+  --> $DIR/well-known-values.rs:59:5
    |
 LL |     target_has_atomic = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -167,7 +167,7 @@ LL |     target_has_atomic = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:60:5
+  --> $DIR/well-known-values.rs:61:5
    |
 LL |     target_has_atomic_equal_alignment = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -176,7 +176,7 @@ LL |     target_has_atomic_equal_alignment = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:62:5
+  --> $DIR/well-known-values.rs:63:5
    |
 LL |     target_has_atomic_load_store = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -185,7 +185,7 @@ LL |     target_has_atomic_load_store = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:64:5
+  --> $DIR/well-known-values.rs:65:5
    |
 LL |     target_os = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -194,7 +194,7 @@ LL |     target_os = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:66:5
+  --> $DIR/well-known-values.rs:67:5
    |
 LL |     target_pointer_width = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -203,7 +203,7 @@ LL |     target_pointer_width = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:68:5
+  --> $DIR/well-known-values.rs:69:5
    |
 LL |     target_thread_local = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^----------------------
@@ -214,7 +214,7 @@ LL |     target_thread_local = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:70:5
+  --> $DIR/well-known-values.rs:71:5
    |
 LL |     target_vendor = "_UNEXPECTED_VALUE",
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -223,7 +223,7 @@ LL |     target_vendor = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:72:5
+  --> $DIR/well-known-values.rs:73:5
    |
 LL |     test = "_UNEXPECTED_VALUE",
    |     ^^^^----------------------
@@ -234,7 +234,18 @@ LL |     test = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:74:5
+  --> $DIR/well-known-values.rs:75:5
+   |
+LL |     ub_checks = "_UNEXPECTED_VALUE",
+   |     ^^^^^^^^^----------------------
+   |              |
+   |              help: remove the value
+   |
+   = note: no expected value for `ub_checks`
+   = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
+
+warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
+  --> $DIR/well-known-values.rs:77:5
    |
 LL |     unix = "_UNEXPECTED_VALUE",
    |     ^^^^----------------------
@@ -245,7 +256,7 @@ LL |     unix = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `_UNEXPECTED_VALUE`
-  --> $DIR/well-known-values.rs:76:5
+  --> $DIR/well-known-values.rs:79:5
    |
 LL |     windows = "_UNEXPECTED_VALUE",
    |     ^^^^^^^----------------------
@@ -256,7 +267,7 @@ LL |     windows = "_UNEXPECTED_VALUE",
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
 warning: unexpected `cfg` condition value: `linuz`
-  --> $DIR/well-known-values.rs:82:7
+  --> $DIR/well-known-values.rs:85:7
    |
 LL | #[cfg(target_os = "linuz")] // testing that we suggest `linux`
    |       ^^^^^^^^^^^^-------
@@ -266,5 +277,5 @@ LL | #[cfg(target_os = "linuz")] // testing that we suggest `linux`
    = note: expected values for `target_os` are: `aix`, `android`, `cuda`, `dragonfly`, `emscripten`, `espidf`, `freebsd`, `fuchsia`, `haiku`, `hermit`, `horizon`, `hurd`, `illumos`, `ios`, `l4re`, `linux`, `macos`, `netbsd`, `none`, `nto`, `openbsd`, `psp`, `redox`, `solaris`, `solid_asp3`, `teeos`, `tvos`, `uefi`, `unknown`, `visionos`, `vita`, `vxworks`, `wasi`, `watchos`, `windows`, `xous`, `zkvm`
    = note: see <https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/check-cfg.html> for more information about checking conditional configuration
 
