src/doc/trpl/macros.md

   1 % Macros
   2
   3 By now you’ve learned about many of the tools Rust provides for abstracting and
   4 reusing code. These units of code reuse have a rich semantic structure. For
   5 example, functions have a type signature, type parameters have trait bounds,
   6 and overloaded functions must belong to a particular trait.
   7
   8 This structure means that Rust’s core abstractions have powerful compile-time
   9 correctness checking. But this comes at the price of reduced flexibility. If
  10 you visually identify a pattern of repeated code, you may find it’s difficult
  11 or cumbersome to express that pattern as a generic function, a trait, or
  12 anything else within Rust’s semantics.
  13
  14 Macros allow us to abstract at a syntactic level. A macro invocation is
  15 shorthand for an "expanded" syntactic form. This expansion happens early in
  16 compilation, before any static checking. As a result, macros can capture many
  17 patterns of code reuse that Rust’s core abstractions cannot.
  18
  19 The drawback is that macro-based code can be harder to understand, because
  20 fewer of the built-in rules apply. Like an ordinary function, a well-behaved
  21 macro can be used without understanding its implementation. However, it can be
  22 difficult to design a well-behaved macro!  Additionally, compiler errors in
  23 macro code are harder to interpret, because they describe problems in the
  24 expanded code, not the source-level form that developers use.
  25
  26 These drawbacks make macros something of a "feature of last resort". That’s not
  27 to say that macros are bad; they are part of Rust because sometimes they’re
  28 needed for truly concise, well-abstracted code. Just keep this tradeoff in
  29 mind.
  30
  31 # Defining a macro
  32
  33 You may have seen the `vec!` macro, used to initialize a [vector][] with any
  34 number of elements.
  35
  36 [vector]: vectors.html
  37
  38 ```rust
  39 let x: Vec<u32> = vec![1, 2, 3];
  40 # assert_eq!(x, [1, 2, 3]);
  41 ```
  42
  43 This can’t be an ordinary function, because it takes any number of arguments.
  44 But we can imagine it as syntactic shorthand for
  45
  46 ```rust
  47 let x: Vec<u32> = {
  48     let mut temp_vec = Vec::new();
  49     temp_vec.push(1);
  50     temp_vec.push(2);
  51     temp_vec.push(3);
  52     temp_vec
  53 };
  54 # assert_eq!(x, [1, 2, 3]);
  55 ```
  56
  57 We can implement this shorthand, using a macro: [^actual]
  58
  59 [^actual]: The actual definition of `vec!` in libcollections differs from the
  60            one presented here, for reasons of efficiency and reusability.
  61
  62 ```rust
  63 macro_rules! vec {
  64     ( $( $x:expr ),* ) => {
  65         {
  66             let mut temp_vec = Vec::new();
  67             $(
  68                 temp_vec.push($x);
  69             )*
  70             temp_vec
  71         }
  72     };
  73 }
  74 # fn main() {
  75 #     assert_eq!(vec![1,2,3], [1, 2, 3]);
  76 # }
  77 ```
  78
  79 Whoa, that’s a lot of new syntax! Let’s break it down.
  80
  81 ```ignore
  82 macro_rules! vec { ... }
  83 ```
  84
  85 This says we’re defining a macro named `vec`, much as `fn vec` would define a
  86 function named `vec`. In prose, we informally write a macro’s name with an
  87 exclamation point, e.g. `vec!`. The exclamation point is part of the invocation
  88 syntax and serves to distinguish a macro from an ordinary function.
  89
  90 ## Matching
  91
  92 The macro is defined through a series of rules, which are pattern-matching
  93 cases. Above, we had
  94
  95 ```ignore
  96 ( $( $x:expr ),* ) => { ... };
  97 ```
  98
  99 This is like a `match` expression arm, but the matching happens on Rust syntax
 100 trees, at compile time. The semicolon is optional on the last (here, only)
 101 case. The "pattern" on the left-hand side of `=>` is known as a ‘matcher’.
