diff --git a/Cargo.lock b/Cargo.lock
index 684dfbca6f1..4441a50fef9 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2022,9 +2022,9 @@ checksum = "9fa0e2a1fcbe2f6be6c42e342259976206b383122fc152e872795338b5a3f3a7"
 
 [[package]]
 name = "libc"
-version = "0.2.171"
+version = "0.2.172"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c19937216e9d3aa9956d9bb8dfc0b0c8beb6058fc4f7a4dc4d850edf86a237d6"
+checksum = "d750af042f7ef4f724306de029d18836c26c1765a54a6a3f094cbd23a7267ffa"
 
 [[package]]
 name = "libdbus-sys"
diff --git a/compiler/rustc_ast/src/expand/autodiff_attrs.rs b/compiler/rustc_ast/src/expand/autodiff_attrs.rs
index 13a7c5a1805..2f918faaf75 100644
--- a/compiler/rustc_ast/src/expand/autodiff_attrs.rs
+++ b/compiler/rustc_ast/src/expand/autodiff_attrs.rs
@@ -50,8 +50,16 @@ pub enum DiffActivity {
     /// with it.
     Dual,
     /// Forward Mode, Compute derivatives for this input/output and *overwrite* the shadow argument
+    /// with it. It expects the shadow argument to be `width` times larger than the original
+    /// input/output.
+    Dualv,
+    /// Forward Mode, Compute derivatives for this input/output and *overwrite* the shadow argument
     /// with it. Drop the code which updates the original input/output for maximum performance.
     DualOnly,
+    /// Forward Mode, Compute derivatives for this input/output and *overwrite* the shadow argument
+    /// with it. Drop the code which updates the original input/output for maximum performance.
+    /// It expects the shadow argument to be `width` times larger than the original input/output.
+    DualvOnly,
     /// Reverse Mode, Compute derivatives for this &T or *T input and *add* it to the shadow argument.
     Duplicated,
     /// Reverse Mode, Compute derivatives for this &T or *T input and *add* it to the shadow argument.
@@ -59,7 +67,15 @@ pub enum DiffActivity {
     DuplicatedOnly,
     /// All Integers must be Const, but these are used to mark the integer which represents the
     /// length of a slice/vec. This is used for safety checks on slices.
-    FakeActivitySize,
+    /// The integer (if given) specifies the size of the slice element in bytes.
+    FakeActivitySize(Option<u32>),
+}
+
+impl DiffActivity {
+    pub fn is_dual_or_const(&self) -> bool {
+        use DiffActivity::*;
+        matches!(self, |Dual| DualOnly | Dualv | DualvOnly | Const)
+    }
 }
 /// We generate one of these structs for each `#[autodiff(...)]` attribute.
 #[derive(Clone, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
@@ -131,11 +147,7 @@ pub fn valid_ret_activity(mode: DiffMode, activity: DiffActivity) -> bool {
     match mode {
         DiffMode::Error => false,
         DiffMode::Source => false,
-        DiffMode::Forward => {
-            activity == DiffActivity::Dual
-                || activity == DiffActivity::DualOnly
-                || activity == DiffActivity::Const
-        }
+        DiffMode::Forward => activity.is_dual_or_const(),
         DiffMode::Reverse => {
             activity == DiffActivity::Const
                 || activity == DiffActivity::Active
@@ -153,10 +165,8 @@ pub fn valid_ret_activity(mode: DiffMode, activity: DiffActivity) -> bool {
 pub fn valid_ty_for_activity(ty: &P<Ty>, activity: DiffActivity) -> bool {
     use DiffActivity::*;
     // It's always allowed to mark something as Const, since we won't compute derivatives wrt. it.
-    if matches!(activity, Const) {
-        return true;
-    }
-    if matches!(activity, Dual | DualOnly) {
+    // Dual variants also support all types.
+    if activity.is_dual_or_const() {
         return true;
     }
     // FIXME(ZuseZ4) We should make this more robust to also
@@ -172,9 +182,7 @@ pub fn valid_input_activity(mode: DiffMode, activity: DiffActivity) -> bool {
     return match mode {
         DiffMode::Error => false,
         DiffMode::Source => false,
-        DiffMode::Forward => {
-            matches!(activity, Dual | DualOnly | Const)
-        }
+        DiffMode::Forward => activity.is_dual_or_const(),
         DiffMode::Reverse => {
             matches!(activity, Active | ActiveOnly | Duplicated | DuplicatedOnly | Const)
         }
@@ -189,10 +197,12 @@ impl Display for DiffActivity {
             DiffActivity::Active => write!(f, "Active"),
             DiffActivity::ActiveOnly => write!(f, "ActiveOnly"),
             DiffActivity::Dual => write!(f, "Dual"),
+            DiffActivity::Dualv => write!(f, "Dualv"),
             DiffActivity::DualOnly => write!(f, "DualOnly"),
+            DiffActivity::DualvOnly => write!(f, "DualvOnly"),
             DiffActivity::Duplicated => write!(f, "Duplicated"),
             DiffActivity::DuplicatedOnly => write!(f, "DuplicatedOnly"),
-            DiffActivity::FakeActivitySize => write!(f, "FakeActivitySize"),
+            DiffActivity::FakeActivitySize(s) => write!(f, "FakeActivitySize({:?})", s),
         }
     }
 }
@@ -220,7 +230,9 @@ impl FromStr for DiffActivity {
             "ActiveOnly" => Ok(DiffActivity::ActiveOnly),
             "Const" => Ok(DiffActivity::Const),
             "Dual" => Ok(DiffActivity::Dual),
+            "Dualv" => Ok(DiffActivity::Dualv),
             "DualOnly" => Ok(DiffActivity::DualOnly),
+            "DualvOnly" => Ok(DiffActivity::DualvOnly),
             "Duplicated" => Ok(DiffActivity::Duplicated),
             "DuplicatedOnly" => Ok(DiffActivity::DuplicatedOnly),
             _ => Err(()),
diff --git a/compiler/rustc_builtin_macros/src/autodiff.rs b/compiler/rustc_builtin_macros/src/autodiff.rs
index 4161829480d..daebd516499 100644
--- a/compiler/rustc_builtin_macros/src/autodiff.rs
+++ b/compiler/rustc_builtin_macros/src/autodiff.rs
@@ -799,8 +799,19 @@ mod llvm_enzyme {
                         d_inputs.push(shadow_arg.clone());
                     }
                 }
-                DiffActivity::Dual | DiffActivity::DualOnly => {
-                    for i in 0..x.width {
+                DiffActivity::Dual
+                | DiffActivity::DualOnly
+                | DiffActivity::Dualv
+                | DiffActivity::DualvOnly => {
+                    // the *v variants get lowered to enzyme_dupv and enzyme_dupnoneedv, which cause
+                    // Enzyme to not expect N arguments, but one argument (which is instead larger).
+                    let iterations =
+                        if matches!(activity, DiffActivity::Dualv | DiffActivity::DualvOnly) {
+                            1
+                        } else {
+                            x.width
+                        };
+                    for i in 0..iterations {
                         let mut shadow_arg = arg.clone();
                         let old_name = if let PatKind::Ident(_, ident, _) = arg.pat.kind {
                             ident.name
@@ -823,7 +834,7 @@ mod llvm_enzyme {
                 DiffActivity::Const => {
                     // Nothing to do here.
                 }
-                DiffActivity::None | DiffActivity::FakeActivitySize => {
+                DiffActivity::None | DiffActivity::FakeActivitySize(_) => {
                     panic!("Should not happen");
                 }
             }
@@ -887,8 +898,8 @@ mod llvm_enzyme {
                 }
             };
 
