resize

    fn resize(
        _: *anyopaque,
        memory: []u8,
        alignment: Alignment,
        new_len: usize,
        return_address: usize,
    ) bool {
        _ = return_address;
        assert(new_len > 0);
        if (new_len <= memory.len) {
            return true; // in-place shrink always works
        }
        const mallocSize = func: {
            if (@TypeOf(c.malloc_size) != void) break :func c.malloc_size;
            if (@TypeOf(c.malloc_usable_size) != void) break :func c.malloc_usable_size;
            if (@TypeOf(c._msize) != void) break :func c._msize;
            return false; // we don't know how much space is actually available
        };
        const usable_len: usize = switch (allocStrat(alignment)) {
            .raw, .posix_memalign => mallocSize(memory.ptr),
            .manual_align => usable_len: {
                const unaligned_ptr = manualAlignHeader(memory.ptr).*;
                const full_len = mallocSize(unaligned_ptr);
                const padding = @intFromPtr(memory.ptr) - @intFromPtr(unaligned_ptr);
                break :usable_len full_len - padding;
            },
        };
        return new_len <= usable_len;
    }