 102 These have [their own little grammar] within the language.
 103
 104 [their own little grammar]: ../reference.html#macros
 105
 106 The matcher `$x:expr` will match any Rust expression, binding that syntax tree
 107 to the ‘metavariable’ `$x`. The identifier `expr` is a ‘fragment specifier’;
 108 the full possibilities are enumerated later in this chapter.
 109 Surrounding the matcher with `$(...),*` will match zero or more expressions,
 110 separated by commas.
 111
 112 Aside from the special matcher syntax, any Rust tokens that appear in a matcher
 113 must match exactly. For example,
 114
 115 ```rust
 116 macro_rules! foo {
 117     (x => $e:expr) => (println!("mode X: {}", $e));
 118     (y => $e:expr) => (println!("mode Y: {}", $e));
 119 }
 120
 121 fn main() {
 122     foo!(y => 3);
 123 }
 124 ```
 125
 126 will print
 127
 128 ```text
 129 mode Y: 3
 130 ```
 131
 132 With
 133
 134 ```rust,ignore
 135 foo!(z => 3);
 136 ```
 137
 138 we get the compiler error
 139
 140 ```text
 141 error: no rules expected the token `z`
 142 ```
 143
 144 ## Expansion
 145
 146 The right-hand side of a macro rule is ordinary Rust syntax, for the most part.
 147 But we can splice in bits of syntax captured by the matcher. From the original
 148 example:
 149
 150 ```ignore
 151 $(
 152     temp_vec.push($x);
 153 )*
 154 ```
 155
 156 Each matched expression `$x` will produce a single `push` statement in the
 157 macro expansion. The repetition in the expansion proceeds in "lockstep" with
 158 repetition in the matcher (more on this in a moment).
 159
 160 Because `$x` was already declared as matching an expression, we don’t repeat
 161 `:expr` on the right-hand side. Also, we don’t include a separating comma as
 162 part of the repetition operator. Instead, we have a terminating semicolon
 163 within the repeated block.
 164
 165 Another detail: the `vec!` macro has *two* pairs of braces on the right-hand
 166 side. They are often combined like so:
 167
 168 ```ignore
 169 macro_rules! foo {
 170     () => {{
 171         ...
 172     }}
 173 }
 174 ```
 175
 176 The outer braces are part of the syntax of `macro_rules!`. In fact, you can use
 177 `()` or `[]` instead. They simply delimit the right-hand side as a whole.
 178
 179 The inner braces are part of the expanded syntax. Remember, the `vec!` macro is
 180 used in an expression context. To write an expression with multiple statements,
 181 including `let`-bindings, we use a block. If your macro expands to a single
 182 expression, you don’t need this extra layer of braces.
 183
 184 Note that we never *declared* that the macro produces an expression. In fact,
 185 this is not determined until we use the macro as an expression. With care, you
 186 can write a macro whose expansion works in several contexts. For example,
 187 shorthand for a data type could be valid as either an expression or a pattern.
 188
 189 ## Repetition
 190
 191 The repetition operator follows two principal rules:
 192
 193 1. `$(...)*` walks through one "layer" of repetitions, for all of the `$name`s
 194    it contains, in lockstep, and
 195 2. each `$name` must be under at least as many `$(...)*`s as it was matched
 196    against. If it is under more, it’ll be duplicated, as appropriate.
 197
 198 This baroque macro illustrates the duplication of variables from outer
 199 repetition levels.
 200
 201 ```rust
 202 macro_rules! o_O {
 203     (
 204         $(
 205             $x:expr; [ $( $y:expr ),* ]
 206         );*
 207     ) => {
 208         &[ $($( $x + $y ),*),* ]
 209     }
 210 }
 211
 212 fn main() {
 213     let a: &[i32]
 214         = o_O!(10; [1, 2, 3];
 215                20; [4, 5, 6]);
 216
 217     assert_eq!(a, [11, 12, 13, 24, 25, 26]);
 218 }
 219 ```
 220
 221 That’s most of the matcher syntax. These examples use `$(...)*`, which is a
 222 "zero or more" match. Alternatively you can write `$(...)+` for a "one or
 223 more" match. Both forms optionally include a separator, which can be any token
 224 except `+` or `*`.
