Lang Rust Library Dev

Rust Library Development

Specialized patterns for creating, maintaining, and publishing Rust libraries and crates. This skill focuses on library-specific concerns like API design, documentation, versioning, and distribution.

Overview

┌──────────────────────────────────────────────────────────────┐
│                Rust Skill Hierarchy                          │
├──────────────────────────────────────────────────────────────┤
│                                                              │
│              ┌─────────────────────┐                         │
│              │   lang-rust-dev     │                         │
│              │   (foundation)      │                         │
│              └──────────┬──────────┘                         │
│                         │                                    │
│     ┌───────────┬───────┼───────┬───────────┐               │
│     │           │       │       │           │               │
│     ▼           ▼       ▼       ▼           ▼               │
│ ┌────────┐ ┌────────┐ ... ┌─────────────────────┐           │
│ │  bin   │ │testing │     │lang-rust-library-dev│ ◄─ HERE   │
│ │  -dev  │ │  -dev  │     │   (library focus)   │           │
│ └────────┘ └────────┘     └─────────────────────┘           │
│                                                              │
└──────────────────────────────────────────────────────────────┘

This skill covers:

• Cargo.toml configuration for libraries
• Crate structure and module organization
• Public API design and stability
• Documentation with rustdoc
• Publishing to crates.io
• Feature flags and conditional compilation
• Semantic versioning (SemVer)
• Dependency management for libraries
• Examples, tests, and benches organization

This skill does NOT cover:

• Binary application development → lang-rust-bin-dev
• Testing strategies → lang-rust-testing-dev
• Async runtime patterns → lang-rust-async-dev
• General Rust syntax → lang-rust-dev

Quick Reference

Skill Routing

Use this table to find the right specialized skill:

Cargo.toml for Libraries

Basic Library Configuration

[package]
name = "my-library"
version = "0.1.0"
edition = "2021"
authors = ["Your Name <you@example.com>"]
license = "MIT OR Apache-2.0"
description = "A brief description of what this library does"
documentation = "https://docs.rs/my-library"
homepage = "https://github.com/user/my-library"
repository = "https://github.com/user/my-library"
readme = "README.md"
keywords = ["keyword1", "keyword2", "keyword3"]
categories = ["category1", "category2"]
rust-version = "1.70.0"  # Minimum supported Rust version (MSRV)

[lib]
name = "my_library"  # Optional: overrides package name
path = "src/lib.rs"  # Default path
crate-type = ["lib"]  # Default for libraries

[dependencies]
# Regular dependencies
serde = "1.0"
# Optional dependencies (for features)
tokio = { version = "1.0", optional = true }

[dev-dependencies]
# Test-only dependencies
criterion = "0.5"
proptest = "1.0"

[features]
# Feature flags
default = ["std"]
std = []
async = ["tokio", "tokio/rt-multi-thread"]
serde = ["dep:serde", "serde/derive"]

[package.metadata.docs.rs]
# Documentation build configuration for docs.rs
all-features = true
rustdoc-args = ["--cfg", "docsrs"]

Dependency Specifications

[dependencies]
# Version requirements
exact = "=1.2.3"           # Exact version
caret = "^1.2.3"           # Compatible: >=1.2.3 <2.0.0 (default)
tilde = "~1.2.3"           # Compatible: >=1.2.3 <1.3.0
wildcard = "1.*"           # Any 1.x.x version
range = ">=1.2, <1.5"      # Version range

# Git dependencies
my-git-dep = { git = "https://github.com/user/repo" }
my-git-dep-tag = { git = "https://github.com/user/repo", tag = "v1.0" }
my-git-dep-rev = { git = "https://github.com/user/repo", rev = "abc123" }

# Path dependencies (local development)
my-local-crate = { path = "../my-local-crate" }

# Optional dependencies (enabled via features)
optional-dep = { version = "1.0", optional = true }

# Dependency features
serde = { version = "1.0", features = ["derive"], default-features = false }

Library Types

[lib]
# Library output types
crate-type = ["lib"]           # Rust library (default)
# crate-type = ["dylib"]       # Dynamic library
# crate-type = ["staticlib"]   # Static library
# crate-type = ["cdylib"]      # C-compatible dynamic library
# crate-type = ["rlib"]        # Rust library (explicit)

