Auto merge of #63319 - Centril:rollup-d89rmey, r=Centril

Rollup of 14 pull requests

Successful merges:

 - #61457 (Implement DoubleEndedIterator for iter::{StepBy, Peekable, Take})
 - #63017 (Remove special code-path for handing unknown tokens)
 - #63184 (Explaining the reason why validation is performed in to_str of path.rs)
 - #63230 (Make use of possibly uninitialized data [E0381] a hard error)
 - #63260 (fix UB in a test)
 - #63264 (Revert "Rollup merge of #62696 - chocol4te:fix_#62194, r=estebank")
 - #63272 (Some more libsyntax::attr cleanup)
 - #63285 (Remove leftover AwaitOrigin)
 - #63287 (Don't store &Span)
 - #63293 (Clarify align_to's requirements and obligations)
 - #63295 (improve align_offset docs)
 - #63299 (Make qualify consts in_projection use PlaceRef)
 - #63312 (doc: fix broken sentence)
 - #63315 (Fix #63313)

Failed merges:

r? @ghost

Author: bors

src/libcore/iter/adapters/mod.rs

@@ -485,6 +485,39 @@ impl<I> Iterator for StepBy<I> where I: Iterator {
     }
 }
 
+impl<I> StepBy<I> where I: ExactSizeIterator {
+    // The zero-based index starting from the end of the iterator of the
+    // last element. Used in the `DoubleEndedIterator` implementation.
+    fn next_back_index(&self) -> usize {
+        let rem = self.iter.len() % (self.step + 1);
+        if self.first_take {
+            if rem == 0 { self.step } else { rem - 1 }
+        } else {
+            rem
+        }
+    }
+}
+
+#[stable(feature = "double_ended_step_by_iterator", since = "1.38.0")]
+impl<I> DoubleEndedIterator for StepBy<I> where I: DoubleEndedIterator + ExactSizeIterator {
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.iter.nth_back(self.next_back_index())
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        // `self.iter.nth_back(usize::MAX)` does the right thing here when `n`
+        // is out of bounds because the length of `self.iter` does not exceed
+        // `usize::MAX` (because `I: ExactSizeIterator`) and `nth_back` is
+        // zero-indexed
+        let n = n
+            .saturating_mul(self.step + 1)
+            .saturating_add(self.next_back_index());
+        self.iter.nth_back(n)
+    }
+}
+
 // StepBy can only make the iterator shorter, so the len will still fit.
 #[stable(feature = "iterator_step_by", since = "1.28.0")]
 impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}
@@ -1158,6 +1191,45 @@ impl<I: Iterator> Iterator for Peekable<I> {
     }
 }
 
+#[stable(feature = "double_ended_peek_iterator", since = "1.38.0")]
+impl<I> DoubleEndedIterator for Peekable<I> where I: DoubleEndedIterator {
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.iter.next_back().or_else(|| self.peeked.take().and_then(|x| x))
+    }
+
+    #[inline]
+    fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
+        Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
+    {
+        match self.peeked.take() {
+            Some(None) => return Try::from_ok(init),
+            Some(Some(v)) => match self.iter.try_rfold(init, &mut f).into_result() {
+                Ok(acc) => f(acc, v),
+                Err(e) => {
+                    self.peeked = Some(Some(v));
+                    Try::from_error(e)
+                }
+            },
+            None => self.iter.try_rfold(init, f),
+        }
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
+        where Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        match self.peeked {
+            Some(None) => return init,
+            Some(Some(v)) => {
+                let acc = self.iter.rfold(init, &mut fold);
+                fold(acc, v)
+            }
+            None => self.iter.rfold(init, fold),
+        }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I> {}
 