 225
 226 This system is based on
 227 "[Macro-by-Example](http://www.cs.indiana.edu/ftp/techreports/TR206.pdf)"
 228 (PDF link).
 229
 230 # Hygiene
 231
 232 Some languages implement macros using simple text substitution, which leads to
 233 various problems. For example, this C program prints `13` instead of the
 234 expected `25`.
 235
 236 ```text
 237 #define FIVE_TIMES(x) 5 * x
 238
 239 int main() {
 240     printf("%d\n", FIVE_TIMES(2 + 3));
 241     return 0;
 242 }
 243 ```
 244
 245 After expansion we have `5 * 2 + 3`, and multiplication has greater precedence
 246 than addition. If you’ve used C macros a lot, you probably know the standard
 247 idioms for avoiding this problem, as well as five or six others. In Rust, we
 248 don’t have to worry about it.
 249
 250 ```rust
 251 macro_rules! five_times {
 252     ($x:expr) => (5 * $x);
 253 }
 254
 255 fn main() {
 256     assert_eq!(25, five_times!(2 + 3));
 257 }
 258 ```
 259
 260 The metavariable `$x` is parsed as a single expression node, and keeps its
 261 place in the syntax tree even after substitution.
 262
 263 Another common problem in macro systems is ‘variable capture’. Here’s a C
 264 macro, using [a GNU C extension] to emulate Rust’s expression blocks.
 265
 266 [a GNU C extension]: https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
 267
 268 ```text
 269 #define LOG(msg) ({ \
 270     int state = get_log_state(); \
 271     if (state > 0) { \
 272         printf("log(%d): %s\n", state, msg); \
 273     } \
 274 })
 275 ```
 276
 277 Here’s a simple use case that goes terribly wrong:
 278
 279 ```text
 280 const char *state = "reticulating splines";
 281 LOG(state)
 282 ```
 283
 284 This expands to
 285
 286 ```text
 287 const char *state = "reticulating splines";
 288 int state = get_log_state();
 289 if (state > 0) {
 290     printf("log(%d): %s\n", state, state);
 291 }
 292 ```
 293
 294 The second variable named `state` shadows the first one.  This is a problem
 295 because the print statement should refer to both of them.
 296
 297 The equivalent Rust macro has the desired behavior.
 298
 299 ```rust
 300 # fn get_log_state() -> i32 { 3 }
 301 macro_rules! log {
 302     ($msg:expr) => {{
 303         let state: i32 = get_log_state();
 304         if state > 0 {
 305             println!("log({}): {}", state, $msg);
 306         }
 307     }};
 308 }
 309
 310 fn main() {
 311     let state: &str = "reticulating splines";
 312     log!(state);
 313 }
 314 ```
 315
 316 This works because Rust has a [hygienic macro system][]. Each macro expansion
 317 happens in a distinct ‘syntax context’, and each variable is tagged with the
 318 syntax context where it was introduced. It’s as though the variable `state`
 319 inside `main` is painted a different "color" from the variable `state` inside
 320 the macro, and therefore they don’t conflict.
 321
 322 [hygienic macro system]: http://en.wikipedia.org/wiki/Hygienic_macro
 323
 324 This also restricts the ability of macros to introduce new bindings at the
 325 invocation site. Code such as the following will not work:
 326
 327 ```rust,ignore
 328 macro_rules! foo {
 329     () => (let x = 3);
 330 }
 331
 332 fn main() {
 333     foo!();
 334     println!("{}", x);
 335 }
 336 ```
 337
 338 Instead you need to pass the variable name into the invocation, so it’s tagged
 339 with the right syntax context.
