rust/library/core/src/fmt/fmt_trait_method_doc.md

Formats the value using the given formatter.

Errors

This function should return [Err] if, and only if, the provided [Formatter] returns [Err]. String formatting is considered an infallible operation; this function only returns a [Result] because writing to the underlying stream might fail and it must provide a way to propagate the fact that an error has occurred back up the stack.