How do you implement a custom global allocator in Rust?

Rust allows replacing the default memory allocator (mimalloc on most platforms or the system allocator) with a custom one by implementing the GlobalAlloc trait and registering it with the #[global_allocator] attribute. The trait requires two unsafe methods: fn alloc(&self, layout: Layout) -> *mut u8 (allocate memory matching the given size and alignment) and fn dealloc(&self, ptr: *mut u8, layout: Layout) (free previously allocated memory). A common use case is switching to a faster allocator like mimalloc or jemalloc: add the mimalloc crate and write #[global_allocator] static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc;. Custom allocators are also used in embedded systems (to use a fixed-size arena instead of dynamic heap), for memory tracking/profiling, and for adding guard pages or canaries around allocations for security.