 340
 341 ```rust
 342 macro_rules! foo {
 343     ($v:ident) => (let $v = 3);
 344 }
 345
 346 fn main() {
 347     foo!(x);
 348     println!("{}", x);
 349 }
 350 ```
 351
 352 This holds for `let` bindings and loop labels, but not for [items][].
 353 So the following code does compile:
 354
 355 ```rust
 356 macro_rules! foo {
 357     () => (fn x() { });
 358 }
 359
 360 fn main() {
 361     foo!();
 362     x();
 363 }
 364 ```
 365
 366 [items]: ../reference.html#items
 367
 368 # Recursive macros
 369
 370 A macro’s expansion can include more macro invocations, including invocations
 371 of the very same macro being expanded.  These recursive macros are useful for
 372 processing tree-structured input, as illustrated by this (simplistic) HTML
 373 shorthand:
 374
 375 ```rust
 376 # #![allow(unused_must_use)]
 377 macro_rules! write_html {
 378     ($w:expr, ) => (());
 379
 380     ($w:expr, $e:tt) => (write!($w, "{}", $e));
 381
 382     ($w:expr, $tag:ident [ $($inner:tt)* ] $($rest:tt)*) => {{
 383         write!($w, "<{}>", stringify!($tag));
 384         write_html!($w, $($inner)*);
 385         write!($w, "</{}>", stringify!($tag));
 386         write_html!($w, $($rest)*);
 387     }};
 388 }
 389
 390 fn main() {
 391 #   // FIXME(#21826)
 392     use std::fmt::Write;
 393     let mut out = String::new();
 394
 395     write_html!(&mut out,
 396         html[
 397             head[title["Macros guide"]]
 398             body[h1["Macros are the best!"]]
 399         ]);
 400
 401     assert_eq!(out,
 402         "<html><head><title>Macros guide</title></head>\
 403          <body><h1>Macros are the best!</h1></body></html>");
 404 }
 405 ```
 406
 407 # Debugging macro code
 408
 409 To see the results of expanding macros, run `rustc --pretty expanded`. The
 410 output represents a whole crate, so you can also feed it back in to `rustc`,
 411 which will sometimes produce better error messages than the original
 412 compilation. Note that the `--pretty expanded` output may have a different
 413 meaning if multiple variables of the same name (but different syntax contexts)
 414 are in play in the same scope. In this case `--pretty expanded,hygiene` will
 415 tell you about the syntax contexts.
 416
 417 `rustc` provides two syntax extensions that help with macro debugging. For now,
 418 they are unstable and require feature gates.
 419
 420 * `log_syntax!(...)` will print its arguments to standard output, at compile
 421   time, and "expand" to nothing.
 422
 423 * `trace_macros!(true)` will enable a compiler message every time a macro is
 424   expanded. Use `trace_macros!(false)` later in expansion to turn it off.
 425
 426 # Syntactic requirements
 427
 428 Even when Rust code contains un-expanded macros, it can be parsed as a full
 429 [syntax tree][ast]. This property can be very useful for editors and other
 430 tools that process code. It also has a few consequences for the design of
 431 Rust’s macro system.
 432
 433 [ast]: glossary.html#abstract-syntax-tree
 434
 435 One consequence is that Rust must determine, when it parses a macro invocation,
 436 whether the macro stands in for
 437
 438 * zero or more items,
 439 * zero or more methods,
 440 * an expression,
 441 * a statement, or
 442 * a pattern.
 443
 444 A macro invocation within a block could stand for some items, or for an
 445 expression / statement. Rust uses a simple rule to resolve this ambiguity. A
 446 macro invocation that stands for items must be either
 447
 448 * delimited by curly braces, e.g. `foo! { ... }`, or
 449 * terminated by a semicolon, e.g. `foo!(...);`
 450
 451 Another consequence of pre-expansion parsing is that the macro invocation must
 452 consist of valid Rust tokens. Furthermore, parentheses, brackets, and braces
 453 must be balanced within a macro invocation. For example, `foo!([)` is
 454 forbidden. This allows Rust to know where the macro invocation ends.
 455
 456 More formally, the macro invocation body must be a sequence of ‘token trees’.
 457 A token tree is defined recursively as either
 458
 459 * a sequence of token trees surrounded by matching `()`, `[]`, or `{}`, or
 460 * any other single token.
 461
 462 Within a matcher, each metavariable has a ‘fragment specifier’, identifying
 463 which syntactic form it matches.
 464
 465 * `ident`: an identifier. Examples: `x`; `foo`.
 466 * `path`: a qualified name. Example: `T::SpecialA`.
 467 * `expr`: an expression. Examples: `2 + 2`; `if true then { 1 } else { 2 }`; `f(42)`.
 468 * `ty`: a type. Examples: `i32`; `Vec<(char, String)>`; `&T`.
 469 * `pat`: a pattern. Examples: `Some(t)`; `(17, 'a')`; `_`.
 470 * `stmt`: a single statement. Example: `let x = 3`.
 471 * `block`: a brace-delimited sequence of statements. Example:
 472   `{ log(error, "hi"); return 12; }`.
 473 * `item`: an [item][]. Examples: `fn foo() { }`; `struct Bar;`.
 474 * `meta`: a "meta item", as found in attributes. Example: `cfg(target_os = "windows")`.
 475 * `tt`: a single token tree.
 476
 477 There are additional rules regarding the next token after a metavariable:
 478
 479 * `expr` variables must be followed by one of: `=> , ;`
 480 * `ty` and `path` variables must be followed by one of: `=> , : = > as`
 481 * `pat` variables must be followed by one of: `=> , =`
 482 * Other variables may be followed by any token.
 483
 484 These rules provide some flexibility for Rust’s syntax to evolve without
 485 breaking existing macros.
 486
 487 The macro system does not deal with parse ambiguity at all. For example, the
 488 grammar `$($t:ty)* $e:expr` will always fail to parse, because the parser would
 489 be forced to choose between parsing `$t` and parsing `$e`. Changing the
 490 invocation syntax to put a distinctive token in front can solve the problem. In
 491 this case, you can write `$(T $t:ty)* E $e:exp`.
 492
 493 [item]: ../reference.html#items
 494
 495 # Scoping and macro import/export
 496
 497 Macros are expanded at an early stage in compilation, before name resolution.
 498 One downside is that scoping works differently for macros, compared to other
 499 constructs in the language.
 500
 501 Definition and expansion of macros both happen in a single depth-first,
 502 lexical-order traversal of a crate’s source. So a macro defined at module scope
 503 is visible to any subsequent code in the same module, which includes the body
 504 of any subsequent child `mod` items.
 505
 506 A macro defined within the body of a single `fn`, or anywhere else not at
 507 module scope, is visible only within that item.
 508
 509 If a module has the `macro_use` attribute, its macros are also visible in its
 510 parent module after the child’s `mod` item. If the parent also has `macro_use`
 511 then the macros will be visible in the grandparent after the parent’s `mod`
 512 item, and so forth.
 513
 514 The `macro_use` attribute can also appear on `extern crate`. In this context
 515 it controls which macros are loaded from the external crate, e.g.
 516
 517 ```rust,ignore
 518 #[macro_use(foo, bar)]
 519 extern crate baz;
 520 ```
 521
 522 If the attribute is given simply as `#[macro_use]`, all macros are loaded. If
 523 there is no `#[macro_use]` attribute then no macros are loaded. Only macros
 524 defined with the `#[macro_export]` attribute may be loaded.
 525
 526 To load a crate’s macros without linking it into the output, use `#[no_link]`
 527 as well.
