Macro custom_heap_default 

macro_rules! custom_heap_default {
    () => { ... };
}

Define the default global allocator.

The default global allocator is enabled only if the calling crate has not disabled it using Cargo features as described below. It is only defined for on-chain targets.

Cargo features

A crate that calls this macro can provide its own custom heap implementation, or allow others to provide their own custom heap implementation, by adding a custom-heap feature to its Cargo.toml. After enabling the feature, one may define their own global allocator in the standard way.

RISC-V heap base

On the RISC-V target the heap base is supplied by the loader at runtime, not fixed at compile time. The entrypoint! and entrypoint_no_alloc! macros record it (via allocator::set_heap_base) before any allocation runs. If you install this allocator behind a hand-rolled entrypoint instead of those macros, you must call both set_heap_base and set_heap_limit at the very start of the entrypoint. Omitting set_heap_base leaves the heap at address 0, so the first allocation hits unmapped memory and traps; omitting set_heap_limit leaves the limit at 0, so the allocator refuses every allocation (null → handle_alloc_error → abort).