Auto merge of rust-lang#139301 - matthiaskrgr:rollup-sa6ali8, r=matthiaskrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#139080 (Experimental feature gate for `super let`)
 - rust-lang#139145 (slice: Remove some uses of unsafe in first/last chunk methods)
 - rust-lang#139149 (unstable book: document import_trait_associated_functions)
 - rust-lang#139273 (Apply requested API changes to `cell_update`)
 - rust-lang#139282 (rustdoc: make settings checkboxes always square)
 - rust-lang#139283 (Rustc dev guide subtree update)
 - rust-lang#139294 (Fix the `f16`/`f128` feature gates on integer literals)

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

Author: bors

Diff for: core/src/cell.rs

@@ -544,7 +544,7 @@ impl<T: Copy> Cell<T> {
         unsafe { *self.value.get() }
     }
 
-    /// Updates the contained value using a function and returns the new value.
+    /// Updates the contained value using a function.
     ///
     /// # Examples
     ///
@@ -554,21 +554,14 @@ impl<T: Copy> Cell<T> {
     /// use std::cell::Cell;
     ///
     /// let c = Cell::new(5);
-    /// let new = c.update(|x| x + 1);
-    ///
-    /// assert_eq!(new, 6);
+    /// c.update(|x| x + 1);
     /// assert_eq!(c.get(), 6);
     /// ```
     #[inline]
     #[unstable(feature = "cell_update", issue = "50186")]
-    pub fn update<F>(&self, f: F) -> T
-    where
-        F: FnOnce(T) -> T,
-    {
+    pub fn update(&self, f: impl FnOnce(T) -> T) {
         let old = self.get();
-        let new = f(old);
-        self.set(new);
-        new
+        self.set(f(old));
     }
 }
 

Diff for: core/src/slice/mod.rs

@@ -382,16 +382,11 @@ impl<T> [T] {
     #[stable(feature = "slice_first_last_chunk", since = "1.77.0")]
     #[rustc_const_stable(feature = "slice_first_last_chunk", since = "1.77.0")]
     pub const fn split_first_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])> {
-        if self.len() < N {
-            None
-        } else {
-            // SAFETY: We manually verified the bounds of the split.
-            let (first, tail) = unsafe { self.split_at_unchecked(N) };
+        let Some((first, tail)) = self.split_at_checked(N) else { return None };
 
-            // SAFETY: We explicitly check for the correct number of elements,
-            //   and do not let the references outlive the slice.
-            Some((unsafe { &*(first.as_ptr().cast::<[T; N]>()) }, tail))
-        }
+        // SAFETY: We explicitly check for the correct number of elements,
+        //   and do not let the references outlive the slice.
+        Some((unsafe { &*(first.as_ptr().cast::<[T; N]>()) }, tail))
     }
 
     /// Returns a mutable array reference to the first `N` items in the slice and the remaining
@@ -419,17 +414,12 @@ impl<T> [T] {
     pub const fn split_first_chunk_mut<const N: usize>(
         &mut self,
     ) -> Option<(&mut [T; N], &mut [T])> {
-        if self.len() < N {
-            None
-        } else {
-            // SAFETY: We manually verified the bounds of the split.
-            let (first, tail) = unsafe { self.split_at_mut_unchecked(N) };
+        let Some((first, tail)) = self.split_at_mut_checked(N) else { return None };
 
-            // SAFETY: We explicitly check for the correct number of elements,
-            //   do not let the reference outlive the slice,
-            //   and enforce exclusive mutability of the chunk by the split.
-            Some((unsafe { &mut *(first.as_mut_ptr().cast::<[T; N]>()) }, tail))
-        }
+        // SAFETY: We explicitly check for the correct number of elements,
+        //   do not let the reference outlive the slice,
+        //   and enforce exclusive mutability of the chunk by the split.
+        Some((unsafe { &mut *(first.as_mut_ptr().cast::<[T; N]>()) }, tail))
     }
 
     /// Returns an array reference to the last `N` items in the slice and the remaining slice.
@@ -452,16 +442,12 @@ impl<T> [T] {
     #[stable(feature = "slice_first_last_chunk", since = "1.77.0")]
     #[rustc_const_stable(feature = "slice_first_last_chunk", since = "1.77.0")]
     pub const fn split_last_chunk<const N: usize>(&self) -> Option<(&[T], &[T; N])> {
-        if self.len() < N {
-            None
-        } else {
-            // SAFETY: We manually verified the bounds of the split.
-            let (init, last) = unsafe { self.split_at_unchecked(self.len() - N) };
+        let Some(index) = self.len().checked_sub(N) else { return None };
+        let (init, last) = self.split_at(index);
 