 528
 529 An example:
 530
 531 ```rust
 532 macro_rules! m1 { () => (()) }
 533
 534 // visible here: m1
 535
 536 mod foo {
 537     // visible here: m1
 538
 539     #[macro_export]
 540     macro_rules! m2 { () => (()) }
 541
 542     // visible here: m1, m2
 543 }
 544
 545 // visible here: m1
 546
 547 macro_rules! m3 { () => (()) }
 548
 549 // visible here: m1, m3
 550
 551 #[macro_use]
 552 mod bar {
 553     // visible here: m1, m3
 554
 555     macro_rules! m4 { () => (()) }
 556
 557     // visible here: m1, m3, m4
 558 }
 559
 560 // visible here: m1, m3, m4
 561 # fn main() { }
 562 ```
 563
 564 When this library is loaded with `#[macro_use] extern crate`, only `m2` will
 565 be imported.
 566
 567 The Rust Reference has a [listing of macro-related
 568 attributes](../reference.html#macro-related-attributes).
 569
 570 # The variable `$crate`
 571
 572 A further difficulty occurs when a macro is used in multiple crates. Say that
 573 `mylib` defines
 574
 575 ```rust
 576 pub fn increment(x: u32) -> u32 {
 577     x + 1
 578 }
 579
 580 #[macro_export]
 581 macro_rules! inc_a {
 582     ($x:expr) => ( ::increment($x) )
 583 }
 584
 585 #[macro_export]
 586 macro_rules! inc_b {
 587     ($x:expr) => ( ::mylib::increment($x) )
 588 }
 589 # fn main() { }
 590 ```
 591
 592 `inc_a` only works within `mylib`, while `inc_b` only works outside the
 593 library. Furthermore, `inc_b` will break if the user imports `mylib` under
 594 another name.
 595
 596 Rust does not (yet) have a hygiene system for crate references, but it does
 597 provide a simple workaround for this problem. Within a macro imported from a
 598 crate named `foo`, the special macro variable `$crate` will expand to `::foo`.
 599 By contrast, when a macro is defined and then used in the same crate, `$crate`
 600 will expand to nothing. This means we can write
 601
 602 ```rust
 603 #[macro_export]
 604 macro_rules! inc {
 605     ($x:expr) => ( $crate::increment($x) )
 606 }
 607 # fn main() { }
 608 ```
 609
 610 to define a single macro that works both inside and outside our library. The
 611 function name will expand to either `::increment` or `::mylib::increment`.
 612
 613 To keep this system simple and correct, `#[macro_use] extern crate ...` may
 614 only appear at the root of your crate, not inside `mod`. This ensures that
 615 `$crate` is a single identifier.
 616
 617 # The deep end
 618
 619 The introductory chapter mentioned recursive macros, but it did not give the
 620 full story. Recursive macros are useful for another reason: Each recursive
 621 invocation gives you another opportunity to pattern-match the macro’s
 622 arguments.
 623
 624 As an extreme example, it is possible, though hardly advisable, to implement
 625 the [Bitwise Cyclic Tag](http://esolangs.org/wiki/Bitwise_Cyclic_Tag) automaton
 626 within Rust’s macro system.
 627
 628 ```rust
 629 macro_rules! bct {
 630     // cmd 0:  d ... => ...
 631     (0, $($ps:tt),* ; $_d:tt)
 632         => (bct!($($ps),*, 0 ; ));
 633     (0, $($ps:tt),* ; $_d:tt, $($ds:tt),*)
 634         => (bct!($($ps),*, 0 ; $($ds),*));
 635
 636     // cmd 1p:  1 ... => 1 ... p
 637     (1, $p:tt, $($ps:tt),* ; 1)
 638         => (bct!($($ps),*, 1, $p ; 1, $p));
 639     (1, $p:tt, $($ps:tt),* ; 1, $($ds:tt),*)
 640         => (bct!($($ps),*, 1, $p ; 1, $($ds),*, $p));
 641
 642     // cmd 1p:  0 ... => 0 ...