-warning: 27 warnings emitted
+warning: 28 warnings emitted
 
diff --git a/tests/ui/feature-gates/feature-gate-cfg_ub_checks.rs b/tests/ui/feature-gates/feature-gate-cfg_ub_checks.rs
new file mode 100644
index 00000000000..c24c0066e18
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-cfg_ub_checks.rs
@@ -0,0 +1,5 @@
+#![crate_type = "lib"]
+
+pub fn ub_checks_are_enabled() -> bool {
+    cfg!(ub_checks) //~ ERROR `cfg(ub_checks)` is experimental
+}
diff --git a/tests/ui/feature-gates/feature-gate-cfg_ub_checks.stderr b/tests/ui/feature-gates/feature-gate-cfg_ub_checks.stderr
new file mode 100644
index 00000000000..aa12ee1db6b
--- /dev/null
+++ b/tests/ui/feature-gates/feature-gate-cfg_ub_checks.stderr
@@ -0,0 +1,13 @@
+error[E0658]: `cfg(ub_checks)` is experimental and subject to change
+  --> $DIR/feature-gate-cfg_ub_checks.rs:4:10
+   |
+LL |     cfg!(ub_checks)
+   |          ^^^^^^^^^
+   |
+   = note: see issue #123499 <https://github.com/rust-lang/rust/issues/123499> for more information
+   = help: add `#![feature(cfg_ub_checks)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/precondition-checks/cfg-ub-checks-default.rs b/tests/ui/precondition-checks/cfg-ub-checks-default.rs
new file mode 100644
index 00000000000..57a19523147
--- /dev/null
+++ b/tests/ui/precondition-checks/cfg-ub-checks-default.rs
@@ -0,0 +1,10 @@
+//@ run-pass
+//@ revisions YES NO
+//@ [YES] compile-flags: -Cdebug-assertions=yes
+//@ [NO] compile-flags: -Cdebug-assertions=no
+
+#![feature(cfg_ub_checks)]
+
+fn main() {
+    assert_eq!(cfg!(ub_checks), cfg!(debug_assertions));
+}
diff --git a/tests/ui/precondition-checks/cfg-ub-checks-no.rs b/tests/ui/precondition-checks/cfg-ub-checks-no.rs
new file mode 100644
index 00000000000..73705a7e632
--- /dev/null
+++ b/tests/ui/precondition-checks/cfg-ub-checks-no.rs
@@ -0,0 +1,19 @@
+//@ run-pass
+//@ compile-flags: -Zub-checks=no
+
+#![feature(cfg_ub_checks)]
+
+fn main() {
+    assert!(!cfg!(ub_checks));
+    assert!(compiles_differently());
+}
+
+#[cfg(ub_checks)]
+fn compiles_differently() -> bool {
+    false
+}
+
+#[cfg(not(ub_checks))]
+fn compiles_differently() -> bool {
+    true
+}
diff --git a/tests/ui/precondition-checks/cfg-ub-checks-yes.rs b/tests/ui/precondition-checks/cfg-ub-checks-yes.rs
new file mode 100644
index 00000000000..410ab1fe4ec
--- /dev/null
+++ b/tests/ui/precondition-checks/cfg-ub-checks-yes.rs
@@ -0,0 +1,19 @@
+//@ run-pass
+//@ compile-flags: -Zub-checks=yes
+
+#![feature(cfg_ub_checks)]
+
+fn main() {
+    assert!(cfg!(ub_checks));
+    assert!(compiles_differently());
+}
+
+#[cfg(ub_checks)]
+fn compiles_differently() -> bool {
+    true
+}
+
+#[cfg(not(ub_checks))]
+fn compiles_differently() -> bool {
+    false
+}
diff --git a/tests/ui/precondition-checks/misaligned-slice.rs b/tests/ui/precondition-checks/misaligned-slice.rs
index 52c149b594e..2963a0b5e63 100644
--- a/tests/ui/precondition-checks/misaligned-slice.rs
+++ b/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
 
diff --git a/tests/ui/precondition-checks/null-slice.rs b/tests/ui/precondition-checks/null-slice.rs
index 61c7d467649..280960358b7 100644
--- a/tests/ui/precondition-checks/null-slice.rs
+++ b/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
 
diff --git a/tests/ui/precondition-checks/out-of-bounds-get-unchecked.rs b/tests/ui/precondition-checks/out-of-bounds-get-unchecked.rs
index ba02c3da7b2..011e92183fa 100644
--- a/tests/ui/precondition-checks/out-of-bounds-get-unchecked.rs
+++ b/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