Cargo Workflows

Cargo Workflows

Manage Rust projects with Cargo workspaces, feature flags, build scripts, and development tooling for productive, well-structured Rust development.

When to Use

• Organizing a multi-crate Rust project
• Adding optional functionality with feature flags
• Running code generation or native compilation with build.rs
• Preparing a crate for crates.io publication
• Setting up CI-friendly lint and format checks

Core Patterns

Pattern 1: Cargo Workspaces

Organize related crates in a single repository with shared dependencies and a unified build.

# Root Cargo.toml
[workspace]
resolver = "2"
members = [
    "crates/core",
    "crates/api",
    "crates/cli",
]

# Share dependency versions across workspace members
[workspace.dependencies]
serde = { version = "1", features = ["derive"] }
tokio = { version = "1", features = ["full"] }
anyhow = "1"
tracing = "0.1"

# Shared package metadata
[workspace.package]
edition = "2021"
rust-version = "1.75"
license = "MIT"
repository = "https://github.com/user/project"

# crates/core/Cargo.toml
[package]
name = "myproject-core"
version = "0.1.0"
edition.workspace = true
license.workspace = true

[dependencies]
serde.workspace = true        # uses workspace version
anyhow.workspace = true

project/
  Cargo.toml                  # workspace root
  Cargo.lock                  # single lock file for reproducibility
  crates/
    core/
      Cargo.toml
      src/lib.rs
    api/
      Cargo.toml
      src/lib.rs
    cli/
      Cargo.toml
      src/main.rs

Pattern 2: Feature Flags

Enable optional functionality without forcing dependencies on all users.

# Cargo.toml
[features]
default = ["json"]                   # enabled unless user opts out
json = ["dep:serde_json"]            # optional dependency
postgres = ["dep:sqlx", "sqlx/postgres"]
full = ["json", "postgres"]          # convenience feature combining others

[dependencies]
serde = { version = "1", features = ["derive"] }
serde_json = { version = "1", optional = true }
sqlx = { version = "0.7", optional = true, features = ["runtime-tokio"] }

// Conditional compilation based on features
pub fn serialize<T: serde::Serialize>(value: &T) -> Result<Vec<u8>, Error> {
    #[cfg(feature = "json")]
    {
        return serde_json::to_vec(value).map_err(Error::Serialization);
    }

    #[cfg(not(feature = "json"))]
    {
        // Fallback to a simpler format
        todo!("non-JSON serialization")
    }
}

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

// Feature-gated trait implementations
#[cfg(feature = "json")]
impl FromStr for Config {
    type Err = serde_json::Error;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        serde_json::from_str(s)
    }
}

Usage: cargo build --features "json,postgres" or cargo build --all-features.

Pattern 3: Build Scripts (build.rs)

Run custom logic before compilation: code generation, native library linking, environment detection.

// build.rs
fn main() {
    // Tell Cargo to re-run if these change
    println!("cargo::rerun-if-changed=proto/");
    println!("cargo::rerun-if-env-changed=DATABASE_URL");

    // Set a cfg flag based on environment
    if std::env::var("CI").is_ok() {
        println!("cargo::rustc-cfg=ci_build");
    }

    // Expose build-time info as environment variables
    let git_hash = std::process::Command::new("git")
        .args(["rev-parse", "--short", "HEAD"])
        .output()
        .ok()
        .and_then(|o| String::from_utf8(o.stdout).ok())
        .unwrap_or_else(|| "unknown".to_string());

    println!("cargo::rustc-env=GIT_HASH={}", git_hash.trim());

    // Link native libraries
    println!("cargo::rustc-link-lib=sqlite3");
}

// In your code, access build-time values
const GIT_HASH: &str = env!("GIT_HASH");

fn version_info() -> String {
    format!("v{} ({})", env!("CARGO_PKG_VERSION"), GIT_HASH)
}

Pattern 4: Publishing to crates.io

Prepare a crate for publication with proper metadata and checks.

[package]
name = "my-library"
version = "0.2.0"
edition = "2021"
description = "A concise description of what this crate does"
license = "MIT OR Apache-2.0"
repository = "https://github.com/user/my-library"
documentation = "https://docs.rs/my-library"
readme = "README.md"
keywords = ["keyword1", "keyword2"]      # max 5
categories = ["development-tools"]
exclude = ["tests/fixtures/*", ".github/"]
rust-version = "1.75"                    # MSRV

# Pre-publish checklist
cargo fmt --check                   # formatting
cargo clippy -- -D warnings         # lints as errors
cargo test                          # all tests pass
cargo doc --no-deps                 # docs build
cargo package --list                # review included files
cargo publish --dry-run             # verify packaging

# Publish
cargo publish

Pattern 5: Clippy and Fmt Configuration

Configure linting and formatting for consistent code quality.

# Cargo.toml -- workspace-level clippy config
[workspace.lints.clippy]
all = { level = "warn", priority = -1 }
pedantic = { level = "warn", priority = -1 }
nursery = { level = "warn", priority = -1 }
# Allow specific lints you disagree with
module_name_repetitions = "allow"
must_use_candidate = "allow"
missing_errors_doc = "allow"

[workspace.lints.rust]
unsafe_code = "forbid"
missing_debug_implementations = "warn"

# crates/core/Cargo.toml
[lints]
workspace = true   # inherit workspace lint config

# rustfmt.toml
edition = "2021"
max_width = 100
use_field_init_shorthand = true
imports_granularity = "Crate"
group_imports = "StdExternalCrate"

# CI commands
cargo fmt --all -- --check          # check formatting
cargo clippy --workspace --all-targets --all-features -- -D warnings
cargo test --workspace --all-features

Anti-Patterns

• Massive single-crate projects -- Split into workspace members when crate compile time becomes a bottleneck. Each member compiles independently.

• Features that change behavior rather than add it -- Features should be additive. A crate with feature A enabled should be a superset of the crate without it. Do not use features to toggle between alternative implementations.

  # BAD: mutually exclusive features
  [features]
  backend_a = []
  backend_b = []
  
  # GOOD: additive features
  [features]
  postgres = ["dep:sqlx"]
  caching = ["dep:redis"]
  

• Skipping cargo clippy in CI -- Clippy catches real bugs. Run it with -D warnings to treat lints as errors in CI.

• Not pinning workspace dependency versions -- Use Cargo.lock for applications (commit it to git). For libraries, let Cargo.lock be gitignored but use workspace dependencies for consistency.

• Overly broad build.rs re-run triggers -- Use cargo::rerun-if-changed and cargo::rerun-if-env-changed to avoid unnecessary rebuilds.

Quick Reference