Rollup merge of #133598 - ChayimFriedman2:get-many-mut-detailed-err, r=scottmcm

Change `GetManyMutError` to match T-libs-api decision

That is, differentiate between out-of-bounds and overlapping indices, and remove the generic parameter `N`.

I also exported `GetManyMutError` from `alloc` (and `std`), which was apparently forgotten.

Changing the error to carry additional details means LLVM no longer generates separate short-circuiting branches for the checks, instead it generates one branch at the end. I therefore changed the  code to use early returns to make LLVM generate jumps. Benchmark results between the approaches are somewhat mixed, but I chose this approach because it is significantly faster with ranges and also faster with `unwrap()`.

Benchmark (`jumps` refer to short-circuiting, `acc` is not short-circuiting):
```rust
use criterion::{black_box, criterion_group, criterion_main, Criterion};
use my_crate::{get_many_check_valid_acc, get_many_check_valid_jumps, GetManyMutError};

mod externs {
    #[unsafe(no_mangle)]
    fn foo() {}
    #[unsafe(no_mangle)]
    fn bar() {}
    #[unsafe(no_mangle)]
    fn baz() {}
}

unsafe extern "C" {
    safe fn foo();
    safe fn bar();
    safe fn baz();
}

fn bench_method(c: &mut Criterion) {
    c.bench_function("jumps two usize", |b| {
        b.iter(|| get_many_check_valid_jumps(&[black_box(1), black_box(5)], black_box(10)))
    });
    c.bench_function("jumps two usize unwrap", |b| {
        b.iter(|| get_many_check_valid_jumps(&[black_box(1), black_box(5)], black_box(10)).unwrap())
    });
    c.bench_function("jumps two usize ok", |b| {
        b.iter(|| get_many_check_valid_jumps(&[black_box(1), black_box(5)], black_box(10)).ok())
    });
    c.bench_function("jumps three usize", |b| {
        b.iter(|| {
            get_many_check_valid_jumps(&[black_box(1), black_box(5), black_box(7)], black_box(10))
        })
    });
    c.bench_function("jumps three usize match", |b| {
        b.iter(|| {
            match get_many_check_valid_jumps(
                &[black_box(1), black_box(5), black_box(7)],
                black_box(10),
            ) {
                Err(GetManyMutError::IndexOutOfBounds) => foo(),
                Err(GetManyMutError::OverlappingIndices) => bar(),
                Ok(()) => baz(),
            }
        })
    });
    c.bench_function("jumps two Range", |b| {
        b.iter(|| {
            get_many_check_valid_jumps(
                &[black_box(1)..black_box(5), black_box(7)..black_box(8)],
                black_box(10),
            )
        })
    });
    c.bench_function("jumps two RangeInclusive", |b| {
        b.iter(|| {
            get_many_check_valid_jumps(
                &[black_box(1)..=black_box(5), black_box(7)..=black_box(8)],
                black_box(10),
            )
        })
    });

    c.bench_function("acc two usize", |b| {
        b.iter(|| get_many_check_valid_acc(&[black_box(1), black_box(5)], black_box(10)))
    });
    c.bench_function("acc two usize unwrap", |b| {
        b.iter(|| get_many_check_valid_acc(&[black_box(1), black_box(5)], black_box(10)).unwrap())
    });
    c.bench_function("acc two usize ok", |b| {
        b.iter(|| get_many_check_valid_acc(&[black_box(1), black_box(5)], black_box(10)).ok())
    });
    c.bench_function("acc three usize", |b| {
        b.iter(|| {
            get_many_check_valid_acc(&[black_box(1), black_box(5), black_box(7)], black_box(10))
        })
    });
    c.bench_function("acc three usize match", |b| {
        b.iter(|| {
            match get_many_check_valid_jumps(
                &[black_box(1), black_box(5), black_box(7)],
                black_box(10),
            ) {
                Err(GetManyMutError::IndexOutOfBounds) => foo(),
                Err(GetManyMutError::OverlappingIndices) => bar(),
                Ok(()) => baz(),
            }
        })
    });
    c.bench_function("acc two Range", |b| {
        b.iter(|| {
            get_many_check_valid_acc(
                &[black_box(1)..black_box(5), black_box(7)..black_box(8)],
                black_box(10),
            )
        })
    });
    c.bench_function("acc two RangeInclusive", |b| {
        b.iter(|| {
            get_many_check_valid_acc(
                &[black_box(1)..=black_box(5), black_box(7)..=black_box(8)],
                black_box(10),
            )
        })
    });
}

criterion_group!(benches, bench_method);
criterion_main!(benches);
```
Benchmark results:
```none
jumps two usize          time:   [586.44 ps 590.20 ps 594.50 ps]
jumps two usize unwrap   time:   [390.44 ps 393.63 ps 397.44 ps]
jumps two usize ok       time:   [585.52 ps 591.74 ps 599.38 ps]
jumps three usize        time:   [976.51 ps 983.79 ps 991.51 ps]
jumps three usize match  time:   [390.82 ps 393.80 ps 397.07 ps]
jumps two Range          time:   [1.2583 ns 1.2640 ns 1.2695 ns]
jumps two RangeInclusive time:   [1.2673 ns 1.2770 ns 1.2877 ns]
acc two usize            time:   [592.63 ps 596.44 ps 600.52 ps]
acc two usize unwrap     time:   [582.65 ps 587.07 ps 591.90 ps]
acc two usize ok         time:   [581.59 ps 587.82 ps 595.71 ps]
acc three usize          time:   [894.69 ps 901.23 ps 908.24 ps]
acc three usize match    time:   [392.68 ps 395.73 ps 399.17 ps]
acc two Range            time:   [1.5531 ns 1.5617 ns 1.5711 ns]
acc two RangeInclusive   time:   [1.5746 ns 1.5840 ns 1.5939 ns]
```