# For FFI libraries
# crate-type = ["cdylib", "rlib"]  # Both C-compatible and Rust library

Crate Structure

Standard Library Layout

my-library/
├── Cargo.toml
├── Cargo.lock          # Commit for binaries, gitignore for libraries
├── README.md
├── LICENSE-MIT
├── LICENSE-APACHE
├── CHANGELOG.md
├── src/
│   ├── lib.rs          # Library root
│   ├── prelude.rs      # Common imports (optional)
│   ├── error.rs        # Error types
│   ├── config.rs       # Configuration
│   ├── module1.rs      # Top-level module
│   ├── module2/        # Module with submodules
│   │   ├── mod.rs      # Module root
│   │   ├── sub1.rs
│   │   └── sub2.rs
│   └── internal/       # Private internals
│       ├── mod.rs
│       └── helper.rs
├── examples/           # Usage examples
│   ├── basic.rs
│   └── advanced.rs
├── tests/              # Integration tests
│   ├── integration_test.rs
│   └── common/         # Test utilities
│       └── mod.rs
├── benches/            # Benchmarks
│   └── benchmark.rs
└── docs/               # Additional documentation
    └── architecture.md

lib.rs - Library Root

//! # My Library
//!
//! This is the top-level documentation for the library.
//! It appears on the crate's main documentation page.
//!
//! ## Quick Start
//!
//! ```
//! use my_library::Something;
//!
//! let thing = Something::new();
//! ```

// Deny warnings to catch issues early
#![warn(missing_docs)]
#![warn(missing_debug_implementations)]
#![deny(unsafe_code)]  // If appropriate

// Feature attributes
#![cfg_attr(not(feature = "std"), no_std)]
#![cfg_attr(docsrs, feature(doc_cfg))]

// Public modules
pub mod config;
pub mod error;

// Private internal modules
mod internal;

// Re-exports for convenience
pub use error::{Error, Result};
pub use config::Config;

// Prelude module (optional)
pub mod prelude {
    //! Common imports for users of this library
    pub use crate::{Config, Error, Result};
    pub use crate::something::Something;
}

Module Organization

// src/module1.rs - Flat module file
//! Module documentation

pub struct PublicStruct {
    pub field: String,
    private_field: u32,
}

impl PublicStruct {
    /// Creates a new instance
    pub fn new(field: String) -> Self {
        Self {
            field,
            private_field: 0,
        }
    }
}

// Private helper
fn internal_helper() -> u32 {
    42
}

// src/module2/mod.rs - Module directory with submodules
//! Module2 documentation

mod sub1;
mod sub2;

// Re-export public items
pub use sub1::PublicType1;
pub use sub2::PublicType2;

// Module-level items
pub struct ModuleStruct;

Public API Design

Visibility and Re-exports

// src/lib.rs
pub mod high_level {
    //! High-level API

    // Re-export from internal modules
    pub use crate::internal::core::CoreType;
    pub use crate::internal::utils::UtilType;
}

// Internal modules (not in public API)
mod internal {
    pub(crate) mod core {
        // pub(crate) = visible within crate
        pub struct CoreType;
    }

    pub(crate) mod utils {
        pub struct UtilType;
    }
}

// Users access via:
// use my_library::high_level::{CoreType, UtilType};

Prelude Pattern

// src/prelude.rs
//! The prelude module re-exports commonly used items

pub use crate::error::{Error, Result};
pub use crate::config::Config;
pub use crate::builder::Builder;
pub use crate::traits::{MyTrait, AnotherTrait};

// Users can import with:
// use my_library::prelude::*;

Builder Pattern

/// Configuration builder for `MyType`
#[derive(Debug, Default)]
pub struct MyTypeBuilder {
    field1: Option<String>,
    field2: Option<u32>,
}

impl MyTypeBuilder {
    /// Creates a new builder
    pub fn new() -> Self {
        Self::default()
    }

    /// Sets field1
    pub fn field1(mut self, value: impl Into<String>) -> Self {
        self.field1 = Some(value.into());
        self
    }

