no_std and Feature Verification 🔴

# Install
cargo install cargo-hack

# Check that every feature compiles individually
cargo hack check --each-feature --workspace

# The nuclear option: test ALL feature combinations (exponential!)
# Only practical for crates with <8 features.
cargo hack check --feature-powerset --workspace

# Practical compromise: test each feature alone + all features + no features
cargo hack check --each-feature --workspace --no-dev-deps
cargo check --workspace --all-features
cargo check --workspace --no-default-features

# Example: features that gate platform code
[features]
default = ["linux"]
linux = []                          # Linux-specific hardware access
windows = ["dep:windows-sys"]       # Windows-specific APIs
direct-ipmi = []                    # unsafe IPMI ioctl (ch05)
direct-accel-api = []                    # unsafe accel-mgmt FFI (ch05)

# Verify all features compile in isolation AND together
cargo hack check --each-feature -p diag_tool
# Catches: "feature 'windows' doesn't compile without 'direct-ipmi'"
# Catches: "#[cfg(feature = \"linux\")] has a typo — it's 'lnux'"

# Add to CI pipeline (fast — just compilation checks)
- name: Feature matrix check
  run: cargo hack check --each-feature --workspace --no-dev-deps

┌─────────────────────────────────────────────────────────────┐
│ std                                                         │
│  Everything in core + alloc, PLUS:                          │
│  • File I/O (std::fs, std::io)                              │
│  • Networking (std::net)                                    │
│  • Threads (std::thread)                                    │
│  • Time (std::time)                                         │
│  • Environment (std::env)                                   │
│  • Process (std::process)                                   │
│  • OS-specific (std::os::unix, std::os::windows)            │
├─────────────────────────────────────────────────────────────┤
│ alloc          (available with #![no_std] + extern crate    │
│                 alloc, if you have a global allocator)       │
│  • String, Vec, Box, Rc, Arc                                │
│  • BTreeMap, BTreeSet                                       │
│  • format!() macro                                          │
│  • Collections and smart pointers that need heap            │
├─────────────────────────────────────────────────────────────┤
│ core           (always available, even in #![no_std])        │
│  • Primitive types (u8, bool, char, etc.)                    │
│  • Option, Result                                           │
│  • Iterator, slice, array, str (slices, not String)         │
│  • Traits: Clone, Copy, Debug, Display, From, Into          │
│  • Atomics (core::sync::atomic)                             │
│  • Cell, RefCell (core::cell)  — Pin (core::pin)            │
│  • core::fmt (formatting without allocation)                │
│  • core::mem, core::ptr (low-level memory operations)       │
│  • Math: core::num, basic arithmetic                        │
└─────────────────────────────────────────────────────────────┘

// src/lib.rs — a no_std library crate
#![no_std]

// Optionally use heap allocation
extern crate alloc;
use alloc::string::String;
use alloc::vec::Vec;
use core::fmt;

/// Temperature reading from a thermal sensor.
/// This struct works in any environment — bare metal to Linux.
#[derive(Clone, Copy, Debug)]
pub struct Temperature {
    /// Raw sensor value (0.0625°C per LSB for typical I2C sensors)
    raw: u16,
}

impl Temperature {
    pub const fn from_raw(raw: u16) -> Self {
        Self { raw }
    }

    /// Convert to degrees Celsius (fixed-point, no FPU required)
    pub const fn millidegrees_c(&self) -> i32 {
        (self.raw as i32) * 625 / 10 // 0.0625°C resolution
    }

    pub fn degrees_c(&self) -> f32 {
        self.raw as f32 * 0.0625
    }
}

impl fmt::Display for Temperature {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let md = self.millidegrees_c();
        // Handle sign correctly for values between -0.999°C and -0.001°C
        // where md / 1000 == 0 but the value is negative.
        if md < 0 && md > -1000 {
            write!(f, "-0.{:03}°C", (-md) % 1000)
        } else {
            write!(f, "{}.{:03}°C", md / 1000, (md % 1000).abs())
        }
    }
}

/// Parse space-separated temperature values.
/// Uses alloc — requires a global allocator.
pub fn parse_temperatures(input: &str) -> Vec<Temperature> {
    input
        .split_whitespace()
        .filter_map(|s| s.parse::<u16>().ok())
        .map(Temperature::from_raw)
        .collect()
}

/// Format without allocation — writes directly to a buffer.
/// Works in `core`-only environments (no alloc, no heap).
pub fn format_temp_into(temp: &Temperature, buf: &mut [u8]) -> usize {
    use core::fmt::Write;
    struct SliceWriter<'a> {
        buf: &'a mut [u8],
        pos: usize,
    }
    impl<'a> Write for SliceWriter<'a> {
        fn write_str(&mut self, s: &str) -> fmt::Result {
            let bytes = s.as_bytes();
            let remaining = self.buf.len() - self.pos;
            if bytes.len() > remaining {
                // Buffer full — signal the error instead of silently truncating.
                // Callers can check the returned pos for partial writes.
                return Err(fmt::Error);
            }
            self.buf[self.pos..self.pos + bytes.len()].copy_from_slice(bytes);
            self.pos += bytes.len();
            Ok(())
        }
    }
    let mut w = SliceWriter { buf, pos: 0 };
    let _ = write!(w, "{}", temp);
    w.pos
}

# Cargo.toml for a no_std crate
[package]
name = "thermal-sensor"
version = "0.1.0"
edition = "2021"

[features]
default = ["alloc"]
alloc = []    # Enable Vec, String, etc.
std = []      # Enable full std (implies alloc)

[dependencies]
# Use no_std-compatible crates
serde = { version = "1.0", default-features = false, features = ["derive"] }
# ↑ default-features = false drops std dependency!

