rust-by-example

Rust by Example — Patterns Through Compilable Snippets

Authority: Rust by Example — the official example-driven tutorial. This skill condenses its 24 chapters into a routing layer that points to topic-specific reference files.

This skill owns "how do I write X" patterns with concrete code. It does not own architectural decisions (rust-api-design, rust-workspace), language semantics (rust-stable), or std API selection (rust-stdlib).

Capability Boundaries

✅ Strengths

Converting between types — From/Into/TryFrom/as/Deref/transmute
Flow control idioms — if let, while let, match, loop with labels, ?
Functions, methods, closures (Fn/FnMut/FnOnce), capture modes
Modules, use, pub, super/self/crate, file-based module resolution
Generics — functions, structs, methods, impl blocks, where clauses
Traits — definition, default methods, associated types, trait objects, object safety
Error handling — ?, Result, Option, custom errors with thiserror, anyhow
Attributes — #[derive], #[cfg], #[inline], #[allow], #[non_exhaustive]
Unsafe — raw pointers, dereferences, unsafe fn/unsafe impl, unions, ABI
Procedural macros — derive, attribute, function-like (high-level routing to rust-macros)
Inline assembly — core::arch::asm! (rare but supported)

⚠️ Prerequisites

Basic Rust syntax — see rust-stable
Project structure — see rust-workspace, rust-module-layout

❌ Out of Scope

API design decisions → rust-api-design
Workspace / module layout → rust-workspace, rust-module-layout
Std API selection (which collection, which pointer) → rust-stdlib
Async runtime → rust-concurrency
Deep unsafe / FFI → rust-unsafe-ffi
Authoring complex procedural macros → rust-macros

Data Privacy

This skill does not collect, store, or transmit user data.

Topic Map (One Section per Pattern Category)

Each section has a short example inline; deeper patterns live in the linked reference file.

1. Type Conversions

// From / Into (lossless)
impl From<i32> for Wrapped { fn from(v: i32) -> Self { Self(v) } }
let w: Wrapped = 42.into();   // Into comes for free

// TryFrom (fallible)
impl TryFrom<&str> for Email {
    type Error = EmailError;
    fn try_from(s: &str) -> Result<Self, Self::Error> { /* */ }
}

// as (numeric widening/narrowing — can lose data)
let big: i64 = 1_000_000;
let small: i32 = big as i32;       // ❌ silent truncation possible

// Deref (smart-pointer coercion, NOT polymorphism)
let s: String = "hi".to_string();
let s_ref: &str = &s;              // Deref<Target=str> kicks in

See references/conversions.md for as vs From vs TryFrom, when each is correct.

2. Flow Control

// if let — single-pattern destructure
if let Some(x) = opt { /* */ }

// while let — loop until None
while let Some(line) = reader.lines().next() { /* */ }

// match with @-binding
match point {
    Point { x: 0, y } => println!("on y-axis at {y}"),
    Point { x, y: 0 } => println!("on x-axis at {x}"),
    p @ Point { x: 10..=20, .. } => println!("in x-band: {p:?}"),
    _ => {}
}

// loop with label
'outer: loop {
    loop { break 'outer; }
}

See references/flow-control.md for match ergonomics, guards, ranges, and when to prefer if let.

3. Functions and Closures

// Closure captures by ref/move based on use
let v = vec![1, 2, 3];
let print = || println!("{:?}", v);       // borrows
let consume = move || { v.into_iter().sum() };  // moves

// Fn / FnMut / FnOnce — how a closure borrows its environment
fn apply(f: impl Fn()) { f() }            // can be called multiple times, immutable capture
fn apply_mut(mut f: impl FnMut()) { f() } // mutable capture
fn apply_once(f: impl FnOnce() -> i32) -> i32 { f() }  // consumes captured

See references/closures.md for move keyword, return-type inference, and Fn/FnMut/FnOnce trait bounds.

4. Modules

// src/lib.rs
pub mod network;                  // src/network.rs OR src/network/mod.rs
pub use network::Client;          // re-export

// src/network.rs
pub struct Client { /* */ }
pub fn connect() { /* */ }

// Using items
use std::collections::HashMap;
use std::io::{self, Read};        // io module + Read trait
use crate::network::Client;       // absolute
use super::helper;                 // parent
use self::inner::Item;            // current

See references/modules.md for pub/pub(crate)/pub(super), file-based vs mod.rs, and crate root re-exports. For deep layout work, route to rust-module-layout.

5. Generics

// Generic function with bound
fn first<T>(xs: &[T]) -> Option<&T> { xs.first() }

// Generic with where clause
fn sum<T>(xs: &[T]) -> T where T: Default + std::ops::AddAssign + Copy {
    let mut acc = T::default();
    for x in xs { acc += *x; }
    acc
}

// Generic struct
struct Pair<A, B> { a: A, b: B }

impl<A: Clone, B: Clone> Pair<A, B> {
    fn swapped(&self) -> Pair<B, A> { Pair { a: self.b.clone(), b: self.a.clone() } }
}

See references/generics-traits.md for monomorphization cost, when to use generics vs trait objects, and associated types vs generics.