-            if let DiffActivity::Dual = x.ret_activity {
-                let kind = if x.width == 1 {
+            if matches!(x.ret_activity, DiffActivity::Dual | DiffActivity::Dualv) {
+                let kind = if x.width == 1 || matches!(x.ret_activity, DiffActivity::Dualv) {
                     // Dual can only be used for f32/f64 ret.
                     // In that case we return now a tuple with two floats.
                     TyKind::Tup(thin_vec![ty.clone(), ty.clone()])
@@ -903,7 +914,7 @@ mod llvm_enzyme {
                 let ty = P(rustc_ast::Ty { kind, id: ty.id, span: ty.span, tokens: None });
                 d_decl.output = FnRetTy::Ty(ty);
             }
-            if let DiffActivity::DualOnly = x.ret_activity {
+            if matches!(x.ret_activity, DiffActivity::DualOnly | DiffActivity::DualvOnly) {
                 // No need to change the return type,
                 // we will just return the shadow in place of the primal return.
                 // However, if we have a width > 1, then we don't return -> T, but -> [T; width]
diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 35134e9f5a0..27f7f95f100 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -123,7 +123,7 @@ impl<'a, 'll, CX: Borrow<SCx<'ll>>> GenericBuilder<'a, 'll, CX> {
 /// Empty string, to be used where LLVM expects an instruction name, indicating
 /// that the instruction is to be left unnamed (i.e. numbered, in textual IR).
 // FIXME(eddyb) pass `&CStr` directly to FFI once it's a thin pointer.
-const UNNAMED: *const c_char = c"".as_ptr();
+pub(crate) const UNNAMED: *const c_char = c"".as_ptr();
 
 impl<'ll, CX: Borrow<SCx<'ll>>> BackendTypes for GenericBuilder<'_, 'll, CX> {
     type Value = <GenericCx<'ll, CX> as BackendTypes>::Value;
diff --git a/compiler/rustc_codegen_llvm/src/builder/autodiff.rs b/compiler/rustc_codegen_llvm/src/builder/autodiff.rs
index 5e7ef27143b..e7c071d05aa 100644
--- a/compiler/rustc_codegen_llvm/src/builder/autodiff.rs
+++ b/compiler/rustc_codegen_llvm/src/builder/autodiff.rs
@@ -10,7 +10,7 @@ use rustc_middle::bug;
 use tracing::{debug, trace};
 
 use crate::back::write::llvm_err;
-use crate::builder::SBuilder;
+use crate::builder::{SBuilder, UNNAMED};
 use crate::context::SimpleCx;
 use crate::declare::declare_simple_fn;
 use crate::errors::{AutoDiffWithoutEnable, LlvmError};
@@ -51,6 +51,7 @@ fn has_sret(fnc: &Value) -> bool {
 // using iterators and peek()?
 fn match_args_from_caller_to_enzyme<'ll>(
     cx: &SimpleCx<'ll>,
+    builder: &SBuilder<'ll, 'll>,
     width: u32,
     args: &mut Vec<&'ll llvm::Value>,
     inputs: &[DiffActivity],
@@ -78,7 +79,9 @@ fn match_args_from_caller_to_enzyme<'ll>(
     let enzyme_const = cx.create_metadata("enzyme_const".to_string()).unwrap();
     let enzyme_out = cx.create_metadata("enzyme_out".to_string()).unwrap();
     let enzyme_dup = cx.create_metadata("enzyme_dup".to_string()).unwrap();
+    let enzyme_dupv = cx.create_metadata("enzyme_dupv".to_string()).unwrap();
     let enzyme_dupnoneed = cx.create_metadata("enzyme_dupnoneed".to_string()).unwrap();
+    let enzyme_dupnoneedv = cx.create_metadata("enzyme_dupnoneedv".to_string()).unwrap();
 
     while activity_pos < inputs.len() {
         let diff_activity = inputs[activity_pos as usize];
@@ -90,13 +93,34 @@ fn match_args_from_caller_to_enzyme<'ll>(
             DiffActivity::Active => (enzyme_out, false),
             DiffActivity::ActiveOnly => (enzyme_out, false),
             DiffActivity::Dual => (enzyme_dup, true),
+            DiffActivity::Dualv => (enzyme_dupv, true),
             DiffActivity::DualOnly => (enzyme_dupnoneed, true),
+            DiffActivity::DualvOnly => (enzyme_dupnoneedv, true),
             DiffActivity::Duplicated => (enzyme_dup, true),
             DiffActivity::DuplicatedOnly => (enzyme_dupnoneed, true),
-            DiffActivity::FakeActivitySize => (enzyme_const, false),
+            DiffActivity::FakeActivitySize(_) => (enzyme_const, false),
         };
         let outer_arg = outer_args[outer_pos];
         args.push(cx.get_metadata_value(activity));
+        if matches!(diff_activity, DiffActivity::Dualv) {
+            let next_outer_arg = outer_args[outer_pos + 1];
+            let elem_bytes_size: u64 = match inputs[activity_pos + 1] {
+                DiffActivity::FakeActivitySize(Some(s)) => s.into(),
+                _ => bug!("incorrect Dualv handling recognized."),
+            };
+            // stride: sizeof(T) * n_elems.
+            // n_elems is the next integer.
+            // Now we multiply `4 * next_outer_arg` to get the stride.
+            let mul = unsafe {
+                llvm::LLVMBuildMul(
+                    builder.llbuilder,
+                    cx.get_const_i64(elem_bytes_size),
+                    next_outer_arg,
+                    UNNAMED,
+                )
+            };
+            args.push(mul);
+        }
         args.push(outer_arg);
         if duplicated {
             // We know that duplicated args by construction have a following argument,
@@ -114,7 +138,7 @@ fn match_args_from_caller_to_enzyme<'ll>(
                 } else {
                     let next_activity = inputs[activity_pos + 1];
                     // We analyze the MIR types and add this dummy activity if we visit a slice.
-                    next_activity == DiffActivity::FakeActivitySize
+                    matches!(next_activity, DiffActivity::FakeActivitySize(_))
                 }
             };
             if slice {
@@ -125,7 +149,10 @@ fn match_args_from_caller_to_enzyme<'ll>(
                 // int2 >= int1, which means the shadow vector is large enough to store the gradient.
                 assert_eq!(cx.type_kind(next_outer_ty), TypeKind::Integer);
 
-                for i in 0..(width as usize) {
+                let iterations =
+                    if matches!(diff_activity, DiffActivity::Dualv) { 1 } else { width as usize };
+
+                for i in 0..iterations {
                     let next_outer_arg2 = outer_args[outer_pos + 2 * (i + 1)];
                     let next_outer_ty2 = cx.val_ty(next_outer_arg2);
                     assert_eq!(cx.type_kind(next_outer_ty2), TypeKind::Pointer);
@@ -136,7 +163,7 @@ fn match_args_from_caller_to_enzyme<'ll>(
                 }
                 args.push(cx.get_metadata_value(enzyme_const));
                 args.push(next_outer_arg);
-                outer_pos += 2 + 2 * width as usize;
+                outer_pos += 2 + 2 * iterations;
                 activity_pos += 2;
             } else {
                 // A duplicated pointer will have the following two outer_fn arguments:
@@ -360,6 +387,7 @@ fn generate_enzyme_call<'ll>(
         let outer_args: Vec<&llvm::Value> = get_params(outer_fn);
         match_args_from_caller_to_enzyme(
             &cx,
+            &builder,
             attrs.width,
             &mut args,
             &attrs.input_activity,
diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index c1948e9f31f..b69bc0918be 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -268,7 +268,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
     /// Call this on things you got out of the MIR (so it is as generic as the current
     /// stack frame), to bring it into the proper environment for this interpreter.
-    pub(super) fn instantiate_from_current_frame_and_normalize_erasing_regions<
+    pub fn instantiate_from_current_frame_and_normalize_erasing_regions<
         T: TypeFoldable<TyCtxt<'tcx>>,
     >(
         &self,
@@ -279,9 +279,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
     /// Call this on things you got out of the MIR (so it is as generic as the provided
     /// stack frame), to bring it into the proper environment for this interpreter.
-    pub(super) fn instantiate_from_frame_and_normalize_erasing_regions<
-        T: TypeFoldable<TyCtxt<'tcx>>,
-    >(
+    pub fn instantiate_from_frame_and_normalize_erasing_regions<T: TypeFoldable<TyCtxt<'tcx>>>(
         &self,
         frame: &Frame<'tcx, M::Provenance, M::FrameExtra>,
         value: T,
diff --git a/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs b/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
index 0f5bdc8d768..94ee34c8b7b 100644
--- a/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
+++ b/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs
@@ -111,6 +111,11 @@ fn wasm_abi_safe<'tcx>(tcx: TyCtxt<'tcx>, arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool
         }
     }
 
+    // Zero-sized types are dropped in both ABIs, so they're safe
+    if arg.layout.is_zst() {
+        return true;
+    }
+
     false
 }
 
diff --git a/compiler/rustc_monomorphize/src/partitioning/autodiff.rs b/compiler/rustc_monomorphize/src/partitioning/autodiff.rs
index ebe0b258c1b..22d593b80b8 100644
--- a/compiler/rustc_monomorphize/src/partitioning/autodiff.rs
+++ b/compiler/rustc_monomorphize/src/partitioning/autodiff.rs
@@ -2,7 +2,7 @@ use rustc_ast::expand::autodiff_attrs::{AutoDiffItem, DiffActivity};
 use rustc_hir::def_id::LOCAL_CRATE;
 use rustc_middle::bug;
 use rustc_middle::mir::mono::MonoItem;
-use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
+use rustc_middle::ty::{self, Instance, PseudoCanonicalInput, Ty, TyCtxt, TypingEnv};
 use rustc_symbol_mangling::symbol_name_for_instance_in_crate;
 use tracing::{debug, trace};
 
