Impertio Studio Rust Claude Skill Package Rust Syntax Pattern Matching

rust-syntax-pattern-matching

The mechanics of pattern matching in Rust: match, if let, let else, while let, if-let chains (edition 2024, stable in 1.88), destructuring of tuples / structs / enums / slices / references, ref and ref mut, or-patterns, guards, @ bindings, .. rest patterns, range patterns, and literal patterns.

Cross-references: [[rust-syntax-edition-2024]] (if-let chain edition-gating, if let temporary scope change) [[rust-syntax-ownership]] (move vs borrow in match arms, when ref matters)

When to use this skill

• User writes a match and gets E0004 ("non-exhaustive patterns") or wants to know which arms are missing
• User asks "should I use match, if let, or let else here"
• User wonders why their if let chain is rejected (edition or version too old)
• User destructures a tuple, struct, enum, slice, or reference and the compiler complains about moves
• User asks what ref / ref mut does and when to reach for it
• User wants or-patterns (A | B), guards (if cond), or @ bindings explained
• User asks about .. rest patterns in structs or slices
• User writes a range pattern (1..=10) or asks about exclusive ranges in patterns (stable in 1.80)
• User gets E0408 (variable not bound in all patterns) or E0409 (variable bound with different mode/type)

For broader edition 2024 changes (RPIT lifetime capture, unsafe extern, never type fallback) see [[rust-syntax-edition-2024]]. For move vs borrow semantics that drive ref see [[rust-syntax-ownership]].

Quick reference table

ALWAYS prefer match when every variant must be handled. ALWAYS prefer if let / let else when only one variant matters.

Decision tree: which construct to use

Need every variant handled (exhaustive)?
├── YES → use `match`
└── NO → only one variant matters?
        ├── failure must short-circuit (return / panic / break)?
        │   └── YES → use `let else`
        ├── two simple branches (matched / not matched)?
        │   └── YES → use `if let { ... } else { ... }`
        ├── multiple conditions chained with &&?
        │   └── YES → use `if let` chain (edition 2024, Rust 1.88+)
        └── loop on consecutive matches?
            └── YES → use `while let`

match : exhaustiveness, arm order, wildcards

enum Shape { Circle(f64), Square(f64), Rect(f64, f64) }

fn area(s: Shape) -> f64 {
    match s {
        Shape::Circle(r)    => std::f64::consts::PI * r * r,
        Shape::Square(s)    => s * s,
        Shape::Rect(w, h)   => w * h,
    }
}

Rules :

1. Exhaustiveness : every possible value of the scrutinee MUST be covered. Missing arms produce E0004 ("non-exhaustive patterns").
2. Arm order : first matching arm wins. Place more specific arms before more general ones.
3. Wildcard _ : the catch-all pattern. Does not bind.
4. Bind-all name : name => ... binds the value to name. Use when the value is still useful.
5. #[non_exhaustive] enums from external crates ALWAYS require a _ arm. The compiler refuses to assume the enum is closed.
6. Empty types (Rust 1.82+) : Result<T, Infallible> can omit the Err arm because Infallible has no constructors.

ALWAYS write _ => unreachable!("...") instead of _ => () when you believe the arm is impossible, this surfaces logic bugs at runtime instead of silently swallowing them.

Source: Rust Book Ch 6.2, The match Control Flow Construct Source: Rust Reference, match expressions

if let : single pattern, optional else

if let Some(value) = optional {
    use_value(value);
}

if let Ok(parsed) = "42".parse::<i32>() {
    println!("{parsed}");
} else {
    println!("not a number");
}

Rules :

1. if let PAT = expr { ... } runs the block only when expr matches PAT. Bindings from PAT are in scope inside the block.
2. else branch is optional. When present, the binding is NOT in scope inside else (the match failed, so it could not be bound).
3. Edition 2024 change : temporaries in the if let scrutinee are dropped at the end of the if let arm, not at the end of the containing scope. This fixes drop-order bugs but can break pre-2024 code that depended on the old behaviour. See [[rust-syntax-edition-2024]].