-            // SAFETY: We explicitly check for the correct number of elements,
-            //   and do not let the references outlive the slice.
-            Some((init, unsafe { &*(last.as_ptr().cast::<[T; N]>()) }))
-        }
+        // SAFETY: We explicitly check for the correct number of elements,
+        //   and do not let the references outlive the slice.
+        Some((init, unsafe { &*(last.as_ptr().cast::<[T; N]>()) }))
     }
 
     /// Returns a mutable array reference to the last `N` items in the slice and the remaining
@@ -489,17 +475,13 @@ impl<T> [T] {
     pub const fn split_last_chunk_mut<const N: usize>(
         &mut self,
     ) -> Option<(&mut [T], &mut [T; N])> {
-        if self.len() < N {
-            None
-        } else {
-            // SAFETY: We manually verified the bounds of the split.
-            let (init, last) = unsafe { self.split_at_mut_unchecked(self.len() - N) };
+        let Some(index) = self.len().checked_sub(N) else { return None };
+        let (init, last) = self.split_at_mut(index);
 
-            // SAFETY: We explicitly check for the correct number of elements,
-            //   do not let the reference outlive the slice,
-            //   and enforce exclusive mutability of the chunk by the split.
-            Some((init, unsafe { &mut *(last.as_mut_ptr().cast::<[T; N]>()) }))
-        }
+        // SAFETY: We explicitly check for the correct number of elements,
+        //   do not let the reference outlive the slice,
+        //   and enforce exclusive mutability of the chunk by the split.
+        Some((init, unsafe { &mut *(last.as_mut_ptr().cast::<[T; N]>()) }))
     }
 
     /// Returns an array reference to the last `N` items in the slice.
@@ -522,17 +504,13 @@ impl<T> [T] {
     #[stable(feature = "slice_first_last_chunk", since = "1.77.0")]
     #[rustc_const_stable(feature = "const_slice_last_chunk", since = "1.80.0")]
     pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]> {
-        if self.len() < N {
-            None
-        } else {
-            // SAFETY: We manually verified the bounds of the slice.
-            // FIXME(const-hack): Without const traits, we need this instead of `get_unchecked`.
-            let last = unsafe { self.split_at_unchecked(self.len() - N).1 };
+        // FIXME(const-hack): Without const traits, we need this instead of `get`.
+        let Some(index) = self.len().checked_sub(N) else { return None };
+        let (_, last) = self.split_at(index);
 
-            // SAFETY: We explicitly check for the correct number of elements,
-            //   and do not let the references outlive the slice.
-            Some(unsafe { &*(last.as_ptr().cast::<[T; N]>()) })
-        }
+        // SAFETY: We explicitly check for the correct number of elements,
+        //   and do not let the references outlive the slice.
+        Some(unsafe { &*(last.as_ptr().cast::<[T; N]>()) })
     }
 
     /// Returns a mutable array reference to the last `N` items in the slice.
@@ -556,18 +534,14 @@ impl<T> [T] {
     #[stable(feature = "slice_first_last_chunk", since = "1.77.0")]
     #[rustc_const_stable(feature = "const_slice_first_last_chunk", since = "1.83.0")]
     pub const fn last_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]> {
-        if self.len() < N {
-            None
-        } else {
-            // SAFETY: We manually verified the bounds of the slice.
-            // FIXME(const-hack): Without const traits, we need this instead of `get_unchecked`.
-            let last = unsafe { self.split_at_mut_unchecked(self.len() - N).1 };
-
-            // SAFETY: We explicitly check for the correct number of elements,
-            //   do not let the reference outlive the slice,
-            //   and require exclusive access to the entire slice to mutate the chunk.
-            Some(unsafe { &mut *(last.as_mut_ptr().cast::<[T; N]>()) })
-        }
+        // FIXME(const-hack): Without const traits, we need this instead of `get`.
+        let Some(index) = self.len().checked_sub(N) else { return None };
+        let (_, last) = self.split_at_mut(index);
+
+        // SAFETY: We explicitly check for the correct number of elements,
+        //   do not let the reference outlive the slice,
+        //   and require exclusive access to the entire slice to mutate the chunk.
+        Some(unsafe { &mut *(last.as_mut_ptr().cast::<[T; N]>()) })
     }
 
     /// Returns a reference to an element or subslice depending on the type of

Diff for: coretests/tests/cell.rs

@@ -50,10 +50,10 @@ fn smoketest_cell() {
 fn cell_update() {
     let x = Cell::new(10);
 
-    assert_eq!(x.update(|x| x + 5), 15);
+    x.update(|x| x + 5);
     assert_eq!(x.get(), 15);
 
-    assert_eq!(x.update(|x| x / 3), 5);
+    x.update(|x| x / 3);
     assert_eq!(x.get(), 5);
 }