6. Traits

// Definition with default method
pub trait Summary {
    fn summarize(&self) -> String;
    fn default_summary(&self) -> String { format!("(no summary)") }
}

// Trait object (dynamic dispatch)
fn print_summary(item: &dyn Summary) { println!("{}", item.summarize()); }

// Generic (static dispatch)
fn print_summary_generic<T: Summary>(item: &T) { println!("{}", item.summarize()); }

// Associated type
trait Container { type Item; fn first(&self) -> Option<&Self::Item>; }

Object safety rules: no Self in method signatures, no generic methods, sized receivers. See references/generics-traits.md.

7. Error Handling

use std::fs;
use std::io;
use std::num::ParseIntError;

// ? propagates with From conversion
fn read_count() -> Result<i32, io::Error> {
    let s: String = fs::read_to_string("count.txt")?;
    Ok(s.trim().parse().unwrap_or(0))   // or proper error mapping
}

// Custom error with thiserror
#[derive(thiserror::Error, Debug)]
enum Error {
    #[error("io: {0}")] Io(#[from] io::Error),
    #[error("parse: {0}")] Parse(#[from] ParseIntError),
    #[error("not found: {key}")] NotFound { key: String },
}

// Application-layer catch-all with anyhow
fn main() -> anyhow::Result<()> {
    let x: i32 = "5".parse()?;
    Ok(())
}

See references/error-handling.md for ? with From, thiserror vs anyhow, and custom error design.

8. Attributes

#[derive(Debug, Clone, PartialEq)]   // generate traits
struct User { id: u32 }

#[cfg(target_os = "linux")]          // conditional compilation
fn setup() { /* */ }

#[inline]                            // hint to inline (rarely matters)
fn fast() {}

#[allow(dead_code)]                  // suppress warning
struct Unused;

#[non_exhaustive]                    // allow adding variants
enum Event { Login }

See references/attributes.md for the full attribute catalog, cfg expressions, and when #[inline] actually helps.

9. Unsafe

// Raw pointer dereference
let x = 42;
let ptr: *const i32 = &x;
let val = unsafe { *ptr };

// Splitting a slice (needs unsafe for non-overlapping aliasing)
let mut v = vec![1, 2, 3, 4];
let (left, right) = v.split_at_mut(2);   // safe wrapper

// Calling C ABI
extern "C" { fn abs(input: i32) -> i32; }
let a = unsafe { abs(-5) };

// Union (unsafe to read)
#[repr(C)]
union Value { i: i32, f: f32 }

See references/unsafe.md for safety contracts, the four unsafe superpowers, and when to reach for it. For deep FFI, route to rust-unsafe-ffi.

10. Procedural Macros (overview only)

// Derive (most common)
#[derive(Debug, Clone, serde::Serialize)]
struct Config { /* */ }

// Attribute macro
#[tokio::main]
async fn main() { /* */ }

// Function-like macro
let html = html! { <div>Hello</div> };

Authoring these is complex — route to rust-macros. This skill only covers using them.

See references/procedural-macros-overview.md for the three flavors and when each applies.

11. Inline Assembly

#[cfg(target_arch = "x86_64")]
fn halt() {
    unsafe { std::arch::asm!("hlt", options(nostack)); }
}

See references/inline-asm.md for asm! syntax, clobbers, and when to use it (kernel/embedded only).

Workflow

Identify the question — "how do I write X" → this skill; "which type" → rust-stdlib; "how to architect" → rust-api-design.
Find the matching reference — Sections 1-11 above, or the file in references/.
Read the minimal example — copy the pattern, adapt to your code.
Verify — cargo check, cargo clippy.
Hand off — deeper questions route to specialized skills.

Decision Shortcuts

Question	Where
How do I convert types?	`references/conversions.md`
How do I write `if let`/`match`?	`references/flow-control.md`
How do closures capture?	`references/closures.md`
How do `mod`/`use`/`pub` work?	`references/modules.md`
Generic fn/struct/impl?	`references/generics-traits.md`
Trait object vs generic?	`references/generics-traits.md`
How do I propagate errors?	`references/error-handling.md`
What `#[derive]`/`#[cfg]` are there?	`references/attributes.md`
When is `unsafe` OK?	`references/unsafe.md`
How do I write a procedural macro?	`rust-macros` skill
What does `asm!` look like?	`references/inline-asm.md`

Coming From Another Language

Pattern in...	...becomes in Rust
Java interface + impl	Rust trait + impl block
Java generics `<T extends Foo>`	Rust `<T: Foo>` or `where T: Foo`
Java `Optional<T>`	Rust `Option<T>`
Java try/catch	Rust `?` + `Result`
Python `with`	Rust `Drop` (RAII)
Python decorators	Rust attribute macros
Go `goroutine`	Rust `thread::spawn` or async task
Go `interface{}`	Rust `dyn Trait` or generics
C++ RAII destructor	Rust `Drop` trait
C++ `reinterpret_cast`	Rust `unsafe { transmute }` (avoid)

See references/migrating-from-other-languages.md for a fuller mapping.

Resources

Type Conversions
Flow Control
Closures
Modules
Generics and Traits
Error Handling
Attributes
Unsafe
Procedural Macros Overview
Inline Assembly
Migrating From Other Languages
examples/golden-by-example/: a crate exercising each pattern

About this skill