1.0 Introduction
Macros allow users to extend Rust syntax by replacing a macro declaration with code specified by the
macro body. There are two useful consequences of using macros:
-
Code can be writen once and applied in many places. Functions do that too, but macros create code in
the same scope as the macro declaration, where functions create code in a child scope. That means that
a locally defined macro can use local data - very like cut and paste code, but with the advantage that
there is only one source for change.
-
Macros can be variadic, i.e., accept an arbitrary number of arguments. That is a consequence of
pattern matching used to define macros.
Locally defined macro
fn main() {
let offset = 5;
macro_rules! add_with_local_offset {
($left:expr , $right:expr) => {
$left + $right + offset
};
}
let (a, b) = (1, 2);
let result = add_with_local_offset!(a, b);
let s = stringify!(1 + 2 + offset);
print!("\n {:?} = {:?}", s, result);
}
Output - locally defined macro
"1 + 2 + offset" = 8
Variadic macro
/* Demonstrate variadic macros */
macro_rules! variadic_macro {
/* terminating rule */
($h:expr) => {
print!("\n processing argument {}", $h);
};
/* process arguments recursively */
($h:expr, $($t:expr),+) => {
/* process head */
variadic_macro! { $h }
/* process tail */
variadic_macro! { $($t),+ }
};
}
fn main() {
variadic_macro!(1, 2, 3, 4);
println!();
variadic_macro!('a', 'b', 'c');
}
Output - Variadic macro
processing argument 1
processing argument 2
processing argument 3
processing argument 4
processing argument a
processing argument b
processing argument c
1.0 Basics:
Rust macros come in two flavors: declarative and procedural. Procedural macros support defining custom
#[derive] macros, function-like macros that run at compile-time, and custom attributes. They can be
defined in a crate, loaded by Rust and executed as the macro is expanded.
Procedure macros operate on proc_macro::TokenStreams. Those are iterators over TokenTrees.
A TokenTree contains identifiers, literals, punctuators, and groups. Developing a proc macro involves
parsing TokenTrees.
We won't consider procedural macros further in this Bite. Instead we focus on declarative macros.
They take the form:
Macro structure
macro_rules! name {
rule0 ;
rule1 ;
// …
ruleN ;
}
Rule structure
(pattern) => {expansion}
They are composed of one or more rules that describe how to generate code based on their inputs.
Each rule has a pattern matcher and an associated block that generates code from the pattern.
Here's an example:
Indented print macro
macro_rules! indent_print {
/* first rule */
/* pattern */
($a:expr) => {
{
/* expansion template */
print!("\n {}", $a)
}
};
/* second rule */
() => {
{
print!("\n no argument to print")
}
}
}
Using code
fn main() {
indent_print!("this is a demo");
indent_print!();
}
Output
this is a demo
no argument to print
It isn't very useful except to illustrate how pattern matching works. In the first rule
the pattern is an expression, &a, that is expanded using a print! macro, just displaying
the input. The second rule has no input and simply prints out that fact.
2.0 Variadic Macros
Patterns and expansions support repetitions using a syntax reminiscent of regular expressions.
We saw this in the second opening example.
Table 1. - Repetition Syntax
| Syntax | Semantics |
| $(...)* | repeat zero or more ... with no seperator |
| $(...),* | repeat zero or more ... with comma seperator |
| $(...)+ | repeat one or more ... with no seperator |
| $(...),+ | repeat one or more ... with comma seperator |
The Example below extends our indent_print macro to accept an arbitry number of
input arguments and simply passes the input repeated sequence to a print macro.
Indented print with multiple arguments
macro_rules! ma_indent_print {
/*--------------------------------------------
pattern matches $fmt expression followed
by zero or more expressions
--------------------------------------------*/
($fmt:expr, $($a:expr),*) => {
/*----------------------------------------
template creates $fmt followed by zero
or more expressions follwed by commas
----------------------------------------*/
print!($fmt, $($a),*) // each $a taken by ref
}
}
Using code
fn main() {
let s = String::from("Rust");
ma_indent_print!(
"\n {}, {}, {}", "hi", 42, s
);
/* s taken by ref so not moved */
print!("\n {}", s);
}
Output
hi, 42, Rust
Rust
That is not too complicated. But we need to explore how repetitive expansions are handled, rather
than simply passing them along to another variadic macro like print. That we do in the
next example.
2.1 Practical Example:
The next example uses inputs more structured than simple expressions. Table 2 provides a partial list
of types that declarative macros can accept and process.
Table 2. - Partial list of Macro Fragments
| Fragment Type | Matches | Examples |
| expr |
an expression
|
1 + 2, v.len(), "a string" |
| ident |
an identifier
|
x, my_struct |
| ty |
a type
|
String, Vec<String> |
| path |
a std::path
|
C:\\temp\\CodeExperiments |
| pat |
a pattern
|
_, Some(ref x) |
| literal |
a literal value
|
2, "a literal string" |
| tt |
token tree
|
let x=2; let y=x;, [0, 1, 2], fn f{...} |
In this example we will construct a timer that accepts a rich set of inputs for which we want performance
data. The inputs are a literal that displays what we are timing and a token tree, tt,
that allows us to specify statements, functions, and arbitrary combinations of them.
