Unwrapping `LayoutError` for overflowed `Rc<[T]>`/`Arc<[T]>` capacity

When constructing a `Rc<[T]>` or `Arc<[T]>` with a capacity exceeding `isize::MAX`, taking into account the size used for their headers, they cause a panic by unwrapping a `LayoutError`. Instead, they should panic with a more helpful error like `Vec<T>` and `Box<[T]>` do. This seems to be a simple oversight.

For example:

```rust
const MAX: usize = isize::MAX as usize;
type Header = (usize, usize);
const S: usize = size_of::<Header>();
const A: usize = align_of::<Header>();
Rc::<[u8]>::new_uninit_slice((MAX / A * A) - S + 1); // layout error
Rc::<[u8]>::new_zeroed_slice((MAX / A * A) - S + 1); // layout error
Arc::<[u8]>::new_uninit_slice((MAX / A * A) - S + 1); // layout error
Arc::<[u8]>::new_zeroed_slice((MAX / A * A) - S + 1); // layout error
```

```
called `Result::unwrap()` on an `Err` value: LayoutError
```

<details><summary>See all <code>LayoutError</code> panic backtraces</summary>

```
thread 'main' panicked at library/alloc/src/rc.rs:298:49:
called `Result::unwrap()` on an `Err` value: LayoutError
stack backtrace:
   3: core::result::Result<T,E>::unwrap
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/core/src/result.rs:1109:23
   4: alloc::rc::rc_inner_layout_for_value_layout
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/rc.rs:298:5
   5: alloc::rc::Rc<T>::allocate_for_layout
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/rc.rs:2057:22
   6: alloc::rc::Rc<[T]>::allocate_for_slice
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/rc.rs:2136:13
   7: alloc::rc::Rc<[T]>::new_uninit_slice
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/rc.rs:1050:31
```

```rust
Rc::<[u8]>::new_zeroed_slice((MAX / A * A) - S + 1);
```

```
thread 'main' panicked at library/alloc/src/rc.rs:298:49:
called `Result::unwrap()` on an `Err` value: LayoutError
stack backtrace:
   3: core::result::Result<T,E>::unwrap
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/core/src/result.rs:1109:23
   4: alloc::rc::rc_inner_layout_for_value_layout
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/rc.rs:298:5
   5: alloc::rc::Rc<T>::allocate_for_layout
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/rc.rs:2057:22
   6: alloc::rc::Rc<[T]>::new_zeroed_slice
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/rc.rs:1078:26
```

```rust
Arc::<[u8]>::new_uninit_slice((MAX / A * A) - S + 1);
```

```
thread 'main' panicked at library/alloc/src/sync.rs:367:50:
called `Result::unwrap()` on an `Err` value: LayoutError
stack backtrace:
   3: core::result::Result<T,E>::unwrap
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/core/src/result.rs:1109:23
   4: alloc::sync::arcinner_layout_for_value_layout
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/sync.rs:367:5
   5: alloc::sync::Arc<T>::allocate_for_layout
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/sync.rs:1950:22
   6: alloc::sync::Arc<[T]>::allocate_for_slice
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/sync.rs:2037:13
   7: alloc::sync::Arc<[T]>::new_uninit_slice
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/sync.rs:1168:32
```

```rust
Arc::<[u8]>::new_zeroed_slice((MAX / A * A) - S + 1);
```

```
thread 'main' panicked at library/alloc/src/sync.rs:367:50:
called `Result::unwrap()` on an `Err` value: LayoutError
stack backtrace:
   3: core::result::Result<T,E>::unwrap
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/core/src/result.rs:1109:23
   4: alloc::sync::arcinner_layout_for_value_layout
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/sync.rs:367:5
   5: alloc::sync::Arc<T>::allocate_for_layout
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/sync.rs:1950:22
   6: alloc::sync::Arc<[T]>::new_zeroed_slice
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/sync.rs:1197:27
```

</details>

In contrast, in this same case, `Vec<T>` and `Box<[T]>` panic with a more useful capacity overflow:

```rust
Vec::<u8>::with_capacity(MAX + 1); // capacity overflow
vec![0u8; MAX + 1]; // capacity overflow
Box::<[u8]>::new_uninit_slice(MAX + 1); // capacity overflow
Box::<[u8]>::new_zeroed_slice(MAX + 1); // capacity overflow
```

```
capacity overflow
```

