Pretty Debug with {:#?}

{:?} requests compact Debug output. {:#?} adds the alternate flag and asks the Debug implementation for its pretty representation, which commonly uses line breaks and indentation.

EXAMPLE: CI-CHECKEDTEST ASSERTED · examples/format-output/src/pretty_debug_page.rs#debug-pretty

#[derive(Debug)]
struct Request {
    method: &'static str,
    retries: u8,
}

let request = Request {
    method: "GET",
    retries: 2,
};
let pretty = format!("{request:#?}");
assert!(pretty.contains('\n'));
assert!(pretty.contains("method"));
assert!(pretty.contains("retries"));

When pretty Debug helps

Compact Debug is convenient for a small value on one line. Nested structs, enums, maps, and collections quickly become hard to scan.

Pretty Debug trades vertical space for structure. It is useful while reading local diagnostics, test failures, and temporary inspection output.

Pretty does not mean stable

The standard library does not promise a stable textual format for derived Debug. Rust may change the representation between versions, and a custom type can change its implementation at any time.

Do not use {:#?} as a serialization format, cache key, protocol, persistent file format, or parser input you control. If exact machine-readable output matters, use an explicit serialization format.

Derived and manual Debug

#[derive(Debug)] supports pretty Debug automatically when all fields implement Debug. A manual implementation can use the Formatter debug builders such as debug_struct, debug_tuple, or related helpers, which also support the alternate pretty form.

A completely custom Debug implementation controls its own representation. Pretty output is therefore a request to the implementation, not a guarantee that every type will have the same layout.

The # flag is trait-dependent

The same # flag means something different with hexadecimal, octal, or binary formatters - there it requests a prefix such as 0x or 0b.

See alternate formatting for the broader rule: # asks the active formatting trait for its alternate representation.