// src/main.rs — a no_std binary (e.g., UEFI diagnostic)
#![no_std]
#![no_main]

extern crate alloc;

use core::panic::PanicInfo;

// Required: what to do on panic (no stack unwinding available)
#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    // In embedded: blink an LED, write to UART, hang
    // In UEFI: write to console, halt
    // Minimal: just loop forever
    loop {
        core::hint::spin_loop();
    }
}

// Required if using alloc: provide a global allocator
use alloc::alloc::{GlobalAlloc, Layout};

struct BumpAllocator {
    // Simple bump allocator for embedded/UEFI
    // In practice, use a crate like `linked_list_allocator` or `embedded-alloc`
}

// WARNING: This is a non-functional placeholder! Calling alloc() will return
// null, causing immediate UB (the global allocator contract requires non-null
// returns for non-zero-sized allocations). In real code, use an established
// allocator crate:
//   - embedded-alloc (embedded targets)
//   - linked_list_allocator (UEFI / OS kernels)
//   - talc (general-purpose no_std)
unsafe impl GlobalAlloc for BumpAllocator {
    unsafe fn alloc(&self, _layout: Layout) -> *mut u8 {
        // PLACEHOLDER — will crash! Replace with real allocation logic.
        core::ptr::null_mut()
    }
    unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {
        // No-op for bump allocator
    }
}

#[global_allocator]
static ALLOCATOR: BumpAllocator = BumpAllocator {};

// Entry point (platform-specific, not fn main)
// For UEFI: #[entry] or efi_main
// For embedded: #[cortex_m_rt::entry]

// Your crate: #![no_std] in src/lib.rs
// But tests run under std automatically:

#[cfg(test)]
mod tests {
    use super::*;
    // std is available here — println!, assert!, Vec all work

    #[test]
    fn test_temperature_conversion() {
        let temp = Temperature::from_raw(800); // 50.0°C
        assert_eq!(temp.millidegrees_c(), 50000);
        assert!((temp.degrees_c() - 50.0).abs() < 0.01);
    }

    #[test]
    fn test_format_into_buffer() {
        let temp = Temperature::from_raw(800);
        let mut buf = [0u8; 32];
        let len = format_temp_into(&temp, &mut buf);
        let s = core::str::from_utf8(&buf[..len]).unwrap();
        assert_eq!(s, "50.000°C");
    }
}

# Use defmt-test for on-device testing (embedded ARM)
# Use uefi-test-runner for UEFI targets
# Use QEMU for cross-architecture tests without hardware

# Run no_std library tests on host (always works):
cargo test --lib

# Verify no_std compilation against a no_std target:
cargo check --target thumbv7em-none-eabihf  # ARM Cortex-M
cargo check --target riscv32imac-unknown-none-elf  # RISC-V

flowchart TD
    START["Does your code need\nthe standard library?"] --> NEED_FS{"File system,\nnetwork, threads?"}
    NEED_FS -->|"Yes"| USE_STD["Use std\nNormal application"]
    NEED_FS -->|"No"| NEED_HEAP{"Need heap allocation?\nVec, String, Box"}
    NEED_HEAP -->|"Yes"| USE_ALLOC["#![no_std]\nextern crate alloc"]
    NEED_HEAP -->|"No"| USE_CORE["#![no_std]\ncore only"]
    
    USE_ALLOC --> VERIFY["cargo-hack\n--each-feature"]
    USE_CORE --> VERIFY
    USE_STD --> VERIFY
    VERIFY --> TARGET{"Target has OS?"}
    TARGET -->|"Yes"| HOST_TEST["cargo test --lib\nStandard testing"]
    TARGET -->|"No"| CROSS_TEST["QEMU / defmt-test\nOn-device testing"]
    
    style USE_STD fill:#91e5a3,color:#000
    style USE_ALLOC fill:#ffd43b,color:#000
    style USE_CORE fill:#ff6b6b,color:#000

cargo install cargo-hack

# Check each feature individually
cargo hack check --each-feature --workspace --no-dev-deps

# If a feature combination fails:
# error[E0433]: failed to resolve: use of undeclared crate or module `std`
# → This means a feature gate is missing a #[cfg] guard

# Check all features + no features + each individually:
cargo hack check --each-feature --workspace
cargo check --workspace --all-features
cargo check --workspace --no-default-features

// lib.rs
#![no_std]

pub struct RingBuffer<const N: usize> {
    data: [u8; N],
    head: usize,
    len: usize,
}

impl<const N: usize> RingBuffer<N> {
    pub const fn new() -> Self {
        Self { data: [0; N], head: 0, len: 0 }
    }

    pub fn push(&mut self, byte: u8) -> bool {
        if self.len == N { return false; }
        let idx = (self.head + self.len) % N;
        self.data[idx] = byte;
        self.len += 1;
        true
    }

    pub fn pop(&mut self) -> Option<u8> {
        if self.len == 0 { return None; }
        let byte = self.data[self.head];
        self.head = (self.head + 1) % N;
        self.len -= 1;
        Some(byte)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn push_pop() {
        let mut rb = RingBuffer::<4>::new();
        assert!(rb.push(1));
        assert!(rb.push(2));
        assert_eq!(rb.pop(), Some(1));
        assert_eq!(rb.pop(), Some(2));
        assert_eq!(rb.pop(), None);
    }
}

rustup target add thumbv7em-none-eabihf
cargo check --target thumbv7em-none-eabihf
# ✅ Compiles for bare-metal ARM