rust-impl-no-std

The no_std Rust skill: how to opt out of the standard library with #![no_std], work against core only, pull in alloc when a heap is available, register a #[panic_handler] and optional #[global_allocator], and design a hybrid library that builds on both std and no_std consumers via a default = ["std"] feature.

Cross-references: [[rust-core-stdlib-overview]] (what std contains that core does not), [[rust-impl-cargo-project]] (features, profiles, panic = "abort"), [[rust-syntax-unsafe]] (#[global_allocator] involves the unsafe GlobalAlloc trait), [[rust-impl-cross-compile]] (thumbv7em-none-eabihf and other bare-metal targets).

When to use this skill

• User adds #![no_std] to lib.rs / main.rs and the compiler starts complaining about missing types.
• User targets an embedded board (Cortex-M, RISC-V MCU) and asks how to structure the crate.
• User wants Vec / Box / String in a no_std crate and is unsure whether alloc is allowed.
• User writes their first #[panic_handler] or hits the "duplicate panic handler" link error.
• User wants to plug in a custom allocator via #[global_allocator].
• User has a library that some users will consume from std applications and others from no_std firmware, and asks how to support both.
• User asks why std::format! doesn't compile in their crate.
• User asks why HashMap cannot be imported from alloc.