@@ -22,23 +22,51 @@ fn adjust_activity_to_abi<'tcx>(tcx: TyCtxt<'tcx>, fn_ty: Ty<'tcx>, da: &mut Vec
     for (i, ty) in sig.inputs().iter().enumerate() {
         if let Some(inner_ty) = ty.builtin_deref(true) {
             if inner_ty.is_slice() {
+                // Now we need to figure out the size of each slice element in memory to allow
+                // safety checks and usability improvements in the backend.
+                let sty = match inner_ty.builtin_index() {
+                    Some(sty) => sty,
+                    None => {
+                        panic!("slice element type unknown");
+                    }
+                };
+                let pci = PseudoCanonicalInput {
+                    typing_env: TypingEnv::fully_monomorphized(),
+                    value: sty,
+                };
+
+                let layout = tcx.layout_of(pci);
+                let elem_size = match layout {
+                    Ok(layout) => layout.size,
+                    Err(_) => {
+                        bug!("autodiff failed to compute slice element size");
+                    }
+                };
+                let elem_size: u32 = elem_size.bytes() as u32;
+
                 // We know that the length will be passed as extra arg.
                 if !da.is_empty() {
                     // We are looking at a slice. The length of that slice will become an
                     // extra integer on llvm level. Integers are always const.
                     // However, if the slice get's duplicated, we want to know to later check the
                     // size. So we mark the new size argument as FakeActivitySize.
+                    // There is one FakeActivitySize per slice, so for convenience we store the
+                    // slice element size in bytes in it. We will use the size in the backend.
                     let activity = match da[i] {
                         DiffActivity::DualOnly
                         | DiffActivity::Dual
+                        | DiffActivity::Dualv
                         | DiffActivity::DuplicatedOnly
-                        | DiffActivity::Duplicated => DiffActivity::FakeActivitySize,
+                        | DiffActivity::Duplicated => {
+                            DiffActivity::FakeActivitySize(Some(elem_size))
+                        }
                         DiffActivity::Const => DiffActivity::Const,
                         _ => bug!("unexpected activity for ptr/ref"),
                     };
                     new_activities.push(activity);
                     new_positions.push(i + 1);
                 }
+
                 continue;
             }
         }
diff --git a/library/core/src/hint.rs b/library/core/src/hint.rs
index f6708cc4bc9..1ca23ab6eea 100644
--- a/library/core/src/hint.rs
+++ b/library/core/src/hint.rs
@@ -4,6 +4,7 @@
 //!
 //! Hints may be compile time or runtime.
 
+use crate::mem::MaybeUninit;
 use crate::{intrinsics, ub_checks};
 
 /// Informs the compiler that the site which is calling this function is not
@@ -735,9 +736,9 @@ pub const fn cold_path() {
     crate::intrinsics::cold_path()
 }
 
-/// Returns either `true_val` or `false_val` depending on the value of `b`,
-/// with a hint to the compiler that `b` is unlikely to be correctly
-/// predicted by a CPU’s branch predictor.
+/// Returns either `true_val` or `false_val` depending on the value of
+/// `condition`, with a hint to the compiler that `condition` is unlikely to be
+/// correctly predicted by a CPU’s branch predictor.
 ///
 /// This method is functionally equivalent to
 /// ```ignore (this is just for illustrative purposes)
@@ -753,10 +754,10 @@ pub const fn cold_path() {
 /// search.
 ///
 /// Note however that this lowering is not guaranteed (on any platform) and
-/// should not be relied upon when trying to write constant-time code. Also
-/// be aware that this lowering might *decrease* performance if `condition`
-/// is well-predictable. It is advisable to perform benchmarks to tell if
-/// this function is useful.
+/// should not be relied upon when trying to write cryptographic constant-time
+/// code. Also be aware that this lowering might *decrease* performance if
+/// `condition` is well-predictable. It is advisable to perform benchmarks to
+/// tell if this function is useful.
 ///
 /// # Examples
 ///
@@ -780,6 +781,17 @@ pub const fn cold_path() {
 /// ```
 #[inline(always)]
 #[unstable(feature = "select_unpredictable", issue = "133962")]
-pub fn select_unpredictable<T>(b: bool, true_val: T, false_val: T) -> T {
-    crate::intrinsics::select_unpredictable(b, true_val, false_val)
+pub fn select_unpredictable<T>(condition: bool, true_val: T, false_val: T) -> T {
+    // FIXME(https://github.com/rust-lang/unsafe-code-guidelines/issues/245):
+    // Change this to use ManuallyDrop instead.
+    let mut true_val = MaybeUninit::new(true_val);
+    let mut false_val = MaybeUninit::new(false_val);
+    // SAFETY: The value that is not selected is dropped, and the selected one
+    // is returned. This is necessary because the intrinsic doesn't drop the
+    // value that is  not selected.
+    unsafe {
+        crate::intrinsics::select_unpredictable(!condition, &mut true_val, &mut false_val)
+            .assume_init_drop();
+        crate::intrinsics::select_unpredictable(condition, true_val, false_val).assume_init()
+    }
 }
diff --git a/library/core/src/intrinsics/mod.rs b/library/core/src/intrinsics/mod.rs
index e5604d277f5..a01efb2adeb 100644
--- a/library/core/src/intrinsics/mod.rs
+++ b/library/core/src/intrinsics/mod.rs
@@ -1327,6 +1327,8 @@ pub const fn unlikely(b: bool) -> bool {
 /// any safety invariants.
 ///
 /// The public form of this instrinsic is [`core::hint::select_unpredictable`].
+/// However unlike the public form, the intrinsic will not drop the value that
+/// is not selected.
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic]
 #[rustc_nounwind]
diff --git a/library/core/src/num/f128.rs b/library/core/src/num/f128.rs
index 08c34e852da..d3d1eebc227 100644
--- a/library/core/src/num/f128.rs
+++ b/library/core/src/num/f128.rs
@@ -224,14 +224,16 @@ impl f128 {
 
     /// Not a Number (NaN).
     ///
-    /// Note that IEEE 754 doesn't define just a single NaN value;
-    /// a plethora of bit patterns are considered to be NaN.
-    /// Furthermore, the standard makes a difference
-    /// between a "signaling" and a "quiet" NaN,
-    /// and allows inspecting its "payload" (the unspecified bits in the bit pattern).
-    /// This constant isn't guaranteed to equal to any specific NaN bitpattern,
-    /// and the stability of its representation over Rust versions
-    /// and target platforms isn't guaranteed.
+    /// Note that IEEE 754 doesn't define just a single NaN value; a plethora of bit patterns are
+    /// considered to be NaN. Furthermore, the standard makes a difference between a "signaling" and
+    /// a "quiet" NaN, and allows inspecting its "payload" (the unspecified bits in the bit pattern)
+    /// and its sign. See the [specification of NaN bit patterns](f32#nan-bit-patterns) for more
+    /// info.
+    ///
+    /// This constant is guaranteed to be a quiet NaN (on targets that follow the Rust assumptions
+    /// that the quiet/signaling bit being set to 1 indicates a quiet NaN). Beyond that, nothing is
+    /// guaranteed about the specific bit pattern chosen here: both payload and sign are arbitrary.
+    /// The concrete bit pattern may change across Rust versions and target platforms.
     #[allow(clippy::eq_op)]
     #[rustc_diagnostic_item = "f128_nan"]
     #[unstable(feature = "f128", issue = "116909")]
diff --git a/library/core/src/num/f16.rs b/library/core/src/num/f16.rs
index a33e5f53014..dceb30177e6 100644
--- a/library/core/src/num/f16.rs
+++ b/library/core/src/num/f16.rs
@@ -219,14 +219,16 @@ impl f16 {
 
     /// Not a Number (NaN).
     ///
-    /// Note that IEEE 754 doesn't define just a single NaN value;
-    /// a plethora of bit patterns are considered to be NaN.
-    /// Furthermore, the standard makes a difference
-    /// between a "signaling" and a "quiet" NaN,
-    /// and allows inspecting its "payload" (the unspecified bits in the bit pattern).
-    /// This constant isn't guaranteed to equal to any specific NaN bitpattern,
-    /// and the stability of its representation over Rust versions
-    /// and target platforms isn't guaranteed.
+    /// Note that IEEE 754 doesn't define just a single NaN value; a plethora of bit patterns are
+    /// considered to be NaN. Furthermore, the standard makes a difference between a "signaling" and
+    /// a "quiet" NaN, and allows inspecting its "payload" (the unspecified bits in the bit pattern)
+    /// and its sign. See the [specification of NaN bit patterns](f32#nan-bit-patterns) for more
+    /// info.
+    ///
+    /// This constant is guaranteed to be a quiet NaN (on targets that follow the Rust assumptions
+    /// that the quiet/signaling bit being set to 1 indicates a quiet NaN). Beyond that, nothing is
+    /// guaranteed about the specific bit pattern chosen here: both payload and sign are arbitrary.
+    /// The concrete bit pattern may change across Rust versions and target platforms.
     #[allow(clippy::eq_op)]
     #[rustc_diagnostic_item = "f16_nan"]
     #[unstable(feature = "f16", issue = "116909")]
diff --git a/library/core/src/num/f32.rs b/library/core/src/num/f32.rs
index e473fac0393..c97dbfb63ae 100644
--- a/library/core/src/num/f32.rs
+++ b/library/core/src/num/f32.rs
@@ -470,14 +470,16 @@ impl f32 {
 