3 files changed, 34 insertions, 28 deletions

diff --git a/library/alloc/src/slice.rs b/library/alloc/src/slice.rs
index e3c7835f1d1..edc8d99f2f9 100644
--- a/library/alloc/src/slice.rs
+++ b/library/alloc/src/slice.rs
@@ -27,6 +27,8 @@ pub use core::slice::ArrayChunksMut;
 pub use core::slice::ArrayWindows;
 #[stable(feature = "inherent_ascii_escape", since = "1.60.0")]
 pub use core::slice::EscapeAscii;
+#[unstable(feature = "get_many_mut", issue = "104642")]
+pub use core::slice::GetManyMutError;
 #[stable(feature = "slice_get_slice", since = "1.28.0")]
 pub use core::slice::SliceIndex;
 #[cfg(not(no_global_oom_handling))]
diff --git a/library/core/src/error.rs b/library/core/src/error.rs
index 95a39cc3aed..91549f49f9f 100644
--- a/library/core/src/error.rs
+++ b/library/core/src/error.rs
@@ -1076,4 +1076,4 @@ impl Error for crate::time::TryFromFloatSecsError {}
 impl Error for crate::ffi::FromBytesUntilNulError {}
 
 #[unstable(feature = "get_many_mut", issue = "104642")]
-impl<const N: usize> Error for crate::slice::GetManyMutError<N> {}
+impl Error for crate::slice::GetManyMutError {}
diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index 191eaccff98..97a7338b0bf 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -4629,13 +4629,11 @@ impl<T> [T] {
     pub fn get_many_mut<I, const N: usize>(
         &mut self,
         indices: [I; N],
-    ) -> Result<[&mut I::Output; N], GetManyMutError<N>>
+    ) -> Result<[&mut I::Output; N], GetManyMutError>
     where
         I: GetManyMutIndex + SliceIndex<Self>,
     {
-        if !get_many_check_valid(&indices, self.len()) {
-            return Err(GetManyMutError { _private: () });
-        }
+        get_many_check_valid(&indices, self.len())?;
         // SAFETY: The `get_many_check_valid()` call checked that all indices
         // are disjunct and in bounds.
         unsafe { Ok(self.get_many_unchecked_mut(indices)) }
@@ -4978,53 +4976,59 @@ impl<T, const N: usize> SlicePattern for [T; N] {
 /// This will do `binomial(N + 1, 2) = N * (N + 1) / 2 = 0, 1, 3, 6, 10, ..`
 /// comparison operations.
 #[inline]
-fn get_many_check_valid<I: GetManyMutIndex, const N: usize>(indices: &[I; N], len: usize) -> bool {
+fn get_many_check_valid<I: GetManyMutIndex, const N: usize>(
+    indices: &[I; N],
+    len: usize,
+) -> Result<(), GetManyMutError> {
     // NB: The optimizer should inline the loops into a sequence
     // of instructions without additional branching.
-    let mut valid = true;
     for (i, idx) in indices.iter().enumerate() {
-        valid &= idx.is_in_bounds(len);
+        if !idx.is_in_bounds(len) {
+            return Err(GetManyMutError::IndexOutOfBounds);
+        }
         for idx2 in &indices[..i] {
-            valid &= !idx.is_overlapping(idx2);
+            if idx.is_overlapping(idx2) {
+                return Err(GetManyMutError::OverlappingIndices);
+            }
         }
     }
-    valid
+    Ok(())
 }
 
-/// The error type returned by [`get_many_mut<N>`][`slice::get_many_mut`].
+/// The error type returned by [`get_many_mut`][`slice::get_many_mut`].
 ///
 /// It indicates one of two possible errors:
 /// - An index is out-of-bounds.
-/// - The same index appeared multiple times in the array.
+/// - The same index appeared multiple times in the array
+///   (or different but overlapping indices when ranges are provided).
 ///
 /// # Examples
 ///
 /// ```
 /// #![feature(get_many_mut)]
+/// use std::slice::GetManyMutError;
 ///
 /// let v = &mut [1, 2, 3];
-/// assert!(v.get_many_mut([0, 999]).is_err());
-/// assert!(v.get_many_mut([1, 1]).is_err());
+/// assert_eq!(v.get_many_mut([0, 999]), Err(GetManyMutError::IndexOutOfBounds));
+/// assert_eq!(v.get_many_mut([1, 1]), Err(GetManyMutError::OverlappingIndices));
 /// ```
 #[unstable(feature = "get_many_mut", issue = "104642")]
-// NB: The N here is there to be forward-compatible with adding more details
-// to the error type at a later point
-#[derive(Clone, PartialEq, Eq)]
-pub struct GetManyMutError<const N: usize> {
-    _private: (),
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum GetManyMutError {
+    /// An index provided was out-of-bounds for the slice.
+    IndexOutOfBounds,
+    /// Two indices provided were overlapping.
+    OverlappingIndices,
 }
 
 #[unstable(feature = "get_many_mut", issue = "104642")]
-impl<const N: usize> fmt::Debug for GetManyMutError<N> {
+impl fmt::Display for GetManyMutError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("GetManyMutError").finish_non_exhaustive()
-    }
-}
-
-#[unstable(feature = "get_many_mut", issue = "104642")]
-impl<const N: usize> fmt::Display for GetManyMutError<N> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Display::fmt("an index is out of bounds or appeared multiple times in the array", f)
+        let msg = match self {
+            GetManyMutError::IndexOutOfBounds => "an index is out of bounds",
+            GetManyMutError::OverlappingIndices => "there were overlapping indices",
+        };
+        fmt::Display::fmt(msg, f)
     }
 }