Function sources 

fn sources(
    path: &Path,
    origin: &str,
    prefix: &str,
    extension: &str,
    into: &mut Vec<String>,
) -> Result<(), Error>

Every file with extension under path, relative to it and /-separated.

Recursive, which the listing it replaced was not. read_dir on the top level alone meant a .rs or .py one directory down was in no pair, in no unpaired, and invisible to the rule that makes silent exclusion impossible — so the one way to add an unexamined example was to put it in a subdirectory (§1.3.152). It is now claimed by its path, sub/example.rs, which a pair can name and which cargo cannot run without an explicit [[example]] path: either way the decision is forced rather than skipped.