Timer Macro
use std::{thread, time};
use std::result::*;
use std::io::prelude::*;
use std::fs::File;
macro_rules! time_it {
/* measure time to process token tree &tt */
($context:literal, $($tt:tt)*) => {
let timer = std::time::Instant::now();
$(
$tt
)*
let t = timer.elapsed();
println!("{}: {:?}", $context, t);
};
/* measure time to scaffold without execution */
($context:literal) => {
let timer = std::time::Instant::now();
let t = timer.elapsed();
println!("{}: {:?}", $context, t);
};
}
Output
empty: 928ns
let _x = 1 + 2: 956ns
mults: 2.636µs
sleep(10) + sleep(20): 30.241214ms
file open: 139.476µs
Using Code
fn do_sleep(ms:u64) {
let millis =
time::Duration::from_millis(ms);
thread::sleep(millis);
}
fn open_file_for_write(file_name:&str)
->Result<(), std::io::Error> {
let mut file = File::create(file_name)?;
file.write_all(b"Hello, Rust BuildOn!")?;
Ok(())
}
fn main() {
time_it!("empty");
time_it!(
"let _x = 1 + 2",
let _x = 1 + 2;
);
time_it!(
"mults",
let mut x:f64 = 1.5;
for _i in 0..10 {
x *= x;
}
);
time_it!(
"sleep(10) + sleep(20)",
do_sleep(10);
do_sleep(20);
);
time_it!(
"file open",
let _ = open_file_for_write("foobar");
);
}
Token trees, tt, may include tokens that are not expressions, like identifiers. We could add another
rule that ignores them, like the empty rule. If a user supplies an identifier as an input, that causes a
compiler warning to be emitted, but no build or execution errors. Since it is clear that an identifier
has no inherent execution I chose not to provide that rule.
Here is another useful macro example that presents a variadic macro:
Macro for Creating Hashmaps
use std::collections::HashMap;
macro_rules! hash_map {
/* expr can be separated only by , ; or => */
($( $key:expr , $value:expr ),*) => {
{
let mut h = HashMap::new();
$( h.insert($key, $value); )*
h
}
};
}
fn main() {
let map = hash_map!(
"zero" , 0,
"one" , 1,
"two" , 2,
"three" , 3,
"four" , 4
);
print!("\n {:?}", map);
}
Output
{
"two": 2, "four": 4, "zero": 0,
"one": 1, "three": 3
}
3.0 Standard Macros
Rust provides a number of frequently used macros. Many of these we have encountered in previous bites:
Table 3. - Partial list of Standard Macros
| std Macro | Description | Example |
| assert |
Asserts that boolean expression is true |
assert!(x==y, "x != y"); |
| assert_eq |
Asserts that expressions are equal |
assert_eq!(x, y, msg); |
| assert_ne |
Asserts expressions are not equal |
assert_ne!(x, y, msg); |
| concat |
concatenates literals into static string slice |
let x = concat!(a,2.5,"lit_str"); |
| debug |
prints and returns value of expression |
dbg!(a*2); |
| file |
returns name of containing file |
file!(); |
| format |
same as print! except returns String |
let s = format!("{:?}",x); |
| line |
returns number of containing line |
let n = line!(); |
| panic |
panics current thread |
panic!(); |
| print |
Prints to std out |
print!(frm, x, y, ...); |
| stringify |
converts argument symbols to strings and concatenates |
stringify!(a + b, "lit str"); |
| vec |
creates new vector |
let v = vec![0, 1, 2]; |
| write |
writes formatted data to buffer |
write!(&mut buff, fmt_str, x, y, ...); |
4.0 Final Thoughts:
One issue with macros that is easy to overlook: macro processing may move non-copy types so they become invalid
after executing the offending macro. In the example below, we provide two versions of a demonstration
macro. The first, show_compare1, uses a rust match operation that takes its arguments by value,
consuming them. So use of its arguments after macro execution will cause a build failure.
The second version, show_compare2 passes its arguments by reference for matching and does not
invalidate its arguments. You can open the playground example and experiment with calling either one for
both copy and non-copy arguments.
Macros
#![allow(unused_macros)]
/*----------------------------------------
macro show_compare1 will move its args
if they are not copy
----------------------------------------*/
macro_rules! show_compare1 {
($left:expr , $right:expr) => {
match ($left, $right) {
(left_val, right_val) => {
if(left_val == right_val) {
print!(
"\n {:?} == {:?}",
left_val, right_val
);
}
else {
print!(
"\n {:?} != {:?}",
left_val, right_val
);
}
}
}
};
}
/*----------------------------------------
macro show_compare2 will not move its
args even if they are not copy
----------------------------------------*/
macro_rules! show_compare2 {
($left:expr , $right:expr) => {
match (&$left, &$right) {
(left_val, right_val) => {
if(*left_val == *right_val) {
print!(
"\n {:?} == {:?}",
left_val, right_val
);
}
else {
print!(
"\n {:?} != {:?}",
left_val, right_val
);
}
}
}
};
}
Using Code
fn main() {
// let (a, b) = (1, 2);
let a = String::from("a string");
let mut b = a.clone();
b.push_str(" and another");
// let a = vec![1, 2, 3];
// let mut b = a.clone();
// b.push(4);
show_compare2!(a,b);
/*---------------------------------------
Statements below fail to compile if
a or b are moved.
---------------------------------------*/
print!("\n {:?}", a);
print!("\n {:?}", b);
}
Output
"a string" != "a string and another"
"a string"
"a string and another"
5.0 References: