Inspecting println!

Inspect the internals of the println! macro.

Inside println!

println! is a macro because it checks the format string at compile time: if the number of placeholders and arguments doesn't match, it's a compile error. It expands roughly like this:

println!("x = {}", 5);
// expansion (simplified):
::std::io::_print(format_args!("x = {}\n", 5));

format_args! is a macro built into the compiler; it parses the format and produces a fmt::Arguments value without allocating. format!, write! and panic! use the same foundation.

Your own format macro

To forward arguments as they are, a tt repetition is used; this way the macro supports all of println!'s forms. stringify! gives an expression as its source text:

macro_rules! kayit {
    ($($arg:tt)*) => {
        println!("[log] {}", format!($($arg)*))
    };
}

macro_rules! goster {
    ($e:expr) => {
        println!("{} = {:?}", stringify!($e), $e)
    };
}

kayit!("x = {}", 5);   // [log] x = 5
goster!(2 + 3);        // 2 + 3 = 5

The dbg! macro is built on exactly this idea.

Exercise Logging macros

Write the kayit! and goster! macros above. Make the calls kayit!("x = {}", 5) and goster!(2 + 3).

Show example solution

macro_rules! kayit {
    ($($arg:tt)*) => {
        println!("[log] {}", format!($($arg)*))
    };
}

macro_rules! goster {
    ($e:expr) => {
        println!("{} = {:?}", stringify!($e), $e)
    };
}

fn main() {
    kayit!("x = {}", 5);
    goster!(2 + 3);
}

Test yourself

What happens with <code>println!("{} {}", 1)</code>?
- It prints "1 "
- Panic
- It prints "1 {}"
- Compile error: 2 placeholders, 1 argument
Show answer

Compile error: 2 placeholders, 1 argument — The format string must be a literal; that's how the compiler can parse and check it.

Source text

println!("{} = {:?}", ____!($e), $e)

Show answer

println!("{} = {:?}", stringify!($e), $e)

Macro and job

format_args!

Format without allocating
format!

Produces a String
stringify!

Turns code into text
dbg!

Prints expression and value

Next lesson: Summary - Declarative macros

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