    /// Sets field2
    pub fn field2(mut self, value: u32) -> Self {
        self.field2 = Some(value);
        self
    }

    /// Builds the final type
    pub fn build(self) -> Result<MyType, BuildError> {
        Ok(MyType {
            field1: self.field1.ok_or(BuildError::MissingField1)?,
            field2: self.field2.unwrap_or(42),
        })
    }
}

/// The main type
pub struct MyType {
    field1: String,
    field2: u32,
}

impl MyType {
    /// Creates a builder for this type
    pub fn builder() -> MyTypeBuilder {
        MyTypeBuilder::new()
    }
}

#[derive(Debug, thiserror::Error)]
pub enum BuildError {
    #[error("field1 is required")]
    MissingField1,
}

Type-State Pattern

use std::marker::PhantomData;
use crate::error::Result;

/// Type states
pub struct Uninitialized;
pub struct Initialized;

/// Connection in different states
pub struct Connection<State = Uninitialized> {
    url: String,
    _state: PhantomData<State>,
}

impl Connection<Uninitialized> {
    /// Creates a new uninitialized connection
    pub fn new(url: impl Into<String>) -> Self {
        Self {
            url: url.into(),
            _state: PhantomData,
        }
    }

    /// Initializes the connection
    pub fn initialize(self) -> Result<Connection<Initialized>> {
        // Perform initialization
        Ok(Connection {
            url: self.url,
            _state: PhantomData,
        })
    }
}

impl Connection<Initialized> {
    /// Send data (only available when initialized)
    pub fn send(&self, data: &[u8]) -> Result<()> {
        // Send implementation
        Ok(())
    }
}

Documentation with Rustdoc

Documentation Comments

/// Single-line documentation
///
/// Multi-line documentation continues on subsequent lines.
/// Use markdown for formatting.
///
/// # Examples
///
/// ```
/// use my_library::MyType;
///
/// let instance = MyType::new("value");
/// assert_eq!(instance.get(), "value");
/// ```
///
/// # Errors
///
/// Returns `Err` when:
/// - The input is empty
/// - The value is invalid
///
/// # Panics
///
/// Panics if the internal state is corrupted.
///
/// # Safety
///
/// This function is unsafe because...
///
pub struct MyType {
    value: String,
}

//! Inner doc comment for modules
//! Documents the item that contains it (e.g., in lib.rs or mod.rs)

/// Method documentation
impl MyType {
    /// Creates a new instance
    ///
    /// # Arguments
    ///
    /// * `value` - The initial value
    ///
    /// # Examples
    ///
    /// ```
    /// # use my_library::MyType;
    /// let instance = MyType::new("test");
    /// ```
    pub fn new(value: impl Into<String>) -> Self {
        Self {
            value: value.into(),
        }
    }

    /// Gets the value
    #[must_use]
    pub fn get(&self) -> &str {
        &self.value
    }
}

Documentation Attributes

// Hide from documentation
#[doc(hidden)]
pub struct InternalType;

// Add documentation link
#[doc(alias = "alternative_name")]
pub struct MyType;

// Inline documentation from another item
#[doc(inline)]
pub use internal::PublicType;

// Conditional documentation (for docs.rs)
#[cfg_attr(docsrs, doc(cfg(feature = "async")))]
#[cfg(feature = "async")]
pub mod async_module {
    // Async functionality
}

Code Examples in Docs

/// Example with hidden setup code
///
/// ```
/// # use my_library::MyType;
/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
/// let instance = MyType::new("value")?;
/// assert_eq!(instance.get(), "value");
/// # Ok(())
/// # }
/// ```
pub fn example() {}

/// No-run example (compiles but doesn't run)
///
/// ```no_run
/// use my_library::connect;
/// let conn = connect("localhost:8080");
/// ```
pub fn no_run_example() {}

/// Ignore example (doesn't compile)
///
/// ```ignore
/// // Pseudo-code
/// let x = something_not_real();
/// ```
pub fn ignore_example() {}

