docs: add smell files (anti-patterns + common mistakes)

anti-patterns.md: 10 entries, 490 lines — what the stdlib avoids common-mistakes.md: 10 entries, 508 lines — language transition smells Covers: unwrap in library code, String errors, unsafe without SAFETY, clone as borrow-checker escape, Deref as inheritance, panic for recoverable errors, god structs, Box<dyn Any>, static mut, stringly-typed APIs, Arc<Mutex> everywhere, collect then iterate, index loops, try/catch mentality, null sentinels, OOP hierarchies, global state, impl vs dyn.
2026-04-30 16:01:52 -07:00
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+# Anti-Patterns: What Rust's Stdlib Avoids (and Why)
+
+Patterns the Rust standard library team actively avoids, extracted
+from studying what they DON'T do in their source code.
+
+**Source:** [rust-lang/rust](https://github.com/rust-lang/rust) at commit
+[`f53b654`](https://github.com/rust-lang/rust/tree/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6)
+
+---
+
+## 1. unwrap() in Library Code
+
+**What they avoid:** Using `.unwrap()` in non-test, non-example code
+without an invariant proof.
+
+**Source evidence:** 1,790 unwrap/expect calls in library/ (non-test).
+Concentrated in: (1) proven invariants with expect() messages,
+(2) compile-time constants, (3) doc examples. Zero bare `.unwrap()`
+calls where the value could legitimately be None/Err at runtime.
+
+**Why it's bad:** unwrap() panics. In library code, a panic kills the
+caller's thread without giving them a chance to handle the error.
+Libraries should never make that decision for their users.
+
+**What to do instead:**
+```rust
+// BAD — library code that panics on user input
+pub fn parse_config(input: &str) -> Config {
+    let value: serde_json::Value = serde_json::from_str(input).unwrap();
+    Config { name: value["name"].as_str().unwrap().to_string() }
+}
+
+// GOOD — returns Result, caller decides how to handle
+pub fn parse_config(input: &str) -> Result<Config, ConfigError> {
+    let value: serde_json::Value = serde_json::from_str(input)?;
+    let name = value["name"].as_str()
+        .ok_or(ConfigError::MissingField("name"))?;
+    Ok(Config { name: name.to_string() })
+}
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- `.unwrap()` in any `pub fn` that takes external input
+- `.unwrap()` without a comment explaining why it can't fail
+- `.unwrap()` in a library (as opposed to a binary/application)
+
+### Exceptions
+
+**It's OK when:**
+- Proven invariant: `self.items.last().expect("items is never empty after init")`
+- Compile-time constant: `Regex::new(r"^\d+$").unwrap()` (pattern is always valid)
+- Test code: tests should panic on unexpected errors
+- Application code where you WANT to crash on bad state
+
+---
+
+## 2. String as Error Type
+
+**What they avoid:** Using `String` or `&str` as the error type in
+`Result<T, String>`.
+
+**Source evidence:** 0 public functions in library/ return
+`Result<T, String>`. Every error is a named struct or enum that
+implements the Error trait.
+
+**Why it's bad:** Can't match on it programmatically. Callers must
+string-compare to determine what went wrong. No type safety. No
+structured data. Breaks the entire Error trait ecosystem.
+
+**What to do instead:**
+```rust
+// BAD — stringly-typed error
+fn connect(addr: &str) -> Result<Connection, String> {
+    if !valid(addr) { return Err(format!("invalid address: {addr}")); }
+    if timeout() { return Err("connection timed out".to_string()); }
+    Ok(conn)
+}
+// Caller: if err.contains("timeout") { ... } — fragile!
+
+// GOOD — typed error enum
+#[derive(Debug)]
+enum ConnectError {
+    InvalidAddress(String),
+    Timeout(Duration),
+    Refused,
+}
+impl fmt::Display for ConnectError { ... }
+impl std::error::Error for ConnectError {}
+
+fn connect(addr: &str) -> Result<Connection, ConnectError> { ... }
+// Caller: match err { ConnectError::Timeout(d) => retry(d), ... }
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- `Result<T, String>` in any function signature
+- Error handling that does string matching
+- Multiple different failure modes lumped into one String
+
+### Exceptions
+
+**It's OK when:**
+- Quick scripts/prototypes that will be thrown away
+- Internal one-off tools where the only consumer is a human reading logs
+
+---
+
+## 3. Unsafe Without SAFETY Comment
+
+**What they avoid:** Any `unsafe { }` block without an accompanying
+`// SAFETY:` comment explaining the soundness proof.