     /// Not a Number (NaN).
     ///
-    /// Note that IEEE 754 doesn't define just a single NaN value;
-    /// a plethora of bit patterns are considered to be NaN.
-    /// Furthermore, the standard makes a difference
-    /// between a "signaling" and a "quiet" NaN,
-    /// and allows inspecting its "payload" (the unspecified bits in the bit pattern).
-    /// This constant isn't guaranteed to equal to any specific NaN bitpattern,
-    /// and the stability of its representation over Rust versions
-    /// and target platforms isn't guaranteed.
+    /// Note that IEEE 754 doesn't define just a single NaN value; a plethora of bit patterns are
+    /// considered to be NaN. Furthermore, the standard makes a difference between a "signaling" and
+    /// a "quiet" NaN, and allows inspecting its "payload" (the unspecified bits in the bit pattern)
+    /// and its sign. See the [specification of NaN bit patterns](f32#nan-bit-patterns) for more
+    /// info.
+    ///
+    /// This constant is guaranteed to be a quiet NaN (on targets that follow the Rust assumptions
+    /// that the quiet/signaling bit being set to 1 indicates a quiet NaN). Beyond that, nothing is
+    /// guaranteed about the specific bit pattern chosen here: both payload and sign are arbitrary.
+    /// The concrete bit pattern may change across Rust versions and target platforms.
     #[stable(feature = "assoc_int_consts", since = "1.43.0")]
     #[rustc_diagnostic_item = "f32_nan"]
     #[allow(clippy::eq_op)]
diff --git a/library/core/src/num/f64.rs b/library/core/src/num/f64.rs
index 6522a80b0b7..91affdb3794 100644
--- a/library/core/src/num/f64.rs
+++ b/library/core/src/num/f64.rs
@@ -469,14 +469,16 @@ impl f64 {
 
     /// Not a Number (NaN).
     ///
-    /// Note that IEEE 754 doesn't define just a single NaN value;
-    /// a plethora of bit patterns are considered to be NaN.
-    /// Furthermore, the standard makes a difference
-    /// between a "signaling" and a "quiet" NaN,
-    /// and allows inspecting its "payload" (the unspecified bits in the bit pattern).
-    /// This constant isn't guaranteed to equal to any specific NaN bitpattern,
-    /// and the stability of its representation over Rust versions
-    /// and target platforms isn't guaranteed.
+    /// Note that IEEE 754 doesn't define just a single NaN value; a plethora of bit patterns are
+    /// considered to be NaN. Furthermore, the standard makes a difference between a "signaling" and
+    /// a "quiet" NaN, and allows inspecting its "payload" (the unspecified bits in the bit pattern)
+    /// and its sign. See the [specification of NaN bit patterns](f32#nan-bit-patterns) for more
+    /// info.
+    ///
+    /// This constant is guaranteed to be a quiet NaN (on targets that follow the Rust assumptions
+    /// that the quiet/signaling bit being set to 1 indicates a quiet NaN). Beyond that, nothing is
+    /// guaranteed about the specific bit pattern chosen here: both payload and sign are arbitrary.
+    /// The concrete bit pattern may change across Rust versions and target platforms.
     #[rustc_diagnostic_item = "f64_nan"]
     #[stable(feature = "assoc_int_consts", since = "1.43.0")]
     #[allow(clippy::eq_op)]
diff --git a/library/coretests/tests/hint.rs b/library/coretests/tests/hint.rs
new file mode 100644
index 00000000000..032bbc1dcc8
--- /dev/null
+++ b/library/coretests/tests/hint.rs
@@ -0,0 +1,23 @@
+#[test]
+fn select_unpredictable_drop() {
+    use core::cell::Cell;
+    struct X<'a>(&'a Cell<bool>);
+    impl Drop for X<'_> {
+        fn drop(&mut self) {
+            self.0.set(true);
+        }
+    }
+
+    let a_dropped = Cell::new(false);
+    let b_dropped = Cell::new(false);
+    let a = X(&a_dropped);
+    let b = X(&b_dropped);
+    assert!(!a_dropped.get());
+    assert!(!b_dropped.get());
+    let selected = core::hint::select_unpredictable(core::hint::black_box(true), a, b);
+    assert!(!a_dropped.get());
+    assert!(b_dropped.get());
+    drop(selected);
+    assert!(a_dropped.get());
+    assert!(b_dropped.get());
+}
diff --git a/library/coretests/tests/lib.rs b/library/coretests/tests/lib.rs
index 7ad154796f6..1c43bfe0ed4 100644
--- a/library/coretests/tests/lib.rs
+++ b/library/coretests/tests/lib.rs
@@ -68,6 +68,7 @@
 #![feature(pointer_is_aligned_to)]
 #![feature(portable_simd)]
 #![feature(ptr_metadata)]
+#![feature(select_unpredictable)]
 #![feature(slice_from_ptr_range)]
 #![feature(slice_internals)]
 #![feature(slice_partition_dedup)]
@@ -147,6 +148,7 @@ mod ffi;
 mod fmt;
 mod future;
 mod hash;
+mod hint;
 mod intrinsics;
 mod io;
 mod iter;
diff --git a/src/doc/rustc-dev-guide/src/tests/best-practices.md b/src/doc/rustc-dev-guide/src/tests/best-practices.md
index 6905ee13283..2bdc7f3a243 100644
--- a/src/doc/rustc-dev-guide/src/tests/best-practices.md
+++ b/src/doc/rustc-dev-guide/src/tests/best-practices.md
@@ -175,6 +175,8 @@ See [compiletest directives] for a listing of directives.
 - For `ignore-*`/`needs-*`/`only-*` directives, unless extremely obvious,
   provide a brief remark on why the directive is needed. E.g. `"//@ ignore-wasi
   (wasi codegens the main symbol differently)"`.
+- When using `//@ ignore-auxiliary`, specify the corresponding main test files,
+  e.g. ``//@ ignore-auxiliary (used by `./foo.rs`)``.
 
 ## FileCheck best practices
 
diff --git a/src/doc/rustc-dev-guide/src/tests/directives.md b/src/doc/rustc-dev-guide/src/tests/directives.md
index 0aad8be982f..dae659e6317 100644
--- a/src/doc/rustc-dev-guide/src/tests/directives.md
+++ b/src/doc/rustc-dev-guide/src/tests/directives.md
@@ -124,6 +124,9 @@ means the test won't be compiled or run.
 * `ignore-X` where `X` is a target detail or other criteria on which to ignore the test (see below)
 * `only-X` is like `ignore-X`, but will *only* run the test on that target or
   stage
+* `ignore-auxiliary` is intended for files that *participate* in one or more other
+  main test files but that `compiletest` should not try to build the file itself.
+  Please backlink to which main test is actually using the auxiliary file.
 * `ignore-test` always ignores the test. This can be used to temporarily disable
   a test if it is currently not working, but you want to keep it in tree to
   re-enable it later.
diff --git a/src/tools/compiletest/src/directive-list.rs b/src/tools/compiletest/src/directive-list.rs
index 086a8a67456..44c52c8c766 100644
--- a/src/tools/compiletest/src/directive-list.rs
+++ b/src/tools/compiletest/src/directive-list.rs
@@ -44,6 +44,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "ignore-arm-unknown-linux-gnueabihf",
     "ignore-arm-unknown-linux-musleabi",
     "ignore-arm-unknown-linux-musleabihf",
+    "ignore-auxiliary",
     "ignore-avr",
     "ignore-beta",
     "ignore-cdb",
diff --git a/src/tools/compiletest/src/header/cfg.rs b/src/tools/compiletest/src/header/cfg.rs
index c369fff97f4..f1f1384afb9 100644
--- a/src/tools/compiletest/src/header/cfg.rs
+++ b/src/tools/compiletest/src/header/cfg.rs
@@ -100,6 +100,10 @@ fn parse_cfg_name_directive<'a>(
         name: "test",
         message: "always"
     }
+    condition! {
+        name: "auxiliary",
+        message: "used by another main test file"
+    }
     condition! {
         name: &config.target,
         allowed_names: &target_cfgs.all_targets,
diff --git a/src/tools/compiletest/src/header/tests.rs b/src/tools/compiletest/src/header/tests.rs
index 3a8c3748de9..2525e0adc83 100644
--- a/src/tools/compiletest/src/header/tests.rs
+++ b/src/tools/compiletest/src/header/tests.rs
@@ -940,3 +940,9 @@ fn test_supported_crate_types() {
         "//@ needs-crate-type: bin, cdylib, dylib, lib, proc-macro, rlib, staticlib"
     ));
 }
+
+#[test]
+fn test_ignore_auxiliary() {
+    let config = cfg().build();
+    assert!(check_ignore(&config, "//@ ignore-auxiliary"));
+}
diff --git a/src/tools/miri/tests/pass/tls/tls_leak_main_thread_allowed.rs b/src/tools/miri/tests/pass/tls/tls_leak_main_thread_allowed.rs
index 341b2280e01..abc0968f7c4 100644
--- a/src/tools/miri/tests/pass/tls/tls_leak_main_thread_allowed.rs
+++ b/src/tools/miri/tests/pass/tls/tls_leak_main_thread_allowed.rs
@@ -13,7 +13,7 @@ pub fn main() {
     TLS.set(Some(Box::leak(Box::new(123))));
 
