Files

The filesystem is the first thing outside your program that you are likely to talk to, and the first place Rust's insistence on types starts paying rent: a path is not a string, a missing file is not an empty one, and a handle that goes out of scope closes itself.

Two of these pages are finished; the other four are stubs — outlines waiting for a runnable example, written so the arc has a shape and a permanent URL before the prose exists. A page graduates by gaining an examples/ program and losing its stub notice, which is the same promise as everywhere else here: no page claims something a program has not printed. A file example has a second obstacle to clear before it can be checked against a recorded answer key — the path differs per machine, and the directory may hold leftovers from an earlier run — and the two finished pages clear it the same way: write only inside a directory made under std::env::temp_dir() and named with the process id, print byte counts, error kinds and contents but never a path, and remove the directory at the end.

Lesson	Level	What it teaches
Opening a file	201	open, create and OpenOptions — three doors and one decision the type system will not make for you: create empties the file at open, append does not create, and the error names the problem but not the file
A file is bytes; a String is a promise	201	write_all takes &[u8], so a &str is widened for free; read_to_string checks UTF-8 once at the door and refuses with InvalidData; write! is two traits; and include_str! moves the check to compile time
Path and PathBuf	201	The same split as &str and String, plus the join that throws your path away
Reading lines efficiently	201	One allocation for the file, one per line, or none per line — and when each is the right answer
Missing is not empty	201	"The file is not there" and "the file is there and empty" are different answers, and only one is an error
Temporary directories in tests	201 → 301	A test that writes to a fixed path cannot run twice at once — and the fix deletes itself if you drop the handle

The string half of this story — String against &str, and the slice that panics mid-character — is in Strings, where it belongs. The seam between the two, where a &str becomes bytes on the way out and bytes have to earn their way back in, is A file is bytes.

Po polsku

System plików to zwykle pierwsza rzecz poza programem, z którą program rozmawia, i pierwsze miejsce, w którym upór Rusta przy typach zaczyna na siebie zarabiać: ścieżka (path) ma własny typ i nie jest łańcuchem znaków, brak pliku to co innego niż plik pusty, a uchwyt do pliku (file handle) zamyka się sam, gdy wychodzi z zasięgu. Ten ostatni punkt zaskakuje najczęściej — nie ma tu ani close(), ani bloku finally, bo zamknięcie pliku to zwykłe wypuszczenie zasobu, czyli dokładnie to, co polskie materiały o C++ nazywają RAII. Dwie strony tego rozdziału są gotowe, cztery pozostałe to wciąż szkice — a powód jest ten sam dla wszystkich: każdy przykład w tej bibliotece jest w CI porównywany z zapisanym wzorcowym wyjściem, a wszystko, co dotyka systemu plików, z natury różni się między maszynami. Gotowe strony radzą sobie z tym jednakowo: piszą wyłącznie do katalogu utworzonego pod std::env::temp_dir() i nazwanego numerem procesu, wypisują liczby bajtów, rodzaje błędów i zawartość, ale nigdy ścieżkę, i na końcu ten katalog usuwają.

Szukaj po polsku: obsługa plików w Ruscie · ścieżka a łańcuch znaków · wypuszczenie zasobu RAII · rust std::fs · rust file closed when dropped