<details><summary>See all capacity overflow backtraces</summary>

```rust
Vec::<u8>::with_capacity(MAX + 1);
```

```
thread 'main' panicked at src/main.rs:11:9:
capacity overflow
stack backtrace:
   2: alloc::raw_vec::capacity_overflow
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/raw_vec.rs:25:5
   3: alloc::raw_vec::handle_error
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/raw_vec.rs:793:29
   4: alloc::raw_vec::RawVecInner<A>::with_capacity_in
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/raw_vec.rs:426:25
   5: alloc::raw_vec::RawVec<T,A>::with_capacity_in
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/raw_vec.rs:186:20
   6: alloc::vec::Vec<T,A>::with_capacity_in
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:803:20
   7: alloc::vec::Vec<T>::with_capacity
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:483:9
```

```rust
vec![0u8; MAX + 1];
```

```
thread 'main' panicked at src/main.rs:12:9:
capacity overflow
stack backtrace:
   2: alloc::raw_vec::capacity_overflow
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/raw_vec.rs:25:5
   3: alloc::raw_vec::handle_error
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/raw_vec.rs:793:29
   4: alloc::raw_vec::RawVecInner<A>::with_capacity_zeroed_in
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/raw_vec.rs:445:25
   5: alloc::raw_vec::RawVec<T,A>::with_capacity_zeroed_in
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/raw_vec.rs:208:20
   6: <u8 as alloc::vec::spec_from_elem::SpecFromElem>::from_elem
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/vec/spec_from_elem.rs:55:31
   7: alloc::vec::from_elem
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:3189:5
```

```rust
Box::<[u8]>::new_uninit_slice(MAX + 1);
```

```
thread 'main' panicked at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:688:18:
capacity overflow
stack backtrace:
   2: alloc::raw_vec::capacity_overflow
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/raw_vec.rs:25:5
   3: alloc::raw_vec::handle_error
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/raw_vec.rs:793:29
   4: alloc::raw_vec::RawVecInner::with_capacity
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/raw_vec.rs:148:25
   5: alloc::raw_vec::RawVec<T>::with_capacity
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/raw_vec.rs:124:23
   6: alloc::boxed::Box<[T]>::new_uninit_slice
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:688:18
```

```rust
Box::<[u8]>::new_zeroed_slice(MAX + 1);
```

```
thread 'main' panicked at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:713:18:
capacity overflow
stack backtrace:
   2: alloc::raw_vec::capacity_overflow
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/raw_vec.rs:25:5
   3: alloc::raw_vec::handle_error
             at /rustc/43ca9d18e333797f0aa3b525501a7cec8d61a96b/library/alloc/src/raw_vec.rs:793:29
   4: alloc::raw_vec::RawVecInner<A>::with_capacity_zeroed_in
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/raw_vec.rs:445:25
   5: alloc::raw_vec::RawVec<T>::with_capacity_zeroed
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/raw_vec.rs:134:20
   6: alloc::boxed::Box<[T]>::new_zeroed_slice
             at ~/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/library/alloc/src/boxed.rs:713:18
```

```rust
Rc::<[u8]>::new_uninit_slice((MAX / A * A) - S + 1);
```

</details>

For completeness, all of these constructors consistently handle failed allocations for valid capacities with `alloc::handle_alloc_error`:

```rust
Vec::<u8>::with_capacity(MAX);
// memory allocation of 9223372036854775807 bytes failed
Rc::<[u8]>::new_zeroed_slice((MAX / A * A) - S);
// memory allocation of 9223372036854775800 bytes failed
```

<details><summary>See all memory allocation failures</summary>

```rust
Vec::<u8>::with_capacity(MAX);
```

```
memory allocation of 9223372036854775807 bytes failed
```

```rust
vec![0u8; MAX];
```

```
memory allocation of 9223372036854775807 bytes failed
```

```rust
Box::<[u8]>::new_uninit_slice(MAX);
```

```
memory allocation of 9223372036854775807 bytes failed
```

```rust
Box::<[u8]>::new_zeroed_slice(MAX);
```

```
memory allocation of 9223372036854775807 bytes failed
```

```rust
Rc::<[u8]>::new_uninit_slice((MAX / A * A) - S);
```

```
memory allocation of 9223372036854775800 bytes failed
```

```rust
Rc::<[u8]>::new_zeroed_slice((MAX / A * A) - S);
```

```
memory allocation of 9223372036854775800 bytes failed
```

```rust
Arc::<[u8]>::new_uninit_slice((MAX / A * A) - S);
```

```
memory allocation of 9223372036854775800 bytes failed
```

```rust
Arc::<[u8]>::new_zeroed_slice((MAX / A * A) - S);
```

```
memory allocation of 9223372036854775800 bytes failed
```

</details>

```rust
$ rustc --version --verbose
rustc 1.86.0-nightly (43ca9d18e 2025-02-08)
binary: rustc
commit-hash: 43ca9d18e333797f0aa3b525501a7cec8d61a96b
commit-date: 2025-02-08
host: x86_64-apple-darwin
release: 1.86.0-nightly
LLVM version: 19.1.7
```

(Similarly on Rust 1.84.1)

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Unwrapping `LayoutError` for overflowed `Rc<[T]>`/`Arc<[T]>` capacity #136797

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Unwrapping LayoutError for overflowed Rc<[T]>/Arc<[T]> capacity #136797

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Unwrapping `LayoutError` for overflowed `Rc<[T]>`/`Arc<[T]>` capacity #136797