     // We can only ignore leaks on targets that use `#[thread_local]` statics to implement
-    // `thread_local!`. Ignore the test on targest that don't.
+    // `thread_local!`. Ignore the test on targets that don't.
     if cfg!(target_thread_local) {
         thread_local! {
             static TLS_KEY: Cell<Option<&'static i32>> = Cell::new(None);
diff --git a/src/tools/rust-analyzer/Cargo.lock b/src/tools/rust-analyzer/Cargo.lock
index 745a8097c8a..2dbb3f5d69c 100644
--- a/src/tools/rust-analyzer/Cargo.lock
+++ b/src/tools/rust-analyzer/Cargo.lock
@@ -902,9 +902,9 @@ checksum = "bbd2bcb4c963f2ddae06a2efc7e9f3591312473c50c6685e1f298068316e66fe"
 
 [[package]]
 name = "libc"
-version = "0.2.169"
+version = "0.2.172"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b5aba8db14291edd000dfcc4d620c7ebfb122c613afb886ca8803fa4e128a20a"
+checksum = "d750af042f7ef4f724306de029d18836c26c1765a54a6a3f094cbd23a7267ffa"
 
 [[package]]
 name = "libloading"
diff --git a/tests/assembly/cstring-merging.rs b/tests/assembly/cstring-merging.rs
index b5c530ac35d..f7d0775f7af 100644
--- a/tests/assembly/cstring-merging.rs
+++ b/tests/assembly/cstring-merging.rs
@@ -1,3 +1,5 @@
+// MIPS assembler uses the label prefix `$anon.` for local anonymous variables
+// other architectures (including ARM and x86-64) use the prefix `.Lanon.`
 //@ only-linux
 //@ assembly-output: emit-asm
 //@ compile-flags: --crate-type=lib -Copt-level=3
@@ -6,13 +8,13 @@
 use std::ffi::CStr;
 
 // CHECK: .section .rodata.str1.{{[12]}},"aMS"
-// CHECK: .Lanon.{{.+}}:
+// CHECK: {{(\.L|\$)}}anon.{{.+}}:
 // CHECK-NEXT: .asciz "foo"
 #[unsafe(no_mangle)]
 static CSTR: &[u8; 4] = b"foo\0";
 
 // CHECK-NOT: .section
-// CHECK: .Lanon.{{.+}}:
+// CHECK: {{(\.L|\$)}}anon.{{.+}}:
 // CHECK-NEXT: .asciz "bar"
 #[unsafe(no_mangle)]
 pub fn cstr() -> &'static CStr {
@@ -20,7 +22,7 @@ pub fn cstr() -> &'static CStr {
 }
 
 // CHECK-NOT: .section
-// CHECK: .Lanon.{{.+}}:
+// CHECK: {{(\.L|\$)}}anon.{{.+}}:
 // CHECK-NEXT: .asciz "baz"
 #[unsafe(no_mangle)]
 pub fn manual_cstr() -> &'static str {
diff --git a/tests/codegen/autodiffv2.rs b/tests/codegen/autodiffv2.rs
new file mode 100644
index 00000000000..a40d19d3be3
--- /dev/null
+++ b/tests/codegen/autodiffv2.rs
@@ -0,0 +1,113 @@
+//@ compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
+//@ no-prefer-dynamic
+//@ needs-enzyme
+//
+// In Enzyme, we test against a large range of LLVM versions (5+) and don't have overly many
+// breakages. One benefit is that we match the IR generated by Enzyme only after running it
+// through LLVM's O3 pipeline, which will remove most of the noise.
+// However, our integration test could also be affected by changes in how rustc lowers MIR into
+// LLVM-IR, which could cause additional noise and thus breakages. If that's the case, we should
+// reduce this test to only match the first lines and the ret instructions.
+//
+// The function tested here has 4 inputs and 5 outputs, so we could either call forward-mode
+// autodiff 4 times, or reverse mode 5 times. Since a forward-mode call is usually faster than
+// reverse mode, we prefer it here. This file also tests a new optimization (batch mode), which
+// allows us to call forward-mode autodiff only once, and get all 5 outputs in a single call.
+//
+// We support 2 different batch modes. `d_square2` has the same interface as scalar forward-mode,
+// but each shadow argument is `width` times larger (thus 16 and 20 elements here).
+// `d_square3` instead takes `width` (4) shadow arguments, which are all the same size as the
+// original function arguments.
+//
+// FIXME(autodiff): We currently can't test `d_square1` and `d_square3` in the same file, since they
+// generate the same dummy functions which get merged by LLVM, breaking pieces of our pipeline which
+// try to rewrite the dummy functions later. We should consider to change to pure declarations both
+// in our frontend and in the llvm backend to avoid these issues.
+
+#![feature(autodiff)]
+
+use std::autodiff::autodiff;
+
+#[no_mangle]
+//#[autodiff(d_square1, Forward, Dual, Dual)]
+#[autodiff(d_square2, Forward, 4, Dualv, Dualv)]
+#[autodiff(d_square3, Forward, 4, Dual, Dual)]
+fn square(x: &[f32], y: &mut [f32]) {
+    assert!(x.len() >= 4);
+    assert!(y.len() >= 5);
+    y[0] = 4.3 * x[0] + 1.2 * x[1] + 3.4 * x[2] + 2.1 * x[3];
+    y[1] = 2.3 * x[0] + 4.5 * x[1] + 1.7 * x[2] + 6.4 * x[3];
+    y[2] = 1.1 * x[0] + 3.3 * x[1] + 2.5 * x[2] + 4.7 * x[3];
+    y[3] = 5.2 * x[0] + 1.4 * x[1] + 2.6 * x[2] + 3.8 * x[3];
+    y[4] = 1.0 * x[0] + 2.0 * x[1] + 3.0 * x[2] + 4.0 * x[3];
+}
+
+fn main() {
+    let x1 = std::hint::black_box(vec![0.0, 1.0, 2.0, 3.0]);
+
+    let dx1 = std::hint::black_box(vec![1.0; 12]);
+
+    let z1 = std::hint::black_box(vec![1.0, 0.0, 0.0, 0.0]);
+    let z2 = std::hint::black_box(vec![0.0, 1.0, 0.0, 0.0]);
+    let z3 = std::hint::black_box(vec![0.0, 0.0, 1.0, 0.0]);
+    let z4 = std::hint::black_box(vec![0.0, 0.0, 0.0, 1.0]);
+
+    let z5 = std::hint::black_box(vec![
+        1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0,
+    ]);
+
+    let mut y1 = std::hint::black_box(vec![0.0; 5]);
+    let mut y2 = std::hint::black_box(vec![0.0; 5]);
+    let mut y3 = std::hint::black_box(vec![0.0; 5]);
+    let mut y4 = std::hint::black_box(vec![0.0; 5]);
+
+    let mut y5 = std::hint::black_box(vec![0.0; 5]);
+
+    let mut y6 = std::hint::black_box(vec![0.0; 5]);
+
+    let mut dy1_1 = std::hint::black_box(vec![0.0; 5]);
+    let mut dy1_2 = std::hint::black_box(vec![0.0; 5]);
+    let mut dy1_3 = std::hint::black_box(vec![0.0; 5]);
+    let mut dy1_4 = std::hint::black_box(vec![0.0; 5]);
+
+    let mut dy2 = std::hint::black_box(vec![0.0; 20]);
+
+    let mut dy3_1 = std::hint::black_box(vec![0.0; 5]);
+    let mut dy3_2 = std::hint::black_box(vec![0.0; 5]);
+    let mut dy3_3 = std::hint::black_box(vec![0.0; 5]);
+    let mut dy3_4 = std::hint::black_box(vec![0.0; 5]);
+
+    // scalar.
+    //d_square1(&x1, &z1, &mut y1, &mut dy1_1);
+    //d_square1(&x1, &z2, &mut y2, &mut dy1_2);
+    //d_square1(&x1, &z3, &mut y3, &mut dy1_3);
+    //d_square1(&x1, &z4, &mut y4, &mut dy1_4);
+
+    // assert y1 == y2 == y3 == y4
+    //for i in 0..5 {
+    //    assert_eq!(y1[i], y2[i]);
+    //    assert_eq!(y1[i], y3[i]);
+    //    assert_eq!(y1[i], y4[i]);
+    //}
+
+    // batch mode A)
+    d_square2(&x1, &z5, &mut y5, &mut dy2);
+
+    // assert y1 == y2 == y3 == y4 == y5
+    //for i in 0..5 {
+    //    assert_eq!(y1[i], y5[i]);
+    //}
+
+    // batch mode B)
+    d_square3(&x1, &z1, &z2, &z3, &z4, &mut y6, &mut dy3_1, &mut dy3_2, &mut dy3_3, &mut dy3_4);
+    for i in 0..5 {
+        assert_eq!(y5[i], y6[i]);
+    }
+
+    for i in 0..5 {
+        assert_eq!(dy2[0..5][i], dy3_1[i]);
+        assert_eq!(dy2[5..10][i], dy3_2[i]);
+        assert_eq!(dy2[10..15][i], dy3_3[i]);
+        assert_eq!(dy2[15..20][i], dy3_4[i]);
+    }
+}
diff --git a/tests/codegen/remap_path_prefix/aux_mod.rs b/tests/codegen/remap_path_prefix/aux_mod.rs
index c37e91c705c..3217e9e51e7 100644
--- a/tests/codegen/remap_path_prefix/aux_mod.rs
+++ b/tests/codegen/remap_path_prefix/aux_mod.rs
@@ -1,4 +1,4 @@
-//@ ignore-test: this is not a test
+//@ ignore-auxiliary (used by `./main.rs`)
 