/// Example that should fail
///
/// ```should_panic
/// use my_library::MyType;
/// MyType::new("");  // Panics on empty string
/// ```
pub fn should_panic_example() {}

/// Example with compile_fail
///
/// ```compile_fail
/// use my_library::MyType;
/// let x: i32 = MyType::new("test");  // Type error
/// ```
pub fn compile_fail_example() {}

Module-Level Documentation

//! # Module Name
//!
//! This module provides...
//!
//! ## Overview
//!
//! Detailed description...
//!
//! ## Examples
//!
//! ```
//! use my_library::module::Type;
//! ```

// Module contents

Feature Flags

Defining Features

# Cargo.toml
[features]
# Default features (enabled automatically)
default = ["std"]

# Feature flag with no dependencies
std = []

# Feature flag that enables optional dependencies
async = ["tokio", "futures"]

# Feature that enables other features
full = ["std", "async", "serde"]

# Feature that enables dependency features
serde = ["dep:serde", "serde/derive"]

[dependencies]
tokio = { version = "1.0", optional = true }
futures = { version = "0.3", optional = true }
serde = { version = "1.0", optional = true }

Using Features in Code

// Conditional compilation based on feature
#[cfg(feature = "async")]
pub mod async_module {
    use tokio::runtime::Runtime;

    pub fn async_function() {
        // Async implementation
    }
}

#[cfg(not(feature = "std"))]
use core::fmt;
#[cfg(feature = "std")]
use std::fmt;

// Function available only with feature
#[cfg(feature = "async")]
pub async fn process_async(data: &[u8]) -> Result<()> {
    // Implementation
    Ok(())
}

// Different implementations based on features
#[cfg(feature = "std")]
pub fn allocate() -> Vec<u8> {
    Vec::new()
}

#[cfg(not(feature = "std"))]
pub fn allocate() -> heapless::Vec<u8, 256> {
    heapless::Vec::new()
}

Feature Documentation

#![cfg_attr(docsrs, feature(doc_cfg))]

/// This type requires the `async` feature
#[cfg(feature = "async")]
#[cfg_attr(docsrs, doc(cfg(feature = "async")))]
pub struct AsyncType;

/// This function requires either `feature1` or `feature2`
#[cfg(any(feature = "feature1", feature = "feature2"))]
#[cfg_attr(docsrs, doc(cfg(any(feature = "feature1", feature = "feature2"))))]
pub fn conditional_function() {}

Semantic Versioning

SemVer Rules

Format: MAJOR.MINOR.PATCH

• MAJOR: Incompatible API changes
• MINOR: Add functionality (backward compatible)
• PATCH: Bug fixes (backward compatible)

Pre-1.0.0: Different rules apply

• 0.0.x: Any change can break compatibility
• 0.y.z: MINOR acts like MAJOR, PATCH acts like MINOR

What Requires a Major Version Bump

// Breaking changes (require MAJOR bump):

// 1. Removing public items
pub struct OldType;  // Removed entirely

// 2. Adding trait bounds to public types
pub struct Generic<T>;  // Changed to:
pub struct Generic<T: Clone>;

// 3. Changing function signatures
pub fn process(x: u32) {}  // Changed to:
pub fn process(x: u64) {}

// 4. Changing trait methods
pub trait MyTrait {
    fn method(&self);  // Changed to:
    fn method(&self) -> i32;
}

// 5. Removing or renaming public fields
pub struct Type {
    pub field: String,  // Removed or renamed
}

// 6. Changing error types
pub fn operation() -> Result<(), OldError> {}  // Changed to:
pub fn operation() -> Result<(), NewError> {}

What is Compatible (MINOR version)

// Compatible changes (MINOR bump):

// 1. Adding new public items
pub struct NewType;
pub fn new_function() {}

// 2. Adding trait implementations
impl Clone for ExistingType {}

// 3. Adding defaulted type parameters
pub struct Type<T = DefaultType>;

// 4. Adding private fields to structs (if not constructable)
pub struct Type {
    existing: String,
    new_private: u32,  // OK if struct is #[non_exhaustive] or has no pub constructor
}

// 5. Making things more public
pub(crate) fn internal() {}  // Changed to:
pub fn internal() {}