 643     (1, $p:tt, $($ps:tt),* ; $($ds:tt),*)
 644         => (bct!($($ps),*, 1, $p ; $($ds),*));
 645
 646     // halt on empty data string
 647     ( $($ps:tt),* ; )
 648         => (());
 649 }
 650 ```
 651
 652 Exercise: use macros to reduce duplication in the above definition of the
 653 `bct!` macro.
 654
 655 # Common macros
 656
 657 Here are some common macros you’ll see in Rust code.
 658
 659 ## panic!
 660
 661 This macro causes the current thread to panic. You can give it a message
 662 to panic with:
 663
 664 ```rust,no_run
 665 panic!("oh no!");
 666 ```
 667
 668 ## vec!
 669
 670 The `vec!` macro is used throughout the book, so you’ve probably seen it
 671 already. It creates `Vec<T>`s with ease:
 672
 673 ```rust
 674 let v = vec![1, 2, 3, 4, 5];
 675 ```
 676
 677 It also lets you make vectors with repeating values. For example, a hundred
 678 zeroes:
 679
 680 ```rust
 681 let v = vec![0; 100];
 682 ```
 683
 684 ## assert! and assert_eq!
 685
 686 These two macros are used in tests. `assert!` takes a boolean, and `assert_eq!`
 687 takes two values and compares them. Truth passes, success `panic!`s. Like
 688 this:
 689
 690 ```rust,no_run
 691 // A-ok!
 692
 693 assert!(true);
 694 assert_eq!(5, 3 + 2);
 695
 696 // nope :(
 697
 698 assert!(5 < 3);
 699 assert_eq!(5, 3);
 700 ```
 701 ## try!
 702
 703 `try!` is used for error handling. It takes something that can return a
 704 `Result<T, E>`, and gives `T` if it’s a `Ok<T>`, and `return`s with the
 705 `Err(E)` if it’s that. Like this:
 706
 707 ```rust,no_run
 708 use std::fs::File;
 709
 710 fn foo() -> std::io::Result<()> {
 711     let f = try!(File::create("foo.txt"));
 712
 713     Ok(())
 714 }
 715 ```
 716
 717 This is cleaner than doing this:
 718
 719 ```rust,no_run
 720 use std::fs::File;
 721
 722 fn foo() -> std::io::Result<()> {
 723     let f = File::create("foo.txt");
 724
 725     let f = match f {
 726         Ok(t) => t,
 727         Err(e) => return Err(e),
 728     };
 729
 730     Ok(())
 731 }
 732 ```
 733
 734 ## unreachable!
 735
 736 This macro is used when you think some code should never execute:
 737
 738 ```rust
 739 if false {
 740     unreachable!();
 741 }
 742 ```
 743
 744 Sometimes, the compiler may make you have a different branch that you know
 745 will never, ever run. In these cases, use this macro, so that if you end
 746 up wrong, you’ll get a `panic!` about it.
 747
 748 ```rust
 749 let x: Option<i32> = None;
 750
 751 match x {
 752     Some(_) => unreachable!(),
 753     None => println!("I know x is None!"),
 754 }
 755 ```
 756
 757 ## unimplemented!
 758
 759 The `unimplemented!` macro can be used when you’re trying to get your functions
 760 to typecheck, and don’t want to worry about writing out the body of the
 761 function. One example of this situation is implementing a trait with multiple
 762 required methods, where you want to tackle one at a time. Define the others
 763 as `unimplemented!` until you’re ready to write them.
 764
 765 # Procedural macros
 766
 767 If Rust’s macro system can’t do what you need, you may want to write a
 768 [compiler plugin](compiler-plugins.html) instead. Compared to `macro_rules!`
 769 macros, this is significantly more work, the interfaces are much less stable,
 770 and bugs can be much harder to track down. In exchange you get the
 771 flexibility of running arbitrary Rust code within the compiler. Syntax
 772 extension plugins are sometimes called ‘procedural macros’ for this reason.