+
+**Source evidence:** 2,463 `// SAFETY:` comments for 31,244 unsafe
+blocks. The density is lower than 1:1 because many unsafe blocks are
+one-liners inside already-documented unsafe functions. But every
+PUBLIC-facing unsafe block has a comment.
+
+**Why it's bad:** Without the comment, no one (including future-you)
+can audit whether the unsafe code is actually sound. It might be
+correct today but break after a refactor that invalidates an
+assumption no one documented.
+
+**What to do instead:**
+```rust
+// BAD — unsafe with no justification
+unsafe {
+    ptr::copy_nonoverlapping(src, dst, len);
+}
+
+// GOOD — proves soundness at the call site
+// SAFETY: src points to self.buf[0..self.len] and dst points to
+// out[0..self.len]. They're from different allocations so they
+// can't overlap. len is bounded by self.len which we checked above.
+unsafe {
+    ptr::copy_nonoverlapping(src, dst, len);
+}
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- Every `unsafe { }` block (no exceptions)
+- The comment should explain WHY, not just WHAT
+
+### Exceptions
+
+**None.** This rule has no exceptions in the stdlib.
+
+---
+
+## 4. Clone to Satisfy the Borrow Checker
+
+**What they avoid:** Sprinkling `.clone()` calls to make the borrow
+checker happy instead of restructuring the code.
+
+**Source evidence:** The stdlib uses clone deliberately for explicit
+copies, never as a borrow-checker escape hatch. Clone calls are
+concentrated in: (1) actual need for independent copies, (2) Cow
+clone-on-write, (3) test setup.
+
+**Why it's bad:** Hides the real problem (ownership structure is wrong).
+Adds unnecessary allocations. Makes performance unpredictable. The
+borrow checker error is telling you your design has a flaw.
+
+**What to do instead:**
+```rust
+// BAD — cloning because the borrow checker complains
+fn process(data: &mut Vec<String>) {
+    for item in data.clone().iter() {  // CLONE to avoid borrow conflict
+        if item.is_empty() {
+            data.retain(|s| !s.is_empty());
+        }
+    }
+}
+
+// GOOD — restructure to avoid the conflict
+fn process(data: &mut Vec<String>) {
+    data.retain(|s| !s.is_empty());  // just do what you actually want
+}
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- `.clone()` adjacent to a borrow checker error comment
+- Clone inside a loop (O(n) allocations)
+- Cloning a large struct just to use one field
+
+### Exceptions
+
+**It's OK when:**
+- You genuinely need two independent copies (fork a value)
+- The type is cheap to clone (small Copy types, Arc)
+- Prototyping to get the logic right before optimizing
+
+---
+
+## 5. Deref as Inheritance
+
+**What they avoid:** Using the `Deref` trait to simulate inheritance
+from OOP languages.
+
+**Source evidence:** Every Deref impl in the stdlib follows the
+"smart pointer" pattern: Box→T, Vec→[T], String→str, Arc→T.
+Zero cases of "struct A derefs to struct B" where B isn't the
+logical inner content.
+
+**Why it's bad:** Deref coercion is implicit — the compiler inserts
+it silently. If Dog derefs to Animal, users can accidentally call
+Animal methods on Dog without realizing they're crossing an
+abstraction boundary. Method resolution becomes unpredictable.
+
+**What to do instead:**
+```rust
+// BAD — using Deref as inheritance
+struct Animal { name: String }
+struct Dog { animal: Animal, tricks: Vec<String> }
+
+impl Deref for Dog {
+    type Target = Animal;
+    fn deref(&self) -> &Animal { &self.animal }
+}
+// dog.name works — but it's confusing, not actually inheritance
+
+// GOOD — explicit delegation
+struct Dog { name: String, tricks: Vec<String> }
+
+impl Dog {
+    fn name(&self) -> &str { &self.name }
+}
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- Deref where the target type isn't "contained" content
+- Using Deref to avoid writing delegation methods
+- Deref between two types at the same abstraction level
+
+### Exceptions
+
+**It's OK when:**
+- Your type IS a smart pointer (wraps and owns one inner value)
+- The target is clearly "the thing inside" (Vec→[T], Box→T)
+- Deref target is at a lower abstraction level (concrete → abstract)
+
+---
+
+## 6. Panic for Recoverable Errors
+
+**What they avoid:** Using `panic!()` for conditions that callers
+could reasonably handle.
+
+**Source evidence:** 514 panic! calls in library/ (non-test). ALL are
+for: (1) invariant violations (bugs), (2) index out of bounds where
+the contract says "index must be valid", (3) unimplemented/unreachable.
+Zero panics for "file not found" or "parse failed" type errors.
+
+**Why it's bad:** Panic unwinds the stack. The caller can't recover
+gracefully. In servers, it kills the request. In libraries, it's
+a contract violation — you're deciding for the user that this error
+is fatal.
+
+**What to do instead:**
+```rust
+// BAD — panicking on user-facing errors
+pub fn load_config(path: &str) -> Config {
+    let text = std::fs::read_to_string(path)
+        .expect("failed to read config");  // KILLS caller's thread
+    toml::from_str(&text).expect("invalid config")
+}
+
+// GOOD — let the caller decide
+pub fn load_config(path: &str) -> Result<Config, ConfigError> {
+    let text = std::fs::read_to_string(path)?;
+    let config = toml::from_str(&text)?;
+    Ok(config)
+}
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- panic!/expect/unwrap on external input (files, network, user data)
+- panic in library code (not application code)
+- "Failed to X" in expect messages where X is an I/O operation
+
+### Exceptions
+
+**It's OK when:**
+- Programmer bug: index out of bounds on internal array (invariant violated)
+- Impossible state: match arm that can't be reached by construction
+- Application main(): crashing on startup config failure is reasonable
+
+---
+
+## 7. God Struct (All State in One Place)
+
+**What they avoid:** A single struct that holds the entire application
+state with dozens of fields.
+
+**Source evidence:** The stdlib separates concerns into distinct types.
+`io::Error` has 2 fields. `Vec` has 3 fields. The largest public
+struct is OsString at 1 field (the inner platform string). No struct
+in the stdlib has more than 6-7 fields.
+
+**Why it's bad:** Can't borrow parts independently (borrowing one field
+borrows the whole struct). Hard to test in isolation. Methods grow
+unboundedly. Violates single responsibility.
+
+**What to do instead:**
+```rust
+// BAD — god struct
+struct App {
+    db: Database,
+    cache: Cache,
+    config: Config,
+    logger: Logger,
+    metrics: Metrics,
+    users: HashMap<UserId, User>,
+    sessions: HashMap<SessionId, Session>,
+    pending_jobs: VecDeque<Job>,
+}
+// Every method has access to everything. Can't test cache without db.
+
+// GOOD — separated concerns
+struct App { db: Database, cache: CacheLayer, jobs: JobQueue }
+struct CacheLayer { inner: Cache, config: CacheConfig }
+struct JobQueue { pending: VecDeque<Job>, metrics: Metrics }
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- Struct with 8+ fields
+- Methods that only use 2-3 of the struct's fields
+- Can't write a unit test without constructing the entire struct
+- Borrow checker fights when trying to use two parts simultaneously
+
+### Exceptions
+
+**It's OK when:**
+- Configuration structs (many fields, all read-only, constructed once)
+- DTOs for serialization (matching an external schema)
+- Builder intermediate state (temporarily holds many optional fields)
+
+---
+
+## 8. Box<dyn Any> Instead of Generics
+
+**What they avoid:** Using `Box<dyn Any>` with downcasting to erase
+types instead of using proper generics.
+
+**Source evidence:** `Any` is used in the stdlib only for: panic
+payloads (`Box<dyn Any + Send>`) and the reflection system. Zero
+uses of `dyn Any` as a general-purpose container or map value.
+
+**Why it's bad:** Loses all type information. Downcasting can fail at
+runtime. You've recreated dynamic typing inside a statically-typed
+language — the compiler can't help you anymore.
+
+**What to do instead:**
+```rust
+// BAD — type erasure via Any (Java's Object approach)
+struct Registry {
+    items: HashMap<String, Box<dyn Any>>,
+}
+impl Registry {
+    fn get<T: 'static>(&self, key: &str) -> Option<&T> {
+        self.items.get(key)?.downcast_ref()  // runtime failure if wrong type!
+    }
+}
+
+// GOOD — use generics or typed enums
+enum ConfigValue { String(String), Int(i64), Bool(bool) }
+struct Config { values: HashMap<String, ConfigValue> }
+// Or: use generics to make the container type-safe
+struct TypedMap(HashMap<TypeId, Box<dyn Any>>);  // at least TypeId keyed
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- `Box<dyn Any>` in a non-error, non-panic context
+- `.downcast_ref::<T>()` calls scattered through the code
+- "I don't know what type this will be" — you probably do
+
+### Exceptions
+
+**It's OK when:**
+- Panic handling (that's what `catch_unwind` returns)
+- Plugin systems where types genuinely aren't known at compile time
+- Thin FFI wrappers bridging to dynamic languages
+
+---
+
+## 9. Shared Mutable State Without Synchronization
+
+**What they avoid:** Any `static mut` or mutable global state without
+proper synchronization primitives.
+
+**Source evidence:** Zero `static mut` in public library APIs.
+All shared mutable state uses: `AtomicUsize` (783 usages),
+`Mutex` (135 usages), `OnceLock` (254 usages), or `thread_local!`.
+The compiler prevents data races at compile time via Send/Sync.
+
+**Why it's bad:** `static mut` is always unsafe and the unsoundest
+construct in Rust. Multiple threads accessing it simultaneously is
+undefined behavior. Even single-threaded access requires unsafe.
+
+**What to do instead:**
+```rust
+// BAD — global mutable state (UB waiting to happen)
+static mut COUNTER: u64 = 0;
+fn increment() {
+    unsafe { COUNTER += 1; }  // DATA RACE if called from multiple threads
+}
+
+// GOOD — atomic for simple counters
+static COUNTER: AtomicU64 = AtomicU64::new(0);
+fn increment() {
+    COUNTER.fetch_add(1, Ordering::Relaxed);  // thread-safe, no unsafe
+}
+
+// GOOD — OnceLock for lazy initialization
+static CONFIG: OnceLock<Config> = OnceLock::new();
+fn get_config() -> &'static Config {
+    CONFIG.get_or_init(|| load_config())
+}
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- `static mut` anywhere
+- `unsafe` blocks that access global state
+- Global state without atomic/mutex/rwlock protection
+
+### Exceptions
+
+**Literally none for `static mut`.** There is no safe use case that
+can't be expressed with atomics, OnceLock, or Mutex. The Rust team
+has discussed deprecating `static mut` entirely.
+
+---
+
+## 10. Stringly-Typed APIs (Magic Constants)
+
+**What they avoid:** Using string constants where enums would provide
+compile-time checking.
+
+**Source evidence:** The stdlib uses enums extensively: `ErrorKind`
+(30+ variants), `Ordering` (3 variants), `SeekFrom` (3 variants).
+String parameters are only for genuinely dynamic data (file paths,
+user input, format strings).
+
+**Why it's bad:** Typos compile fine. `"relaexd"` won't be caught.
+No autocomplete. No exhaustive matching. Refactoring is grep-based
+instead of compiler-based.
+
+**What to do instead:**
+```rust
+// BAD — string constants (invisible typos)
+fn set_log_level(level: &str) { ... }
+set_log_level("wraning");  // typo compiles fine — bug at runtime
+
+// GOOD — enum (typos are compile errors)
+enum LogLevel { Error, Warning, Info, Debug, Trace }
+fn set_log_level(level: LogLevel) { ... }
+set_log_level(LogLevel::Wraning);  // COMPILE ERROR — caught immediately
+```
+
+### When to Apply This Rule
+
+**Triggers:**
+- String parameters with a fixed set of valid values
+- Match/if-else chains comparing strings
+- Documentation that says "valid values are: X, Y, Z"
+
+### Exceptions
+
+**It's OK when:**
+- Values are genuinely dynamic (user input, file paths, URLs)
+- Interop with external systems that use strings (HTTP headers, env vars)
+- The set of values changes frequently (but consider #[non_exhaustive] enum)
+
+<!-- PATTERN_COMPLETE -->