Embedded and no_std Rust development constraints, patterns, and critical safety rules for microcontroller firmware.

• Enforces no dynamic allocation (heap-free design using heapless collections), no_std compilation, and static memory buffers for deterministic resource usage on resource-constrained devices
• Covers interrupt safety through critical sections and Mutex<RefCell> patterns to prevent race conditions when shared state is accessed from ISRs
• Provides hardware ownership patterns (singleton peripherals, HAL-based access) to prevent conflicting peripheral access and unsafe register manipulation
• Includes framework guidance for RTIC (priority-based), Embassy (async), and bare-metal approaches, plus key crates (cortex-m-rt, heapless, embedded-hal, defmt, probe-run)
• Documents common mistakes (Vec instead of heapless::Vec, missing critical sections, blocking in ISRs) with fixes tied to design patterns