Macro std::dbg

macro_rules! dbg {
    () => { ... };
    ($val : expr $(,) ?) => { ... };
    ($($val : expr), + $(,) ?) => { ... };
}

Prints and returns the value of a given expression for quick and dirty debugging.

An example:

let a = 2;
let b = dbg!(a * 2) + 1;
//      ^-- prints: [src/main.rs:2] a * 2 = 4
assert_eq!(b, 5);

The macro works by using the Debug implementation of the type of the given expression to print the value to stderr along with the source location of the macro invocation as well as the source code of the expression.

Invoking the macro on an expression moves and takes ownership of it before returning the evaluated expression unchanged. If the type of the expression does not implement Copy and you don’t want to give up ownership, you can instead borrow with dbg!(&expr) for some expression expr.

The dbg! macro works exactly the same in release builds. This is useful when debugging issues that only occur in release builds or when debugging in release mode is significantly faster.

Note that the macro is intended as a debugging tool and therefore you should avoid having uses of it in version control for long periods (other than in tests and similar). Debug output from production code is better done with other facilities such as the debug! macro from the log crate.

Stability

The exact output printed by this macro should not be relied upon and is subject to future changes.

Panics

Panics if writing to io::stderr fails.

Further examples

With a method call:

fn foo(n: usize) {
    if let Some(_) = dbg!(n.checked_sub(4)) {
        // ...
    }
}

foo(3)

This prints to stderr:

[src/main.rs:4] n.checked_sub(4) = None

Naive factorial implementation:

fn factorial(n: u32) -> u32 {
    if dbg!(n <= 1) {
        dbg!(1)
    } else {
        dbg!(n * factorial(n - 1))
    }
}

dbg!(factorial(4));

This prints to stderr:

[src/main.rs:3] n <= 1 = false
[src/main.rs:3] n <= 1 = false
[src/main.rs:3] n <= 1 = false
[src/main.rs:3] n <= 1 = true
[src/main.rs:4] 1 = 1
[src/main.rs:5] n * factorial(n - 1) = 2
[src/main.rs:5] n * factorial(n - 1) = 6
[src/main.rs:5] n * factorial(n - 1) = 24
[src/main.rs:11] factorial(4) = 24

The dbg!(..) macro moves the input:

/// A wrapper around `usize` which importantly is not Copyable.
#[derive(Debug)]
struct NoCopy(usize);

let a = NoCopy(42);
let _ = dbg!(a); // <-- `a` is moved here.
let _ = dbg!(a); // <-- `a` is moved again; error!

You can also use dbg!() without a value to just print the file and line whenever it’s reached.

Finally, if you want to dbg!(..) multiple values, it will treat them as a tuple (and return it, too):

assert_eq!(dbg!(1usize, 2u32), (1, 2));

However, a single argument with a trailing comma will still not be treated as a tuple, following the convention of ignoring trailing commas in macro invocations. You can use a 1-tuple directly if you need one:

assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
assert_eq!((1,), dbg!((1u32,))); // 1-tuple