TIL: Optimizing Rust Compile Times

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// Rust's compilation model leads to inherent trade-offs
// Let's examine what makes compilation slow and how to optimize it

// 1. Monomorphization - Generic code is compiled for each concrete type
use std::collections::HashMap;

// This generic function will be compiled separately for each type used
fn process_collection<T: Clone + std::fmt::Debug>(items: &[T]) -> Vec<T> {
    println!("Processing {} items", items.len());
    items.iter().cloned().collect()
}

// Each usage creates a new monomorphized version
fn demonstrate_monomorphization() {
    let numbers = vec![1, 2, 3, 4, 5];
    let strings = vec!["a", "b", "c"];
    let floats = vec![1.0, 2.0, 3.0];
    
    // Three separate compiled versions of process_collection
    let _processed_numbers = process_collection(&numbers);     // process_collection<i32>
    let _processed_strings = process_collection(&strings);     // process_collection<&str>
    let _processed_floats = process_collection(&floats);       // process_collection<f64>
}

// 2. Dependency graph complexity - Each crate must be compiled in order
// Visualization of a typical dependency chain:
/*
your_app
├── serde (1.0.126)
│   └── serde_derive (1.0.126)
├── tokio (1.8.1)
│   ├── tokio-macros (1.3.0)
│   ├── pin-project-lite (0.2.6)
│   ├── bytes (1.0.1)
│   └── parking_lot (0.11.1)
│       └── parking_lot_core (0.8.3)
│           └── smallvec (1.6.1)
└── clap (2.33.3)
    ├── bitflags (1.2.1)
    ├── textwrap (0.11.0)
    └── ansi_term (0.11.0)
*/

// 3. Trait resolution complexity
trait ComplexTrait<T> {
    type Output;
    fn complex_method(&self, input: T) -> Self::Output;
}

// The compiler must resolve all these relationships
impl<T: Clone + std::fmt::Debug> ComplexTrait<T> for Vec<T> {
    type Output = String;
    
    fn complex_method(&self, _input: T) -> Self::Output {
        format!("Processed {} items", self.len())
    }
}

// Usage requires complex trait resolution
fn use_complex_trait<C, T>(container: &C, item: T) -> C::Output 
where 
    C: ComplexTrait<T>,
    T: Clone + std::fmt::Debug,
{
    container.complex_method(item)
}

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# Cargo.toml optimization strategies

[dependencies]
# Use specific features instead of default-features
serde = { version = "1.0", default-features = false, features = ["derive"] }
tokio = { version = "1.0", default-features = false, features = ["rt-multi-thread", "net"] }

# Avoid unnecessary dependencies
# Instead of pulling in a large crate for one function:
# regex = "1.0"  # Heavy dependency

# Consider lighter alternatives or std library functions
# Use std::str for simple pattern matching when possible

[dev-dependencies]
# Keep test dependencies separate to avoid including them in release builds
criterion = "0.3"
proptest = "1.0"

[build-dependencies]
# Only include build dependencies when necessary
cc = "1.0"

# Profile-specific dependencies
[target.'cfg(unix)'.dependencies]
libc = "0.2"

[target.'cfg(windows)'.dependencies]
winapi = { version = "0.3", features = ["processenv"] }

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// lib.rs - Use feature flags to conditionally compile code
#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

#[derive(Debug, Clone)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
pub struct MyStruct {
    pub name: String,
    pub value: i32,
}

impl MyStruct {
    pub fn new(name: String, value: i32) -> Self {
        Self { name, value }
    }
    
    #[cfg(feature = "json")]
    pub fn to_json(&self) -> Result<String, serde_json::Error> {
        serde_json::to_string(self)
    }
    
    #[cfg(feature = "yaml")]
    pub fn to_yaml(&self) -> Result<String, serde_yaml::Error> {
        serde_yaml::to_string(self)
    }
}

// Optional expensive computations
#[cfg(feature = "advanced-math")]
pub mod advanced_math {
    pub fn complex_calculation(input: f64) -> f64 {
        // Expensive mathematical operations
        input.powi(10) + input.ln() + input.sin().cos()
    }
}

#[cfg(not(feature = "advanced-math"))]
pub mod advanced_math {
    pub fn complex_calculation(input: f64) -> f64 {
        // Simple fallback
        input * 2.0
    }
}

