Auto merge of rust-lang#116483 - GuillaumeGomez:rollup-z65pno1, r=GuillaumeGomez

Rollup of 6 pull requests

Successful merges:

 - rust-lang#115454 (Clarify example in docs of str::char_slice)
 - rust-lang#115522 (Clarify ManuallyDrop bit validity)
 - rust-lang#115588 (Fix a comment in std::iter::successors)
 - rust-lang#116198 (Add more diagnostic items for clippy)
 - rust-lang#116329 (update some comments around swap())
 - rust-lang#116475 (rustdoc-search: fix bug with multi-item impl trait)

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

alloc/src/borrow.rs

@@ -55,6 +55,7 @@ pub trait ToOwned {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use = "cloning is often expensive and is not expected to have side effects"]
+    #[cfg_attr(not(test), rustc_diagnostic_item = "to_owned_method")]
     fn to_owned(&self) -> Self::Owned;
 
     /// Uses borrowed data to replace owned data, usually by cloning.

alloc/src/rc.rs

@@ -2701,6 +2701,7 @@ impl<T, I: iter::TrustedLen<Item = T>> ToRcSlice<T> for I {
 ///
 /// [`upgrade`]: Weak::upgrade
 #[stable(feature = "rc_weak", since = "1.4.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "RcWeak")]
 pub struct Weak<
     T: ?Sized,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,

alloc/src/string.rs

@@ -2435,6 +2435,7 @@ pub trait ToString {
     /// ```
     #[rustc_conversion_suggestion]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg_attr(not(test), rustc_diagnostic_item = "to_string_method")]
     fn to_string(&self) -> String;
 }
 

alloc/src/sync.rs

@@ -311,6 +311,7 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
 ///
 /// [`upgrade`]: Weak::upgrade
 #[stable(feature = "arc_weak", since = "1.4.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "ArcWeak")]
 pub struct Weak<
     T: ?Sized,
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,

core/src/array/iter.rs

@@ -13,6 +13,7 @@ use crate::{
 /// A by-value [array] iterator.
 #[stable(feature = "array_value_iter", since = "1.51.0")]
 #[rustc_insignificant_dtor]
+#[rustc_diagnostic_item = "ArrayIntoIter"]
 pub struct IntoIter<T, const N: usize> {
     /// This is the array we are iterating over.
     ///

core/src/cell.rs

@@ -1423,6 +1423,7 @@ impl Clone for BorrowRef<'_> {
 /// See the [module-level documentation](self) for more.
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
+#[rustc_diagnostic_item = "RefCellRef"]
 pub struct Ref<'b, T: ?Sized + 'b> {
     // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
     // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
@@ -1804,6 +1805,7 @@ impl<'b> BorrowRefMut<'b> {
 /// See the [module-level documentation](self) for more.
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
+#[rustc_diagnostic_item = "RefCellRefMut"]
 pub struct RefMut<'b, T: ?Sized + 'b> {
     // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
     // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.

core/src/cmp.rs

@@ -809,6 +809,7 @@ pub trait Ord: Eq + PartialOrd<Self> {
     /// ```
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_diagnostic_item = "ord_cmp_method"]
     fn cmp(&self, other: &Self) -> Ordering;
 
     /// Compares and returns the maximum of two values.

core/src/convert/mod.rs

@@ -100,6 +100,7 @@ pub use num::FloatToInt;
 #[stable(feature = "convert_id", since = "1.33.0")]
 #[rustc_const_stable(feature = "const_identity", since = "1.33.0")]
 #[inline(always)]
+#[rustc_diagnostic_item = "convert_identity"]
 pub const fn identity<T>(x: T) -> T {
     x
 }
@@ -642,6 +643,7 @@ pub trait TryFrom<T>: Sized {
 
     /// Performs the conversion.
     #[stable(feature = "try_from", since = "1.34.0")]
+    #[rustc_diagnostic_item = "try_from_fn"]
     fn try_from(value: T) -> Result<Self, Self::Error>;
 }
 

core/src/default.rs

@@ -130,6 +130,7 @@ pub trait Default: Sized {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_diagnostic_item = "default_fn"]
     fn default() -> Self;
 }
 

core/src/fmt/builders.rs

@@ -84,6 +84,7 @@ impl fmt::Write for PadAdapter<'_, '_> {
 #[must_use = "must eventually call `finish()` on Debug builders"]
 #[allow(missing_debug_implementations)]
 #[stable(feature = "debug_builders", since = "1.2.0")]
+#[rustc_diagnostic_item = "DebugStruct"]
 pub struct DebugStruct<'a, 'b: 'a> {
     fmt: &'a mut fmt::Formatter<'b>,
     result: fmt::Result,

