[rust-lang/rust] GlobalAlloc and Allocator traits missing `base + size < usize::MAX` from the allocation conditions

### ROOT CAUSE The `GlobalAlloc` and `Allocator` traits in Rust's standard library do not document the requirement that allocations must satisfy `base + size <= usize::MAX`. This omission can lead to unsafe implementations where an allocator returns a pointer that causes pointer wrap-around (e.g., allocating 1 byte at `usize::MAX`). While the Rust memory model already requires this condition via `ptr::add`, it is not explicitly stated in the trait documentation, creating a potential gap in safety guarantees. ### CODE FIX Update the documentation of the `GlobalAlloc` and `Allocator` traits to include the condition that the allocation must not cause pointer wrap-around. Specifically: 1. For `GlobalAlloc::alloc`: Add to the `Safety` section: ```rust - The allocation must not cause a pointer wrap-around, i.e., `base + size <= usize::MAX`. ``` 2. For `Allocator::alloc`: Add to the `Safety` section: ```rust - The allocation must not cause a pointer wrap-around, i.e., `base + size <= usize::MAX`. ``` These changes should be made in the Rust source code in the following files: - `src/liballoc/alloc.rs` - `src/liballoc/allocator.rs` This ensures consistency with Rust's memory safety guarantees and clarifies the requirements for implementers.