Source: Rust Book Ch 6.3, Concise Control Flow with if let Source: edition 2024 if-let scope

if let chains : edition 2024, Rust 1.88+

Stabilized in Rust 1.88.0 (2025-06-26). Only available on edition 2024 because the feature depends on the if let temporary scope change.

// 2024 edition only, Rust 1.88+
if let Some(user) = lookup(id)
    && user.is_active
    && let Some(email) = user.email
{
    send(email);
}

Rules :

1. ANY combination of let PAT = expr and boolean expressions, joined by &&.
2. ALL conditions must evaluate to true (and all let-patterns must match) for the body to run.
3. Bindings from earlier let-patterns are in scope in later conditions and in the body.
4. || is NOT allowed across let conditions, only &&.
5. NEVER use this on edition 2021 or earlier, it will not compile. Migrate the crate or use nested if let.

ALWAYS prefer an if let chain over deep nesting when targeting edition 2024.

Source: Rust 1.88.0 release notes, let chains stabilization

let else : extract one variant or diverge

Stabilized in Rust 1.65 (Nov 2022).

fn parse_config(s: &str) -> Config {
    let Ok(cfg) = serde_json::from_str(s) else {
        return Config::default();
    };
    cfg
}

Rules :

1. The else branch MUST diverge : return, break, continue, panic!, unreachable!, std::process::exit, or any expression of type !.
2. Falling through the else block is a hard compile error.
3. Bindings from the pattern are in scope after the statement, in the surrounding block, unlike if let, which scopes them inside the block only.
4. Use let else to flatten the happy path. Replaces let x = match opt { Some(x) => x, None => return };.

NEVER write let Some(x) = opt else { /* nothing */ };, the compiler rejects it. The else must diverge.

Source: Rust 1.65 release notes, let-else Source: Rust Reference, let statements

while let : loop while pattern matches

let mut stack = vec![1, 2, 3];
while let Some(top) = stack.pop() {
    println!("{top}");
}

Rules :

1. The loop runs the body each iteration as long as the pattern matches.
2. The loop stops on the first non-match.
3. The binding is in scope inside the loop body only.
4. NEVER use while let for an iterator when for works, for x in iter desugars to a loop + match and handles Iterator::next returning None correctly without the visual overhead.

USE while let when the source is not a regular iterator (e.g. VecDeque::pop_front, Receiver::recv returning a Result, repeated next_if).

Source: Rust Book Ch 6.3, while let

Destructuring

Patterns can take apart compound values in let, match, function parameters, and closures.

Tuples and tuple structs

let (x, y, z) = (1, 2, 3);

struct Point(i32, i32);
let Point(a, b) = Point(3, 4);

Structs (named fields)

struct User { name: String, age: u32 }

let u = User { name: "Sam".into(), age: 42 };
let User { name, age } = u;             // shorthand: same field names
let User { name: n, age: a } = u;       // rebind to local names
let User { name, .. } = u;              // ignore remaining fields with `..`

Rule : if a struct has more fields than you destructure and you OMIT .., the compiler emits E0027 ("pattern does not mention field"). ALWAYS use .. when you intentionally ignore fields.

Enums

match result {
    Ok(value) => use_value(value),
    Err(e)    => log_error(e),
}

Slices and arrays

match slice {
    []                   => println!("empty"),
    [only]               => println!("one: {only}"),
    [first, .., last]    => println!("{first} ... {last}"),
    [first, rest @ ..]   => println!("first={first}, rest={rest:?}"),
}

Rule : .. matches any number of elements (zero or more). Combined with @ it binds the rest as a sub-slice.