core/src/fmt/mod.rs

@@ -239,6 +239,7 @@ impl<W: Write + ?Sized> Write for &mut W {
 /// documentation of the methods defined on `Formatter` below.
 #[allow(missing_debug_implementations)]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "Formatter"]
 pub struct Formatter<'a> {
     flags: u32,
     fill: char,

core/src/intrinsics.rs

@@ -1509,12 +1509,14 @@ extern "rust-intrinsic" {
     ///
     /// This intrinsic does not have a stable counterpart.
     #[rustc_nounwind]
+    #[rustc_diagnostic_item = "intrinsics_unaligned_volatile_load"]
     pub fn unaligned_volatile_load<T>(src: *const T) -> T;
     /// Performs a volatile store to the `dst` pointer.
     /// The pointer is not required to be aligned.
     ///
     /// This intrinsic does not have a stable counterpart.
     #[rustc_nounwind]
+    #[rustc_diagnostic_item = "intrinsics_unaligned_volatile_store"]
     pub fn unaligned_volatile_store<T>(dst: *mut T, val: T);
 
     /// Returns the square root of an `f32`
@@ -2666,6 +2668,7 @@ pub(crate) fn is_nonoverlapping<T>(src: *const T, dst: *const T, count: usize) -
 #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[rustc_diagnostic_item = "ptr_copy_nonoverlapping"]
 pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize) {
     extern "rust-intrinsic" {
         #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
@@ -2761,6 +2764,7 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
 #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[rustc_diagnostic_item = "ptr_copy"]
 pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
     extern "rust-intrinsic" {
         #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
@@ -2834,6 +2838,7 @@ pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
 #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[rustc_diagnostic_item = "ptr_write_bytes"]
 pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
     extern "rust-intrinsic" {
         #[rustc_const_unstable(feature = "const_ptr_write", issue = "86302")]

core/src/iter/adapters/peekable.rs

@@ -12,6 +12,7 @@ use crate::ops::{ControlFlow, Try};
 #[derive(Clone, Debug)]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "IterPeekable"]
 pub struct Peekable<I: Iterator> {
     iter: I,
     /// Remember a peeked value, even if it was None.

core/src/iter/sources/empty.rs

@@ -27,6 +27,7 @@ pub const fn empty<T>() -> Empty<T> {
 /// This `struct` is created by the [`empty()`] function. See its documentation for more.
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "iter_empty", since = "1.2.0")]
+#[rustc_diagnostic_item = "IterEmpty"]
 pub struct Empty<T>(marker::PhantomData<fn() -> T>);
 
 #[stable(feature = "core_impl_debug", since = "1.9.0")]

core/src/iter/sources/once.rs

@@ -61,6 +61,7 @@ pub fn once<T>(value: T) -> Once<T> {
 /// This `struct` is created by the [`once()`] function. See its documentation for more.
 #[derive(Clone, Debug)]
 #[stable(feature = "iter_once", since = "1.2.0")]
+#[rustc_diagnostic_item = "IterOnce"]
 pub struct Once<T> {
     inner: crate::option::IntoIter<T>,
 }

core/src/iter/sources/successors.rs

@@ -17,7 +17,7 @@ where
     F: FnMut(&T) -> Option<T>,
 {
     // If this function returned `impl Iterator<Item=T>`
-    // it could be based on `unfold` and not need a dedicated type.
+    // it could be based on `from_fn` and not need a dedicated type.
     // However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.
     Successors { next: first, succ }
 }

core/src/iter/traits/collect.rs

@@ -146,6 +146,7 @@ pub trait FromIterator<A>: Sized {
     /// assert_eq!(v, vec![5, 5, 5, 5, 5]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_diagnostic_item = "from_iter_fn"]
     fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self;
 }
 

core/src/mem/manually_drop.rs

@@ -4,12 +4,12 @@ use crate::ptr;
 /// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
 /// This wrapper is 0-cost.
 ///
-/// `ManuallyDrop<T>` is guaranteed to have the same layout as `T`, and is subject
-/// to the same layout optimizations as `T`. As a consequence, it has *no effect*
-/// on the assumptions that the compiler makes about its contents. For example,
-/// initializing a `ManuallyDrop<&mut T>` with [`mem::zeroed`] is undefined
-/// behavior. If you need to handle uninitialized data, use [`MaybeUninit<T>`]
-/// instead.
+/// `ManuallyDrop<T>` is guaranteed to have the same layout and bit validity as
+/// `T`, and is subject to the same layout optimizations as `T`. As a consequence,
+/// it has *no effect* on the assumptions that the compiler makes about its
+/// contents. For example, initializing a `ManuallyDrop<&mut T>` with [`mem::zeroed`]
+/// is undefined behavior. If you need to handle uninitialized data, use
+/// [`MaybeUninit<T>`] instead.
 ///
 /// Note that accessing the value inside a `ManuallyDrop<T>` is safe.
 /// This means that a `ManuallyDrop<T>` whose content has been dropped must not