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# Cargo.toml feature configuration
[features]
default = ["std"]
std = []
serde = ["dep:serde"]
json = ["serde", "dep:serde_json"]
yaml = ["serde", "dep:serde_yaml"]
advanced-math = []

# Optional dependencies (only compiled when features are enabled)
[dependencies]
serde = { version = "1.0", optional = true }
serde_json = { version = "1.0", optional = true }
serde_yaml = { version = "0.8", optional = true }

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# Set up environment for faster compilation
export CARGO_INCREMENTAL=1
export RUSTC_WRAPPER=sccache  # Use sccache for compilation caching

# Create .cargo/config.toml for project-specific settings
mkdir -p .cargo
cat > .cargo/config.toml << 'EOF'
[build]
# Use all available CPU cores
jobs = 0

# Enable incremental compilation
incremental = true

# Use faster linker on Linux
[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=lld"]

# Use faster linker on macOS
[target.x86_64-apple-darwin]
rustflags = ["-C", "link-arg=-fuse-ld=lld"]

[cargo-new]
vcs = "git"
EOF

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# Cargo.toml - Optimized compilation profiles

[profile.dev]
# Faster debug builds
opt-level = 1           # Some optimization for better debug performance
debug = true           # Keep debug info
incremental = true     # Enable incremental compilation
codegen-units = 256    # Parallel code generation (trades runtime speed for compile speed)

[profile.dev.package."*"]
# Compile dependencies with optimization even in debug mode
opt-level = 3
debug = false

[profile.release]
# Production optimizations
opt-level = 3
lto = "thin"           # Link-time optimization (slower compile, faster runtime)
codegen-units = 1      # Better runtime optimization
panic = "abort"        # Smaller binary size
strip = true           # Remove debug symbols

[profile.bench]
# Benchmarking profile
inherits = "release"
debug = true           # Keep debug info for profiling
lto = "fat"           # Aggressive LTO for maximum performance

[profile.test]
# Fast test compilation
inherits = "dev"
opt-level = 1

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# Workspace Cargo.toml for multi-crate projects
[workspace]
members = [
    "core",
    "cli",
    "web", 
    "utils"
]

# Shared dependencies across workspace
[workspace.dependencies]
serde = { version = "1.0", features = ["derive"] }
tokio = { version = "1.0", features = ["rt-multi-thread"] }
anyhow = "1.0"
thiserror = "1.0"

# Individual crate Cargo.toml can inherit workspace dependencies
# core/Cargo.toml
[dependencies]
serde = { workspace = true }
tokio = { workspace = true }
anyhow = { workspace = true }

# Local dependencies
utils = { path = "../utils" }

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// build.rs - Efficient build scripts
use std::env;
use std::path::PathBuf;

fn main() {
    // Only rebuild when specific files change
    println!("cargo:rerun-if-changed=proto/");
    println!("cargo:rerun-if-changed=build.rs");
    
    // Skip expensive operations in debug builds
    let profile = env::var("PROFILE").unwrap_or_default();
    if profile == "debug" {
        println!("cargo:warning=Skipping expensive build operations in debug mode");
        return;
    }
    
    // Use parallel processing when possible
    let num_jobs = env::var("NUM_JOBS")
        .ok()
        .and_then(|s| s.parse().ok())
        .unwrap_or_else(|| std::thread::available_parallelism().unwrap().get());
    
    // Example: Protocol buffer compilation
    let proto_files = ["proto/api.proto", "proto/messages.proto"];
    
    // Configure protobuf compilation
    let mut config = prost_build::Config::new();
    config.out_dir("src/generated/");
    
    // Only recompile if proto files changed
    for proto_file in &proto_files {
        if is_newer_than_target(proto_file) {
            config.compile_protos(&[proto_file], &["proto/"]).unwrap();
        }
    }
}

fn is_newer_than_target(source: &str) -> bool {
    use std::fs;
    use std::time::SystemTime;
    
    let source_time = fs::metadata(source)
        .and_then(|m| m.modified())
        .unwrap_or(SystemTime::UNIX_EPOCH);
    
    let target_time = fs::metadata("src/generated/")
        .and_then(|m| m.modified())
        .unwrap_or(SystemTime::UNIX_EPOCH);
    
    source_time > target_time
}

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// Advanced techniques for compile-time optimization