References

let value = 5;
let r = &value;
let &v = r;     // destructure to get owned i32 (Copy required)

Source: Rust Book Ch 18, Patterns and Matching Source: Rust Reference, Patterns

ref and ref mut : bind by reference

By default, a binding pattern moves or copies the matched value. ref makes the binding a reference instead. ref mut makes it a mutable reference.

let s = String::from("hi");
match s {
    ref borrowed => println!("{borrowed}"),
}
// s is still usable here, no move

Modern style : ALWAYS prefer to take a reference at the scrutinee instead of using ref in arm patterns. Match ergonomics (RFC 2005) handles the rest.

// Idiomatic
match &s {
    text => println!("{text}"),
}

Use ref only when you cannot take a reference at the scrutinee, typically inside an if let chain or destructuring a value you cannot reshape.

WARNING : Edition 2024 restricts some match-ergonomics combinations with & patterns to keep room for future syntax. If you hit a confusing error on a complex destructure, see [[rust-syntax-edition-2024]].

Source: Rust Reference, identifier patterns

Or-patterns, guards, @ bindings, .. rest, ranges, literals

Or-patterns

match n {
    1 | 2 | 3 => "small",
    _         => "other",
}

Rule (E0408 / E0409) : every alternative MUST bind the same variables with the same types. Or-patterns with different bindings are rejected.

Guards

match pair {
    (x, y) if x == y => "equal",
    (x, y) if x > y  => "first bigger",
    _                => "other",
}

Rule : the compiler does NOT consider guard expressions for exhaustiveness. A guarded arm is treated as potentially failing. ALWAYS provide a fallback arm.

@ bindings

match age {
    n @ 0..=12       => println!("child {n}"),
    n @ 13..=19      => println!("teen {n}"),
    n                => println!("adult {n}"),
}

name @ pattern binds name to the entire value while also testing it against pattern.

.. rest

struct Big { a: u8, b: u8, c: u8, d: u8 }
let Big { a, .. } = big;

In structs : .. ignores all unmentioned fields. In tuples and slices : .. ignores any number of elements.

Range patterns

match digit {
    '0'..='9' => "digit",
    'a'..='z' => "lower",
    _         => "other",
}

// Exclusive ranges (Rust 1.80+), pattern matches 0,1,2,...,9 (NOT 10)
match n {
    0..10 => "single digit",
    _     => "two or more digits",
}

ALWAYS use ..= for inclusive ranges. .. is exclusive (1.80+ in patterns).

Literal patterns

match cmd.as_str() {
    "quit" => exit(0),
    "help" => print_help(),
    other  => unknown(other),
}

Strings (&str), characters, integers, floats (deprecated in patterns), and bool can be literal patterns. ALWAYS bind the wildcard so you can include the value in error output.

Source: Rust Reference, Patterns Source: Rust 1.80 release notes, exclusive ranges in patterns

Common pitfalls

For the full list (with code examples and fixes) see references/anti-patterns.md. Highlights :

• Missing _ arm on #[non_exhaustive] enums from external crates, compile error
• let else body that does not diverge, compile error
• Using nested if let { if let { ... } } on edition 2024 instead of a chain, readability regression
• match value { ref x => ... } instead of match &value { x => ... }, works but reads awkwardly
• Or-pattern arms that bind variables of different types (Some(n) | Ok(n) where n has two unrelated types), E0409
• Omitting .. when destructuring a struct with extra fields, E0027
• Relying on guard exhaustiveness, the compiler does not see through guards
• Forgetting that if let chains need edition 2024 AND Rust 1.88+

Further reading

• references/methods.md : full list of pattern syntax and constructs with one-line definitions
• references/examples.md : copy-pasteable end-to-end examples (state machine, parser dispatch, slice patterns, complex destructure)
• references/anti-patterns.md : the eight pitfalls in full, with compiler error text and fix

Sources

Last verified : 2026-05-19.

Source: Impertio-Studio/Rust-Claude-Skill-Package — distributed by TomeVault.