core/src/mem/mod.rs

@@ -723,15 +723,12 @@ pub unsafe fn uninitialized<T>() -> T {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
+#[rustc_diagnostic_item = "mem_swap"]
 pub const fn swap<T>(x: &mut T, y: &mut T) {
     // NOTE(eddyb) SPIR-V's Logical addressing model doesn't allow for arbitrary
     // reinterpretation of values as (chunkable) byte arrays, and the loop in the
     // block optimization in `swap_slice` is hard to rewrite back
     // into the (unoptimized) direct swapping implementation, so we disable it.
-    // FIXME(eddyb) the block optimization also prevents MIR optimizations from
-    // understanding `mem::replace`, `Option::take`, etc. - a better overall
-    // solution might be to make `ptr::swap_nonoverlapping` into an intrinsic, which
-    // a backend can choose to implement using the block optimization, or not.
     #[cfg(not(any(target_arch = "spirv")))]
     {
         // For types that are larger multiples of their alignment, the simple way
@@ -768,11 +765,14 @@ pub(crate) const fn swap_simple<T>(x: &mut T, y: &mut T) {
     // And LLVM actually optimizes it to 3×memcpy if called with
     // a type larger than it's willing to keep in a register.
     // Having typed reads and writes in MIR here is also good as
-    // it lets MIRI and CTFE understand them better, including things
+    // it lets Miri and CTFE understand them better, including things
     // like enforcing type validity for them.
     // Importantly, read+copy_nonoverlapping+write introduces confusing
     // asymmetry to the behaviour where one value went through read+write
     // whereas the other was copied over by the intrinsic (see #94371).
+    // Furthermore, using only read+write here benefits limited backends
+    // such as SPIR-V that work on an underlying *typed* view of memory,
+    // and thus have trouble with Rust's untyped memory operations.
 
     // SAFETY: exclusive references are always valid to read/write,
     // including being aligned, and nothing here panics so it's drop-safe.

core/src/ops/deref.rs

@@ -180,6 +180,7 @@ impl<T: ?Sized> Deref for &mut T {
 pub trait DerefMut: Deref {
     /// Mutably dereferences the value.
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_diagnostic_item = "deref_mut_method"]
     fn deref_mut(&mut self) -> &mut Self::Target;
 }
 

core/src/ops/range.rs

@@ -758,6 +758,7 @@ impl<T: Clone> Bound<&T> {
 /// `RangeBounds` is implemented by Rust's built-in range types, produced
 /// by range syntax like `..`, `a..`, `..b`, `..=c`, `d..e`, or `f..=g`.
 #[stable(feature = "collections_range", since = "1.28.0")]
+#[rustc_diagnostic_item = "RangeBounds"]
 pub trait RangeBounds<T: ?Sized> {
     /// Start index bound.
     ///

core/src/ptr/mod.rs

@@ -494,6 +494,7 @@ mod mut_ptr;
 #[stable(feature = "drop_in_place", since = "1.8.0")]
 #[lang = "drop_in_place"]
 #[allow(unconditional_recursion)]
+#[rustc_diagnostic_item = "ptr_drop_in_place"]
 pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
     // Code here does not matter - this is replaced by the
     // real drop glue by the compiler.
@@ -740,6 +741,7 @@ pub const fn from_mut<T: ?Sized>(r: &mut T) -> *mut T {
 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
 #[rustc_const_stable(feature = "const_slice_from_raw_parts", since = "1.64.0")]
 #[rustc_allow_const_fn_unstable(ptr_metadata)]
+#[rustc_diagnostic_item = "ptr_slice_from_raw_parts"]
 pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
     from_raw_parts(data.cast(), len)
 }
@@ -772,6 +774,7 @@ pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
 #[inline]
 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
 #[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
+#[rustc_diagnostic_item = "ptr_slice_from_raw_parts_mut"]
 pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
     from_raw_parts_mut(data.cast(), len)
 }
@@ -850,6 +853,7 @@ pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
+#[rustc_diagnostic_item = "ptr_swap"]
 pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
     // Give ourselves some scratch space to work with.
     // We do not have to worry about drops: `MaybeUninit` does nothing when dropped.
@@ -911,6 +915,7 @@ pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
 #[inline]
 #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
+#[rustc_diagnostic_item = "ptr_swap_nonoverlapping"]
 pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
     #[allow(unused)]
     macro_rules! attempt_swap_as_chunks {
@@ -1022,6 +1027,7 @@ const unsafe fn swap_nonoverlapping_simple_untyped<T>(x: *mut T, y: *mut T, coun
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_replace", issue = "83164")]
+#[rustc_diagnostic_item = "ptr_replace"]
 pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
     // SAFETY: the caller must guarantee that `dst` is valid to be
     // cast to a mutable reference (valid for writes, aligned, initialized),
@@ -1147,6 +1153,7 @@ pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
 #[rustc_const_stable(feature = "const_ptr_read", since = "1.71.0")]
 #[rustc_allow_const_fn_unstable(const_mut_refs, const_maybe_uninit_as_mut_ptr)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[rustc_diagnostic_item = "ptr_read"]
 pub const unsafe fn read<T>(src: *const T) -> T {
     // It would be semantically correct to implement this via `copy_nonoverlapping`
     // and `MaybeUninit`, as was done before PR #109035. Calling `assume_init`
@@ -1264,6 +1271,7 @@ pub const unsafe fn read<T>(src: *const T) -> T {
 #[rustc_const_stable(feature = "const_ptr_read", since = "1.71.0")]
 #[rustc_allow_const_fn_unstable(const_mut_refs, const_maybe_uninit_as_mut_ptr)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[rustc_diagnostic_item = "ptr_read_unaligned"]
 pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
     let mut tmp = MaybeUninit::<T>::uninit();
     // SAFETY: the caller must guarantee that `src` is valid for reads.
@@ -1539,6 +1547,7 @@ pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
 #[inline]
 #[stable(feature = "volatile", since = "1.9.0")]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
+#[rustc_diagnostic_item = "ptr_read_volatile"]
 pub unsafe fn read_volatile<T>(src: *const T) -> T {
     // SAFETY: the caller must uphold the safety contract for `volatile_load`.
     unsafe {
@@ -1865,6 +1874,7 @@ pub(crate) const unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usiz
 #[stable(feature = "ptr_eq", since = "1.17.0")]
 #[inline(always)]
 #[must_use = "pointer comparison produces a value"]
+#[rustc_diagnostic_item = "ptr_eq"]
 pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
     a == b
 }