// 1. Reduce monomorphization by using trait objects
trait ProcessItem {
    fn process(&self) -> String;
}

// Instead of generic functions that get monomorphized:
fn process_items_generic<T: ProcessItem>(items: &[T]) -> Vec<String> {
    items.iter().map(|item| item.process()).collect()
}

// Use trait objects to reduce compilation:
fn process_items_dynamic(items: &[&dyn ProcessItem]) -> Vec<String> {
    items.iter().map(|item| item.process()).collect()
}

// 2. Use const generics sparingly
struct Matrix<const N: usize, const M: usize> {
    data: [[f64; M]; N],
}

// This creates many monomorphized versions - use judiciously
impl<const N: usize, const M: usize> Matrix<N, M> {
    fn new() -> Self {
        Self { data: [[0.0; M]; N] }
    }
}

// 3. Prefer composition over deep trait bounds
// Instead of:
// fn complex_function<T: Clone + Debug + Send + Sync + Serialize + DeserializeOwned>

// Consider creating a marker trait:
trait ComplexRequirements: Clone + std::fmt::Debug + Send + Sync {}

// Automatically implement for types that satisfy requirements
impl<T> ComplexRequirements for T 
where T: Clone + std::fmt::Debug + Send + Sync {}

fn complex_function<T: ComplexRequirements>(input: T) -> T {
    input
}

// 4. Use macros to reduce boilerplate without runtime cost
macro_rules! impl_display_for_enum {
    ($enum_name:ident { $($variant:ident),* }) => {
        impl std::fmt::Display for $enum_name {
            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                match self {
                    $(Self::$variant => write!(f, stringify!($variant)),)*
                }
            }
        }
    };
}

#[derive(Debug)]
enum Status {
    Ready,
    Processing, 
    Complete,
    Error,
}

impl_display_for_enum!(Status { Ready, Processing, Complete, Error });

// 5. Strategic use of inline annotations
#[inline]
fn small_frequently_called_function(x: i32) -> i32 {
    x * 2 + 1
}

#[inline(never)]
fn large_rarely_called_function() -> Vec<String> {
    // Large function that shouldn't be inlined
    (0..1000).map(|i| format!("Item {}", i)).collect()
}

#[inline(always)]
fn tiny_critical_function(x: u32) -> u32 {
    x.rotate_left(1)
}

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# Measure compilation times
cargo clean
time cargo build

# Analyze what takes the most time
cargo build --timings
# This creates a cargo-timing.html file with detailed analysis

# Profile individual crate compilation
cargo build -Z timings --package your_crate_name

# Use cargo-llvm-lines to see which functions generate the most LLVM IR
cargo install cargo-llvm-lines
cargo llvm-lines | head -20

# Use cargo-bloat to analyze binary size
cargo install cargo-bloat
cargo bloat --release

# Analyze compilation bottlenecks
cargo +nightly build -Z time-passes 2>&1 | head -20

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// analyze_deps.rs - Script to analyze dependency compilation impact
use std::process::Command;
use std::time::Instant;
use std::collections::HashMap;

fn main() {
    let dependencies = vec![
        "serde",
        "tokio", 
        "clap",
        "reqwest",
        "diesel",
    ];
    
    let mut compile_times = HashMap::new();
    
    for dep in dependencies {
        println!("Testing compilation time for {}", dep);
        
        // Create minimal test project
        let output = Command::new("cargo")
            .args(&["new", "--bin", &format!("test-{}", dep)])
            .output()
            .expect("Failed to create test project");
        
        if !output.status.success() {
            eprintln!("Failed to create project for {}", dep);
            continue;
        }
        
        // Add dependency
        let cargo_toml = format!(
            r#"
[package]
name = "test-{}"
version = "0.1.0"
edition = "2021"

[dependencies]
{} = "*"
"#,
            dep, dep
        );
        
        std::fs::write(format!("test-{}/Cargo.toml", dep), cargo_toml)
            .expect("Failed to write Cargo.toml");
        
        // Measure compilation time
        let start = Instant::now();
        let output = Command::new("cargo")
            .args(&["build"])
            .current_dir(&format!("test-{}", dep))
            .output()
            .expect("Failed to compile");
        
        let compile_time = start.elapsed();
        
        if output.status.success() {
            compile_times.insert(dep.to_string(), compile_time);
            println!("{}: {:?}", dep, compile_time);
        } else {
            eprintln!("Failed to compile {}", dep);
        }
        