 #[inline]
 pub fn some_aux_mod_function() -> i32 {
diff --git a/tests/debuginfo/drop-locations.rs b/tests/debuginfo/drop-locations.rs
index a55cf7b50a8..91b3da5c34a 100644
--- a/tests/debuginfo/drop-locations.rs
+++ b/tests/debuginfo/drop-locations.rs
@@ -1,5 +1,7 @@
 //@ ignore-android
-//@ ignore-test: #128971
+
+// FIXME: stepping with "next" in a debugger skips past end-of-scope drops
+//@ ignore-test (broken, see #128971)
 
 #![allow(unused)]
 
diff --git a/tests/ui/borrowck/move-error-snippets-ext.rs b/tests/ui/borrowck/move-error-snippets-ext.rs
index f8103228cf8..6dd68438f17 100644
--- a/tests/ui/borrowck/move-error-snippets-ext.rs
+++ b/tests/ui/borrowck/move-error-snippets-ext.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./move-error-snippets.rs`)
 
 macro_rules! aaa {
     ($c:ident) => {{
diff --git a/tests/ui/codemap_tests/two_files_data.rs b/tests/ui/codemap_tests/two_files_data.rs
index a4e4cf7e896..82852f6cfbd 100644
--- a/tests/ui/codemap_tests/two_files_data.rs
+++ b/tests/ui/codemap_tests/two_files_data.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./two_files.rs`)
 
 trait Foo { }
 
diff --git a/tests/ui/conditional-compilation/module_with_cfg.rs b/tests/ui/conditional-compilation/module_with_cfg.rs
index 29eb6d43aa7..a96f8a3e6e9 100644
--- a/tests/ui/conditional-compilation/module_with_cfg.rs
+++ b/tests/ui/conditional-compilation/module_with_cfg.rs
@@ -1,3 +1,3 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./inner-cfg-non-inline-mod.rs`)
 
 #![cfg_attr(all(), cfg(false))]
diff --git a/tests/ui/cross/cross-file-errors/underscore.rs b/tests/ui/cross/cross-file-errors/underscore.rs
index 9d075735393..73eb36cec24 100644
--- a/tests/ui/cross/cross-file-errors/underscore.rs
+++ b/tests/ui/cross/cross-file-errors/underscore.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./main.rs`)
 #![crate_type = "lib"]
 
 macro_rules! underscore {
diff --git a/tests/ui/directory_ownership/macro_expanded_mod_helper/foo/bar.rs b/tests/ui/directory_ownership/macro_expanded_mod_helper/foo/bar.rs
index 1d832a36ef5..2ccdd798c73 100644
--- a/tests/ui/directory_ownership/macro_expanded_mod_helper/foo/bar.rs
+++ b/tests/ui/directory_ownership/macro_expanded_mod_helper/foo/bar.rs
@@ -1 +1 @@
-//@ ignore-test not a test, auxiliary
+//@ ignore-auxiliary (used by `../../macro-expanded-mod.rs`)
diff --git a/tests/ui/directory_ownership/macro_expanded_mod_helper/foo/mod.rs b/tests/ui/directory_ownership/macro_expanded_mod_helper/foo/mod.rs
index 08349ba6747..9009f80c691 100644
--- a/tests/ui/directory_ownership/macro_expanded_mod_helper/foo/mod.rs
+++ b/tests/ui/directory_ownership/macro_expanded_mod_helper/foo/mod.rs
@@ -1,3 +1,3 @@
-//@ ignore-test not a test, auxiliary
+//@ ignore-auxiliary (used by `../../macro-expanded-mod.rs`)
 
 mod_decl!(bar);
diff --git a/tests/ui/directory_ownership/mod_file_not_owning_aux1.rs b/tests/ui/directory_ownership/mod_file_not_owning_aux1.rs
deleted file mode 100644
index 6d6884fef04..00000000000
--- a/tests/ui/directory_ownership/mod_file_not_owning_aux1.rs
+++ /dev/null
@@ -1,6 +0,0 @@
-//@ ignore-test this is not a test
-
-macro_rules! m {
-    () => { mod mod_file_not_owning_aux2; }
-}
-m!();
diff --git a/tests/ui/directory_ownership/mod_file_not_owning_aux2.rs b/tests/ui/directory_ownership/mod_file_not_owning_aux2.rs
deleted file mode 100644
index 76f1c1a7276..00000000000
--- a/tests/ui/directory_ownership/mod_file_not_owning_aux2.rs
+++ /dev/null
@@ -1 +0,0 @@
-//@ ignore-test this is not a test
diff --git a/tests/ui/directory_ownership/mod_file_not_owning_aux3.rs b/tests/ui/directory_ownership/mod_file_not_owning_aux3.rs
deleted file mode 100644
index 96a5780d971..00000000000
--- a/tests/ui/directory_ownership/mod_file_not_owning_aux3.rs
+++ /dev/null
@@ -1,3 +0,0 @@
-//@ ignore-test this is not a test
-
-mod mod_file_not_owning_aux2;
diff --git a/tests/ui/lint/expansion-time-include.rs b/tests/ui/lint/expansion-time-include.rs
index 3ecc01b045c..cbe3510f04a 100644
--- a/tests/ui/lint/expansion-time-include.rs
+++ b/tests/ui/lint/expansion-time-include.rs
@@ -1,4 +1,4 @@
-//@ ignore-test auxiliary file for expansion-time.rs
+//@ ignore-auxiliary (used by `./expansion-time.rs`)
 
 1
 2
diff --git a/tests/ui/lint/known-tool-in-submodule/submodule.rs b/tests/ui/lint/known-tool-in-submodule/submodule.rs
index 0bb2b93d53b..9c24964e94f 100644
--- a/tests/ui/lint/known-tool-in-submodule/submodule.rs
+++ b/tests/ui/lint/known-tool-in-submodule/submodule.rs
@@ -1,4 +1,4 @@
-//@ ignore-test: not a test
+//@ ignore-auxiliary (used by `./root.rs`)
 