core/src/ptr/non_null.rs

@@ -68,6 +68,7 @@ use crate::slice::{self, SliceIndex};
 #[repr(transparent)]
 #[rustc_layout_scalar_valid_range_start(1)]
 #[rustc_nonnull_optimization_guaranteed]
+#[rustc_diagnostic_item = "NonNull"]
 pub struct NonNull<T: ?Sized> {
     pointer: *const T,
 }

core/src/slice/iter.rs

@@ -59,6 +59,7 @@ impl<'a, T> IntoIterator for &'a mut [T] {
 /// [slices]: slice
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
+#[rustc_diagnostic_item = "SliceIter"]
 pub struct Iter<'a, T: 'a> {
     /// The pointer to the next element to return, or the past-the-end location
     /// if the iterator is empty.

core/src/slice/raw.rs

@@ -90,6 +90,7 @@ use crate::ptr;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "const_slice_from_raw_parts", since = "1.64.0")]
 #[must_use]
+#[rustc_diagnostic_item = "slice_from_raw_parts"]
 pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
     // SAFETY: the caller must uphold the safety contract for `from_raw_parts`.
     unsafe {
@@ -136,6 +137,7 @@ pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
 #[must_use]
+#[rustc_diagnostic_item = "slice_from_raw_parts_mut"]
 pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T] {
     // SAFETY: the caller must uphold the safety contract for `from_raw_parts_mut`.
     unsafe {

core/src/str/mod.rs

@@ -808,7 +808,7 @@ impl str {
     /// assert_eq!(Some((0, 'y')), char_indices.next()); // not (0, 'y̆')
     /// assert_eq!(Some((1, '\u{0306}')), char_indices.next());
     ///
-    /// // note the 3 here - the last character took up two bytes
+    /// // note the 3 here - the previous character took up two bytes
     /// assert_eq!(Some((3, 'e')), char_indices.next());
     /// assert_eq!(Some((4, 's')), char_indices.next());
     ///

core/src/str/traits.rs

@@ -624,6 +624,7 @@ pub trait FromStr: Sized {
     /// assert_eq!(5, x);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_diagnostic_item = "from_str_method"]
     fn from_str(s: &str) -> Result<Self, Self::Err>;
 }
 

core/tests/array.rs

@@ -663,7 +663,7 @@ fn array_mixed_equality_nans() {
 
 #[test]
 fn array_into_iter_fold() {
-    // Strings to help MIRI catch if we double-free or something
+    // Strings to help Miri catch if we double-free or something
     let a = ["Aa".to_string(), "Bb".to_string(), "Cc".to_string()];
     let mut s = "s".to_string();
     a.into_iter().for_each(|b| s += &b);
@@ -679,7 +679,7 @@ fn array_into_iter_fold() {
 
 #[test]
 fn array_into_iter_rfold() {
-    // Strings to help MIRI catch if we double-free or something
+    // Strings to help Miri catch if we double-free or something
     let a = ["Aa".to_string(), "Bb".to_string(), "Cc".to_string()];
     let mut s = "s".to_string();
     a.into_iter().rev().for_each(|b| s += &b);