        // Cleanup
        std::fs::remove_dir_all(format!("test-{}", dep))
            .unwrap_or_default();
    }
    
    // Sort by compilation time
    let mut sorted_times: Vec<_> = compile_times.iter().collect();
    sorted_times.sort_by_key(|(_, time)| *time);
    
    println!("\nCompilation times (fastest to slowest):");
    for (dep, time) in sorted_times {
        println!("{}: {:?}", dep, time);
    }
}

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# .github/workflows/ci.yml - Optimized CI for Rust projects
name: CI

on: [push, pull_request]

env:
  CARGO_TERM_COLOR: always
  RUSTFLAGS: "-D warnings"

jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        rust-version: [stable, beta]
    
    steps:
    - uses: actions/checkout@v3
    
    # Cache Rust toolchain
    - name: Cache Rust toolchain
      uses: actions/cache@v3
      with:
        path: |
          ~/.cargo/registry
          ~/.cargo/git
          ~/.rustup
        key: ${{ runner.os }}-rust-${{ matrix.rust-version }}-${{ hashFiles('**/Cargo.lock') }}
        restore-keys: |
          ${{ runner.os }}-rust-${{ matrix.rust-version }}-
          ${{ runner.os }}-rust-
    
    # Install Rust
    - name: Install Rust
      uses: actions-rs/toolchain@v1
      with:
        toolchain: ${{ matrix.rust-version }}
        profile: minimal
        override: true
        components: rustfmt, clippy
    
    # Cache cargo build
    - name: Cache cargo build
      uses: actions/cache@v3
      with:
        path: target
        key: ${{ runner.os }}-cargo-${{ matrix.rust-version }}-${{ hashFiles('**/Cargo.lock') }}
        restore-keys: |
          ${{ runner.os }}-cargo-${{ matrix.rust-version }}-
          ${{ runner.os }}-cargo-
    
    # Check formatting first (fastest)
    - name: Check formatting
      run: cargo fmt --all -- --check
    
    # Run clippy (catches issues early)
    - name: Run clippy
      run: cargo clippy --all-targets --all-features -- -D warnings
    
    # Build (use --locked to ensure reproducible builds)
    - name: Build
      run: cargo build --locked --all-features
    
    # Test
    - name: Run tests
      run: cargo test --all-features
    
    # Build documentation
    - name: Build docs
      run: cargo doc --no-deps --all-features

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// benches/compilation_benchmark.rs
use criterion::{black_box, criterion_group, criterion_main, Criterion};

// Benchmark functions that might affect compile time
fn expensive_generic_function<T: Clone + std::fmt::Debug>(items: &[T]) -> Vec<T> {
    items.iter().cloned().collect()
}

fn optimized_function(items: &[i32]) -> Vec<i32> {
    items.to_vec()
}

fn benchmark_monomorphization(c: &mut Criterion) {
    let data = vec![1, 2, 3, 4, 5];
    
    c.bench_function("generic function", |b| {
        b.iter(|| expensive_generic_function(black_box(&data)))
    });
    
    c.bench_function("specialized function", |b| {
        b.iter(|| optimized_function(black_box(&data)))
    });
}

criterion_group!(benches, benchmark_monomorphization);
criterion_main!(benches);

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// Example of a compile-time optimized Rust project structure
// src/lib.rs

#![warn(missing_docs)]
#![cfg_attr(not(feature = "std"), no_std)]

// Feature-gated modules
#[cfg(feature = "serde")]
pub mod serialization;

#[cfg(feature = "async")]
pub mod async_utils;

// Core functionality always available
pub mod core;
pub mod utils;

// Re-exports for convenience
pub use core::*;

// Conditional re-exports
#[cfg(feature = "serde")]
pub use serialization::*;

#[cfg(feature = "async")]
pub use async_utils::*;

// Type aliases to reduce monomorphization
pub type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>;
pub type HashMap<K, V> = std::collections::HashMap<K, V, ahash::RandomState>;

// Prelude module for common imports
pub mod prelude {
    pub use crate::core::*;
    pub use crate::Result;
    
    #[cfg(feature = "serde")]
    pub use crate::serialization::*;
}