// 6. Relaxing trait bounds
pub fn process<T: Clone + Send>(x: T) {}  // Changed to:
pub fn process<T: Clone>(x: T) {}

Preventing Breaking Changes

// Use #[non_exhaustive] to reserve right to add fields
#[non_exhaustive]
pub struct Config {
    pub field1: String,
    pub field2: u32,
}

// Use builder pattern to avoid breaking changes
impl Config {
    pub fn builder() -> ConfigBuilder {
        ConfigBuilder::default()
    }
}

// Use sealed trait pattern to prevent external implementations
mod sealed {
    pub trait Sealed {}
}

pub trait MyTrait: sealed::Sealed {
    // Methods
}

impl sealed::Sealed for MyType {}
impl MyTrait for MyType {
    // Implementation
}

Publishing to crates.io

Pre-Publication Checklist

# 1. Verify package builds
cargo build --release

# 2. Run all tests
cargo test --all-features

# 3. Check documentation
cargo doc --no-deps --all-features --open

# 4. Run clippy
cargo clippy --all-features -- -D warnings

# 5. Format code
cargo fmt --check

# 6. Verify package contents
cargo package --list

# 7. Do a dry-run publish
cargo publish --dry-run

Required Files

my-library/
├── Cargo.toml       # Must have required metadata
├── README.md        # Shown on crates.io
├── LICENSE-MIT      # or LICENSE-APACHE or LICENSE
├── CHANGELOG.md     # Version history (recommended)
└── src/
    └── lib.rs       # Library code

Cargo.toml Metadata for Publishing

[package]
name = "my-library"
version = "0.1.0"
edition = "2021"
authors = ["Your Name <email@example.com>"]
license = "MIT OR Apache-2.0"
description = "A short description (max 256 chars)"
documentation = "https://docs.rs/my-library"
homepage = "https://github.com/user/my-library"
repository = "https://github.com/user/my-library"
readme = "README.md"
keywords = ["cli", "tool", "utility"]  # Max 5, each max 20 chars
categories = ["command-line-utilities"]  # From crates.io list
exclude = [
    "tests/fixtures/*",
    ".github/*",
    "*.png",
]

[badges]
# CI badge
github-actions = { repository = "user/repo", workflow = "CI" }

Publishing Workflow

# 1. Create crates.io account and get API token
# Visit https://crates.io/me

# 2. Login to crates.io
cargo login <your-api-token>

# 3. Publish the crate
cargo publish

# 4. Verify publication
# Visit https://crates.io/crates/my-library

Yanking Versions

# Yank a published version (prevents new projects from using it)
cargo yank --version 0.1.0

# Unyank a version
cargo yank --vers 0.1.0 --undo

Publishing Updates

# 1. Update version in Cargo.toml
# 2. Update CHANGELOG.md
# 3. Commit changes
git add Cargo.toml CHANGELOG.md
git commit -m "chore: bump version to 0.2.0"

# 4. Tag release
git tag -a v0.2.0 -m "Release v0.2.0"

# 5. Publish
cargo publish

# 6. Push commits and tags
git push origin main --tags

Examples and Tests

Examples Directory

// examples/basic.rs
//! Basic usage example

use my_library::MyType;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Simple example showing basic usage
    let instance = MyType::new("example")?;
    println!("Created: {:?}", instance);

    // Show key functionality
    let result = instance.process()?;
    println!("Result: {}", result);

    Ok(())
}

# Run examples
cargo run --example basic
cargo run --example advanced --features async

Integration Tests

// tests/integration_test.rs
use my_library::MyType;

#[test]
fn test_basic_usage() {
    let instance = MyType::new("test").unwrap();
    assert_eq!(instance.get(), "test");
}

#[test]
#[cfg(feature = "async")]
fn test_async_feature() {
    // Test async functionality
}

Benchmarks

// benches/benchmark.rs
use criterion::{black_box, criterion_group, criterion_main, Criterion};
use my_library::MyType;

fn benchmark_operation(c: &mut Criterion) {
    c.bench_function("my_operation", |b| {
        let instance = MyType::new("bench");
        b.iter(|| {
            black_box(instance.process())
        });
    });
}