 #[allow(tool::lint)]
 pub fn foo() {}
diff --git a/tests/ui/lint/lint_pre_expansion_extern_module_aux.rs b/tests/ui/lint/lint_pre_expansion_extern_module_aux.rs
index 6e16a796ff1..10e3c0f9564 100644
--- a/tests/ui/lint/lint_pre_expansion_extern_module_aux.rs
+++ b/tests/ui/lint/lint_pre_expansion_extern_module_aux.rs
@@ -1,3 +1,3 @@
-//@ ignore-test: not a test
+//@ ignore-auxiliary (used by `./lint-pre-expansion-extern-module.rs`)
 
 pub fn try() {}
diff --git a/tests/ui/lint/unknown-lints/other.rs b/tests/ui/lint/unknown-lints/other.rs
index 25333584916..7770bc59108 100644
--- a/tests/ui/lint/unknown-lints/other.rs
+++ b/tests/ui/lint/unknown-lints/other.rs
@@ -1,6 +1,4 @@
-//@ ignore-test (auxiliary)
-
-// Companion to allow-in-other-module.rs
+//@ ignore-auxiliary (used by `./allow-in-other-module.rs`)
 
 // This should not warn.
 #![allow(not_a_real_lint)]
diff --git a/tests/ui/lint/wasm_c_abi_transition.rs b/tests/ui/lint/wasm_c_abi_transition.rs
index 1fe81679e65..6a933a0de03 100644
--- a/tests/ui/lint/wasm_c_abi_transition.rs
+++ b/tests/ui/lint/wasm_c_abi_transition.rs
@@ -39,3 +39,9 @@ pub fn call_other_fun(x: MyType) {
     unsafe { other_fun(x) } //~ERROR: wasm ABI transition
     //~^WARN: previously accepted
 }
+
+// Zero-sized types are safe in both ABIs
+#[repr(C)]
+pub struct MyZstType;
+#[allow(improper_ctypes_definitions)]
+pub extern "C" fn zst_safe(_x: (), _y: MyZstType) {}
diff --git a/tests/ui/macros/auxiliary/macro-include-items-expr.rs b/tests/ui/macros/auxiliary/macro-include-items-expr.rs
index 7394f194b80..d00491fd7e5 100644
--- a/tests/ui/macros/auxiliary/macro-include-items-expr.rs
+++ b/tests/ui/macros/auxiliary/macro-include-items-expr.rs
@@ -1,3 +1 @@
-// ignore-test: this is not a test
-
 1
diff --git a/tests/ui/macros/auxiliary/macro-include-items-item.rs b/tests/ui/macros/auxiliary/macro-include-items-item.rs
index 7d54745e03b..761cd002189 100644
--- a/tests/ui/macros/auxiliary/macro-include-items-item.rs
+++ b/tests/ui/macros/auxiliary/macro-include-items-item.rs
@@ -1,3 +1 @@
-// ignore-test: this is not a test
-
 fn foo() { bar() }
diff --git a/tests/ui/macros/include-single-expr-helper-1.rs b/tests/ui/macros/include-single-expr-helper-1.rs
index ddeeb982f64..6802719afa1 100644
--- a/tests/ui/macros/include-single-expr-helper-1.rs
+++ b/tests/ui/macros/include-single-expr-helper-1.rs
@@ -1,4 +1,4 @@
-//@ ignore-test auxiliary file for include-single-expr.rs
+//@ ignore-auxiliary (used by `./include-single-expr.rs`)
 
 0
 
diff --git a/tests/ui/macros/include-single-expr-helper.rs b/tests/ui/macros/include-single-expr-helper.rs
index e8ad9746b02..bd75bbbd583 100644
--- a/tests/ui/macros/include-single-expr-helper.rs
+++ b/tests/ui/macros/include-single-expr-helper.rs
@@ -1,4 +1,4 @@
-//@ ignore-test auxiliary file for include-single-expr.rs
+//@ ignore-auxiliary (used by `./include-single-expr.rs`)
 
 0
 10
diff --git a/tests/ui/macros/issue-69838-dir/bar.rs b/tests/ui/macros/issue-69838-dir/bar.rs
index 4433005b85f..6f91f8e2ffa 100644
--- a/tests/ui/macros/issue-69838-dir/bar.rs
+++ b/tests/ui/macros/issue-69838-dir/bar.rs
@@ -1,3 +1,3 @@
-//@ ignore-test -- this is an auxiliary file as part of another test.
+//@ ignore-auxiliary (used by `../issue-69838-mods-relative-to-included-path.rs`)
 
 pub fn i_am_in_bar() {}
diff --git a/tests/ui/macros/issue-69838-dir/included.rs b/tests/ui/macros/issue-69838-dir/included.rs
index 11fcd3eff72..328334d5e66 100644
--- a/tests/ui/macros/issue-69838-dir/included.rs
+++ b/tests/ui/macros/issue-69838-dir/included.rs
@@ -1,3 +1,3 @@
-//@ ignore-test -- this is an auxiliary file as part of another test.
+//@ ignore-auxiliary (used by `../issue-69838-mods-relative-to-included-path.rs`)
 
 pub mod bar;
diff --git a/tests/ui/macros/macro-expanded-include/foo/mod.rs b/tests/ui/macros/macro-expanded-include/foo/mod.rs
index 926d84c93e5..4e6d9e4aea4 100644
--- a/tests/ui/macros/macro-expanded-include/foo/mod.rs
+++ b/tests/ui/macros/macro-expanded-include/foo/mod.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `../test.rs`)
 
 macro_rules! m {
     () => { include!("file.txt"); }
diff --git a/tests/ui/missing_non_modrs_mod/foo.rs b/tests/ui/missing_non_modrs_mod/foo.rs
index dd3e970b8c6..afdc5e39b84 100644
--- a/tests/ui/missing_non_modrs_mod/foo.rs
+++ b/tests/ui/missing_non_modrs_mod/foo.rs
@@ -1,4 +1,3 @@
-//
-//@ ignore-test this is just a helper for the real test in this dir
+//@ ignore-auxiliary (used by `./missing_non_modrs_mod.rs`)
 
 mod missing;
diff --git a/tests/ui/missing_non_modrs_mod/foo_inline.rs b/tests/ui/missing_non_modrs_mod/foo_inline.rs
index 9d46e9bdd0c..ed6d3a49101 100644
--- a/tests/ui/missing_non_modrs_mod/foo_inline.rs
+++ b/tests/ui/missing_non_modrs_mod/foo_inline.rs
@@ -1,4 +1,4 @@
-//@ ignore-test this is just a helper for the real test in this dir
+//@ ignore-auxiliary (used by `./missing_non_modrs_mod_inline.rs`)
 
 mod inline {
     mod missing;
diff --git a/tests/ui/missing_non_modrs_mod/missing_non_modrs_mod.stderr b/tests/ui/missing_non_modrs_mod/missing_non_modrs_mod.stderr
index 4e48799318b..c084fbf00c2 100644
--- a/tests/ui/missing_non_modrs_mod/missing_non_modrs_mod.stderr
+++ b/tests/ui/missing_non_modrs_mod/missing_non_modrs_mod.stderr
@@ -1,5 +1,5 @@
 error[E0583]: file not found for module `missing`
-  --> $DIR/foo.rs:4:1
+  --> $DIR/foo.rs:3:1
    |
 LL | mod missing;
    | ^^^^^^^^^^^^
diff --git a/tests/ui/modules/mod_file_aux.rs b/tests/ui/modules/mod_file_aux.rs
index f37296b3af0..eec38d189b4 100644
--- a/tests/ui/modules/mod_file_aux.rs
+++ b/tests/ui/modules/mod_file_aux.rs
@@ -1,4 +1,3 @@
-//@ run-pass
-//@ ignore-test Not a test. Used by other tests
+//@ ignore-auxiliary (used by `./mod_file_with_path_attr.rs` and `mod_file.rs`)
 
 pub fn foo() -> isize { 10 }
diff --git a/tests/ui/modules_and_files_visibility/mod_file_aux.rs b/tests/ui/modules_and_files_visibility/mod_file_aux.rs
index 77390da75f8..6fac8dae3d7 100644
--- a/tests/ui/modules_and_files_visibility/mod_file_aux.rs
+++ b/tests/ui/modules_and_files_visibility/mod_file_aux.rs
@@ -1,3 +1,3 @@
-//@ ignore-test Not a test. Used by other tests
+//@ ignore-auxiliary (used by `./mod_file_correct_spans.rs`)
 
