Use Hadris without the standard library

Disable default features, then select the platform, I/O mode, and capabilities that the target needs:

[dependencies.hadris]
version = "3.0.0-rc.2"
default-features = false
features = ["alloc", "sync", "fat"]

Stable format crates have no read feature: reading is always compiled. FatFs and ExFatFs read and write with alloc, which holds their node table, and the write feature adds format and, with alloc, the tree writer write. FAT and exFAT format and check run on an unmounted device without an allocator, and the ISO 9660, UDF and NTFS readers and the CPIO reader need no allocator either.

Add alloc for the FAT and exFAT drivers, for the image writers (ISO 9660, UDF, the ISO/UDF bridge, CPIO, FAT and exFAT), which take a hadris::fs::Tree, and for owned names, copy_tree, read_tree and the async Volume. std implies alloc but does not select sync or async.

All storage I/O flows through hadris-storage block devices (every filesystem driver) or hadris-io streams (CPIO), so callers adapt firmware, kernel, memory, or device-specific readers rather than depending on std::io.

hadris::fs::sync::Volume requires std; allocation-only sync applications use the bare driver. The async Volume requires alloc instead. For FAT without an allocator, use hadris::fat::embedded, not FatFs.

Choose the narrowest tier

Need	Features
Allocation-free synchronous reader	sync
Allocation-free asynchronous reader	async
FAT read and write or exFAT read without an allocator (the embedded API)	sync or async
FAT or exFAT shared drivers (FatFs, ExFatFs)	Add alloc
FAT or exFAT formatting	Add write; no allocator required
Image writers, owned names or buffers	Add alloc
Several I/O modes	Enable each; the APIs live in separate namespaces

See the complete feature and capability matrix.

For integration examples, see Adapt a custom device and Use asynchronous I/O. Firmware on a microcontroller should read Use FAT and exFAT on a microcontroller, which has the flash and stack budget per target.