Auto merge of rust-lang#101017 - JohnTitor:rollup-73f2fhb, r=JohnTitor

Rollup of 8 pull requests

Successful merges:

 - rust-lang#99064 (distinguish the method and associated function diagnostic information)
 - rust-lang#99920 (Custom allocator support in `rustc_serialize`)
 - rust-lang#100034 ( Elaborate all box dereferences in `ElaborateBoxDerefs`)
 - rust-lang#100076 (make slice::{split_at,split_at_unchecked} const functions)
 - rust-lang#100604 (Remove unstable Result::into_ok_or_err)
 - rust-lang#100933 (Reduce code size of `assert_matches_failed`)
 - rust-lang#100978 (Handle `Err` in `ast::LitKind::to_token_lit`.)
 - rust-lang#101010 (rustdoc: remove unused CSS for `.multi-column`)

Failed merges:

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

Author: bors

core/src/lib.rs

@@ -149,6 +149,7 @@
 #![feature(maybe_uninit_uninit_array)]
 #![feature(ptr_metadata)]
 #![feature(slice_ptr_get)]
+#![feature(slice_split_at_unchecked)]
 #![feature(str_internals)]
 #![feature(utf16_extra)]
 #![feature(utf16_extra_const)]

core/src/panicking.rs

@@ -190,11 +190,11 @@ pub fn assert_matches_failed<T: fmt::Debug + ?Sized>(
     right: &str,
     args: Option<fmt::Arguments<'_>>,
 ) -> ! {
-    // Use the Display implementation to display the pattern.
+    // The pattern is a string so it can be displayed directly.
     struct Pattern<'a>(&'a str);
     impl fmt::Debug for Pattern<'_> {
         fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-            fmt::Display::fmt(self.0, f)
+            f.write_str(self.0)
         }
     }
     assert_failed_inner(AssertKind::Match, &left, &Pattern(right), args);

core/src/result.rs

@@ -1776,40 +1776,6 @@ impl<T, E> Result<Result<T, E>, E> {
     }
 }
 
-impl<T> Result<T, T> {
-    /// Returns the [`Ok`] value if `self` is `Ok`, and the [`Err`] value if
-    /// `self` is `Err`.
-    ///
-    /// In other words, this function returns the value (the `T`) of a
-    /// `Result<T, T>`, regardless of whether or not that result is `Ok` or
-    /// `Err`.
-    ///
-    /// This can be useful in conjunction with APIs such as
-    /// [`Atomic*::compare_exchange`], or [`slice::binary_search`], but only in
-    /// cases where you don't care if the result was `Ok` or not.
-    ///
-    /// [`Atomic*::compare_exchange`]: crate::sync::atomic::AtomicBool::compare_exchange
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(result_into_ok_or_err)]
-    /// let ok: Result<u32, u32> = Ok(3);
-    /// let err: Result<u32, u32> = Err(4);
-    ///
-    /// assert_eq!(ok.into_ok_or_err(), 3);
-    /// assert_eq!(err.into_ok_or_err(), 4);
-    /// ```
-    #[inline]
-    #[unstable(feature = "result_into_ok_or_err", reason = "newly added", issue = "82223")]
-    pub const fn into_ok_or_err(self) -> T {
-        match self {
-            Ok(v) => v,
-            Err(v) => v,
-        }
-    }
-}
-
 // This is a separate function to reduce the code size of the methods
 #[cfg(not(feature = "panic_immediate_abort"))]
 #[inline(never)]

core/src/slice/mod.rs

@@ -1541,13 +1541,14 @@ impl<T> [T] {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_split_at_not_mut", issue = "none")]
     #[inline]
     #[track_caller]
     #[must_use]
-    pub fn split_at(&self, mid: usize) -> (&[T], &[T]) {
+    pub const fn split_at(&self, mid: usize) -> (&[T], &[T]) {
         assert!(mid <= self.len());
         // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
-        // fulfills the requirements of `from_raw_parts_mut`.
+        // fulfills the requirements of `split_at_unchecked`.
         unsafe { self.split_at_unchecked(mid) }
     }
 
@@ -1626,11 +1627,19 @@ impl<T> [T] {
     /// }
     /// ```
     #[unstable(feature = "slice_split_at_unchecked", reason = "new API", issue = "76014")]
+    #[rustc_const_unstable(feature = "slice_split_at_unchecked", issue = "76014")]
     #[inline]
     #[must_use]
-    pub unsafe fn split_at_unchecked(&self, mid: usize) -> (&[T], &[T]) {
+    pub const unsafe fn split_at_unchecked(&self, mid: usize) -> (&[T], &[T]) {
+        // HACK: the const function `from_raw_parts` is used to make this
+        // function const; previously the implementation used
+        // `(self.get_unchecked(..mid), self.get_unchecked(mid..))`
+
+        let len = self.len();
+        let ptr = self.as_ptr();
+
         // SAFETY: Caller has to check that `0 <= mid <= self.len()`
-        unsafe { (self.get_unchecked(..mid), self.get_unchecked(mid..)) }
+        unsafe { (from_raw_parts(ptr, mid), from_raw_parts(ptr.add(mid), len - mid)) }
     }
 
     /// Divides one mutable slice into two at an index, without doing bounds checking.

core/tests/lib.rs

@@ -75,7 +75,6 @@
 #![feature(const_pin)]
 #![feature(never_type)]
 #![feature(unwrap_infallible)]
-#![feature(result_into_ok_or_err)]
 #![feature(pointer_byte_offsets)]
 #![feature(portable_simd)]
 #![feature(ptr_metadata)]

core/tests/result.rs

@@ -95,15 +95,6 @@ fn test_unwrap_or() {
     assert_eq!(ok_err.unwrap_or(50), 50);
 }
 
-#[test]
-fn test_ok_or_err() {
-    let ok: Result<isize, isize> = Ok(100);
-    let err: Result<isize, isize> = Err(200);
-
-    assert_eq!(ok.into_ok_or_err(), 100);
-    assert_eq!(err.into_ok_or_err(), 200);
-}
-
 #[test]
 fn test_unwrap_or_else() {
     fn handler(msg: &'static str) -> isize {