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

# Run benchmarks
cargo bench

Error Handling

Library Error Types

use std::fmt;

/// Library error type
#[derive(Debug)]
pub enum Error {
    /// IO error occurred
    Io(std::io::Error),
    /// Invalid input
    InvalidInput(String),
    /// Configuration error
    Config(ConfigError),
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Error::Io(e) => write!(f, "IO error: {}", e),
            Error::InvalidInput(msg) => write!(f, "Invalid input: {}", msg),
            Error::Config(e) => write!(f, "Config error: {}", e),
        }
    }
}

impl std::error::Error for Error {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Error::Io(e) => Some(e),
            Error::Config(e) => Some(e),
            _ => None,
        }
    }
}

// Conversions from other error types
impl From<std::io::Error> for Error {
    fn from(err: std::io::Error) -> Self {
        Error::Io(err)
    }
}

impl From<ConfigError> for Error {
    fn from(err: ConfigError) -> Self {
        Error::Config(err)
    }
}

/// Result type alias for convenience
pub type Result<T> = std::result::Result<T, Error>;

#[derive(Debug)]
pub struct ConfigError {
    message: String,
}

impl fmt::Display for ConfigError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.message)
    }
}

impl std::error::Error for ConfigError {}

Using thiserror

// Add to Cargo.toml:
// thiserror = "1.0"

use thiserror::Error;

#[derive(Error, Debug)]
pub enum Error {
    #[error("IO error: {0}")]
    Io(#[from] std::io::Error),

    #[error("Invalid input: {0}")]
    InvalidInput(String),

    #[error("Configuration error")]
    Config(#[from] ConfigError),

    #[error("Not found: {0}")]
    NotFound(String),
}

pub type Result<T> = std::result::Result<T, Error>;

Troubleshooting

Documentation Build Failures

Problem: Broken doc links

// Error: unresolved link to `NonExistent`
/// See [`NonExistent`] for details

// Fix: Use correct paths
/// See [`crate::actual::Type`] for details
/// Or: See [Type](crate::actual::Type)

Problem: Doc tests fail

# Run doc tests to see failures
cargo test --doc

# Fix: Add necessary imports to doc examples
/// ```
/// # use my_library::MyType;  # Hidden line
/// let x = MyType::new();
/// ```

Publishing Failures

Problem: Version already published

# Error: crate version `0.1.0` is already uploaded
# Fix: Bump version in Cargo.toml

Problem: Missing required fields

# Error: missing required field `description`
# Fix: Add to Cargo.toml
description = "A library that does X"
license = "MIT OR Apache-2.0"

Problem: Package too large

# Error: package size exceeds 10 MB limit
# Fix: Exclude unnecessary files
exclude = [
    "tests/fixtures/*.bin",
    "docs/images/*.png",
    ".github/",
]

Best Practices

API Design

1. Prefer explicit over implicit

   • Clear function names
   • Explicit error types
   • Documented behavior

2. Use builders for complex construction

   • Many optional parameters → builder pattern
   • Complex validation → builder with build() method

3. Provide type safety

   • Use type-state pattern where applicable
   • Leverage the type system to prevent invalid states

4. Follow naming conventions

   • new() for constructors
   • with_* for builders
   • into_* for consuming conversions
   • as_* for cheap references
   • to_* for expensive conversions

Versioning

1. Follow SemVer strictly
2. Maintain a CHANGELOG.md
3. Use #[non_exhaustive] for extensible types
4. Document MSRV (Minimum Supported Rust Version)

Documentation

1. Document all public items
2. Include examples in documentation
3. Use doc tests to verify examples
4. Add crate-level documentation in lib.rs

Testing

1. Test public API in integration tests
2. Provide examples for common use cases
3. Use doc tests for simple examples
4. Benchmark performance-critical code

References

• The Cargo Book
• API Guidelines
• The Rustdoc Book
• crates.io Publishing Guide
• Semantic Versioning

See Also

• lang-rust-dev - Rust fundamentals and syntax
• lang-rust-bin-dev - Binary application development
• lang-rust-testing-dev - Testing strategies