@@ -1627,6 +1699,51 @@ impl<I> Iterator for Take<I> where I: Iterator{
     }
 }
 
+#[stable(feature = "double_ended_take_iterator", since = "1.38.0")]
+impl<I> DoubleEndedIterator for Take<I> where I: DoubleEndedIterator + ExactSizeIterator {
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.n == 0 {
+            None
+        } else {
+            let n = self.n;
+            self.n -= 1;
+            self.iter.nth_back(self.iter.len().saturating_sub(n))
+        }
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.iter.len();
+        if self.n > n {
+            let m = len.saturating_sub(self.n) + n;
+            self.n -= n + 1;
+            self.iter.nth_back(m)
+        } else {
+            if len > 0 {
+                self.iter.nth_back(len - 1);
+            }
+            None
+        }
+    }
+
+    #[inline]
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
+        Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok = Acc>
+    {
+        if self.n == 0 {
+            Try::from_ok(init)
+        } else {
+            let len = self.iter.len();
+            if len > self.n && self.iter.nth_back(len - self.n - 1).is_none() {
+                Try::from_ok(init)
+            } else {
+                self.iter.try_rfold(init, fold)
+            }
+        }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> ExactSizeIterator for Take<I> where I: ExactSizeIterator {}
 

src/libcore/ptr/mod.rs

@@ -1606,10 +1606,12 @@ impl<T: ?Sized> *const T {
     /// `align`.
     ///
     /// If it is not possible to align the pointer, the implementation returns
-    /// `usize::max_value()`.
+    /// `usize::max_value()`. It is permissible for the implementation to *always*
+    /// return `usize::max_value()`. Only your algorithm's performance can depend
+    /// on getting a usable offset here, not its correctness.
     ///
     /// The offset is expressed in number of `T` elements, and not bytes. The value returned can be
-    /// used with the `add` method.
+    /// used with the `wrapping_add` method.
     ///
     /// There are no guarantees whatsoever that offsetting the pointer will not overflow or go
     /// beyond the allocation that the pointer points into. It is up to the caller to ensure that
@@ -2407,10 +2409,12 @@ impl<T: ?Sized> *mut T {
     /// `align`.
     ///
     /// If it is not possible to align the pointer, the implementation returns
-    /// `usize::max_value()`.
+    /// `usize::max_value()`. It is permissible for the implementation to *always*
+    /// return `usize::max_value()`. Only your algorithm's performance can depend
+    /// on getting a usable offset here, not its correctness.
     ///
     /// The offset is expressed in number of `T` elements, and not bytes. The value returned can be
-    /// used with the `add` method.
+    /// used with the `wrapping_add` method.
     ///
     /// There are no guarantees whatsoever that offsetting the pointer will not overflow or go
     /// beyond the allocation that the pointer points into. It is up to the caller to ensure that

src/libcore/slice/mod.rs

@@ -2308,9 +2308,10 @@ impl<T> [T] {
     /// maintained.
     ///
     /// This method splits the slice into three distinct slices: prefix, correctly aligned middle
-    /// slice of a new type, and the suffix slice. The method does a best effort to make the
-    /// middle slice the greatest length possible for a given type and input slice, but only
-    /// your algorithm's performance should depend on that, not its correctness.
+    /// slice of a new type, and the suffix slice. The method may make the middle slice the greatest
+    /// length possible for a given type and input slice, but only your algorithm's performance
+    /// should depend on that, not its correctness. It is permissible for all of the input data to
+    /// be returned as the prefix or suffix slice.
     ///
     /// This method has no purpose when either input element `T` or output element `U` are
     /// zero-sized and will return the original slice without splitting anything.
@@ -2361,9 +2362,10 @@ impl<T> [T] {
     /// maintained.
     ///
     /// This method splits the slice into three distinct slices: prefix, correctly aligned middle
-    /// slice of a new type, and the suffix slice. The method does a best effort to make the
-    /// middle slice the greatest length possible for a given type and input slice, but only
-    /// your algorithm's performance should depend on that, not its correctness.
+    /// slice of a new type, and the suffix slice. The method may make the middle slice the greatest
+    /// length possible for a given type and input slice, but only your algorithm's performance
+    /// should depend on that, not its correctness. It is permissible for all of the input data to
+    /// be returned as the prefix or suffix slice.
     ///
     /// This method has no purpose when either input element `T` or output element `U` are
     /// zero-sized and will return the original slice without splitting anything.