 pub fn foo() -> isize { 10 }
diff --git a/tests/ui/modules_and_files_visibility/mod_file_disambig_aux.rs b/tests/ui/modules_and_files_visibility/mod_file_disambig_aux.rs
index e00b5629c08..9a0b1c4b0d8 100644
--- a/tests/ui/modules_and_files_visibility/mod_file_disambig_aux.rs
+++ b/tests/ui/modules_and_files_visibility/mod_file_disambig_aux.rs
@@ -1 +1 @@
-//@ ignore-test not a test. aux file
+//@ ignore-auxiliary (used by `./mod_file_disambig.rs`)
diff --git a/tests/ui/modules_and_files_visibility/mod_file_disambig_aux/mod.rs b/tests/ui/modules_and_files_visibility/mod_file_disambig_aux/mod.rs
index e00b5629c08..232c933c4cb 100644
--- a/tests/ui/modules_and_files_visibility/mod_file_disambig_aux/mod.rs
+++ b/tests/ui/modules_and_files_visibility/mod_file_disambig_aux/mod.rs
@@ -1 +1 @@
-//@ ignore-test not a test. aux file
+//@ ignore-auxiliary (used by `../mod_file_disambig.rs`)
diff --git a/tests/ui/non_modrs_mods/foors_mod.rs b/tests/ui/non_modrs_mods/foors_mod.rs
index b215e5f09e9..dfaa11bfe13 100644
--- a/tests/ui/non_modrs_mods/foors_mod.rs
+++ b/tests/ui/non_modrs_mods/foors_mod.rs
@@ -1,6 +1,4 @@
-//@ run-pass
-//
-//@ ignore-test: not a test, used by non_modrs_mods.rs
+//@ ignore-auxiliary (used by `./non_modrs_mods.rs`)
 
 pub mod inner_modrs_mod;
 pub mod inner_foors_mod;
diff --git a/tests/ui/non_modrs_mods_and_inline_mods/x.rs b/tests/ui/non_modrs_mods_and_inline_mods/x.rs
index c4548d39fad..38ff011d409 100644
--- a/tests/ui/non_modrs_mods_and_inline_mods/x.rs
+++ b/tests/ui/non_modrs_mods_and_inline_mods/x.rs
@@ -1,4 +1,4 @@
-//@ ignore-test: not a test
+//@ ignore-auxiliary (used by `./non_modrs_mods_and_inline_mods.rs`)
 
 pub mod y {
     pub mod z;
diff --git a/tests/ui/non_modrs_mods_and_inline_mods/x/y/z/mod.rs b/tests/ui/non_modrs_mods_and_inline_mods/x/y/z/mod.rs
index ec7b7de78d8..cac5e274fbe 100644
--- a/tests/ui/non_modrs_mods_and_inline_mods/x/y/z/mod.rs
+++ b/tests/ui/non_modrs_mods_and_inline_mods/x/y/z/mod.rs
@@ -1 +1 @@
-//@ ignore-test: not a test
+//@ ignore-auxiliary (used by `../../../non_modrs_mods_and_inline_mods.rs`)
diff --git a/tests/ui/numbers-arithmetic/saturating-float-casts-impl.rs b/tests/ui/numbers-arithmetic/saturating-float-casts-impl.rs
index 4b176ef5caa..4e578f6132f 100644
--- a/tests/ui/numbers-arithmetic/saturating-float-casts-impl.rs
+++ b/tests/ui/numbers-arithmetic/saturating-float-casts-impl.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./saturating-float-casts.rs` and `./saturating-float-casts-wasm.rs`)
 
 // Tests saturating float->int casts. See u128-as-f32.rs for the opposite direction.
 //
diff --git a/tests/ui/parser/circular_modules_hello.rs b/tests/ui/parser/circular_modules_hello.rs
index eb0284d8b41..540752ea231 100644
--- a/tests/ui/parser/circular_modules_hello.rs
+++ b/tests/ui/parser/circular_modules_hello.rs
@@ -1,4 +1,4 @@
-//@ ignore-test: this is an auxiliary file for circular-modules-main.rs
+//@ ignore-auxiliary (used by `./circular-modules-main.rs`)
 
 #[path = "circular_modules_main.rs"]
 mod circular_modules_main;
diff --git a/tests/ui/parser/issues/circular-module-with-doc-comment-issue-97589/recursive.rs b/tests/ui/parser/issues/circular-module-with-doc-comment-issue-97589/recursive.rs
index 3d758be8c05..2e9a15eb06d 100644
--- a/tests/ui/parser/issues/circular-module-with-doc-comment-issue-97589/recursive.rs
+++ b/tests/ui/parser/issues/circular-module-with-doc-comment-issue-97589/recursive.rs
@@ -1,4 +1,4 @@
-//@ ignore-test: this is an auxiliary file for circular-module-with-doc-comment-issue-97589.rs
+//@ ignore-auxiliary (used by `./circular-module-with-doc-comment-issue-97589.rs`)
 
 //! this comment caused the circular dependency checker to break
 
diff --git a/tests/ui/parser/issues/issue-48508-aux.rs b/tests/ui/parser/issues/issue-48508-aux.rs
index 0f2b4427383..0bf6490edf4 100644
--- a/tests/ui/parser/issues/issue-48508-aux.rs
+++ b/tests/ui/parser/issues/issue-48508-aux.rs
@@ -1,5 +1,4 @@
-//@ run-pass
-//@ ignore-test Not a test. Used by issue-48508.rs
+//@ ignore-auxiliary (used by `./issue-48508.rs`)
 
 pub fn other() -> f64 {
     let µ = 1.0;
diff --git a/tests/ui/proc-macro/module.rs b/tests/ui/proc-macro/module.rs
index 210c05988bf..5878f1b7ddd 100644
--- a/tests/ui/proc-macro/module.rs
+++ b/tests/ui/proc-macro/module.rs
@@ -1 +1 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `./attributes-on-modules-fail.rs`)
diff --git a/tests/ui/proc-macro/module_with_attrs.rs b/tests/ui/proc-macro/module_with_attrs.rs
index 8a4ca92e44b..7e4ec978736 100644
--- a/tests/ui/proc-macro/module_with_attrs.rs
+++ b/tests/ui/proc-macro/module_with_attrs.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `../inner-attr-non-inline-mod.rs`)
 
 #![rustfmt::skip]
 #![print_attr]
diff --git a/tests/ui/proc-macro/outer/inner.rs b/tests/ui/proc-macro/outer/inner.rs
index 210c05988bf..d0f2087321f 100644
--- a/tests/ui/proc-macro/outer/inner.rs
+++ b/tests/ui/proc-macro/outer/inner.rs
@@ -1 +1 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `../attributes-on-modules-fail.rs`)
diff --git a/tests/ui/proc-macro/pretty-print-hack/allsorts-rental-0.5.6/src/lib.rs b/tests/ui/proc-macro/pretty-print-hack/allsorts-rental-0.5.6/src/lib.rs
index f89098f3a5e..a27176a38e2 100644
--- a/tests/ui/proc-macro/pretty-print-hack/allsorts-rental-0.5.6/src/lib.rs
+++ b/tests/ui/proc-macro/pretty-print-hack/allsorts-rental-0.5.6/src/lib.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `../../../pretty-print-hack-show.rs`)
 
 #[derive(Print)]
 enum ProceduralMasqueradeDummyType {
diff --git a/tests/ui/proc-macro/pretty-print-hack/rental-0.5.5/src/lib.rs b/tests/ui/proc-macro/pretty-print-hack/rental-0.5.5/src/lib.rs
index f89098f3a5e..a27176a38e2 100644
--- a/tests/ui/proc-macro/pretty-print-hack/rental-0.5.5/src/lib.rs
+++ b/tests/ui/proc-macro/pretty-print-hack/rental-0.5.5/src/lib.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `../../../pretty-print-hack-show.rs`)
 
 #[derive(Print)]
 enum ProceduralMasqueradeDummyType {
diff --git a/tests/ui/proc-macro/pretty-print-hack/rental-0.5.6/src/lib.rs b/tests/ui/proc-macro/pretty-print-hack/rental-0.5.6/src/lib.rs
index f89098f3a5e..765ee4be656 100644
--- a/tests/ui/proc-macro/pretty-print-hack/rental-0.5.6/src/lib.rs
+++ b/tests/ui/proc-macro/pretty-print-hack/rental-0.5.6/src/lib.rs
@@ -1,4 +1,4 @@
-//@ ignore-test (auxiliary, used by other tests)
+//@ ignore-auxiliary (used by `../../../pretty-print-hack/hide.rs`)
 
 #[derive(Print)]
 enum ProceduralMasqueradeDummyType {
diff --git a/tests/ui/runtime/backtrace-debuginfo-aux.rs b/tests/ui/runtime/backtrace-debuginfo-aux.rs
index 24180ed2196..4493fa51f95 100644
--- a/tests/ui/runtime/backtrace-debuginfo-aux.rs
+++ b/tests/ui/runtime/backtrace-debuginfo-aux.rs
@@ -1,5 +1,4 @@
-//@ run-pass
-//@ ignore-test: not a test, used by backtrace-debuginfo.rs to test file!()
+//@ ignore-auxiliary (used by `./backtrace-debuginfo.rs` to test `file!()`)
 
 #[inline(never)]
 pub fn callback<F>(f: F) where F: FnOnce((&'static str, u32)) {