Function format_failure_detail 

pub fn format_failure_detail(failure: &TestFailure) -> String

Render a single failure as one or more indented detail lines, terminated by a final newline. The result is always whitespace-prefixed on every line so the runner’s collect_failure_block attaches it to the preceding ... FAILED header.

Pure (no I/O, no globals) so the output is unit-testable.