What is Rust term for this

i keep seeing this pattern


what does it mean? or what is the correct terminology?

It's a formatting parameter that means "use the fmt::Debug implementation to format this one". So if something has a Debug implementation (which can usually be easily derived), but not a Display implementation, you'll need to use {:?}. (Even if it has a Display implementation, sometimes the Debug implementation will give you more information... which may be useful for debugging, say.)

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I would say it's a "formatting (sub)string" or placeholder inside a formatting string, with a "next argument" specifier {} and a formatting paramater ? (after a colon inside the curly braces) to select Debug formatting.

See: std::fmt - Rust