Reading a compilation failure

Level: 101 → 201 · newcomer

One line: Before reading the message, work out which stage produced it — a parse error, a type error, a borrow error and a link error come from four different programs with four different vocabularies, and the fix you are looking for is different in each.

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

What it has to cover

• The four dialects side by side, each with a real transcript: expected one of, E0308 mismatched types, E0502 cannot borrow, undefined symbols
• rustc --explain E0308 — the fuller writeup with an example, available offline, and the codes that are worth learning by heart
• Reading a diagnostic's structure: the --> location, the | span, the help: versus note: distinction, and which suggestions are safe to apply blind
• Why the first error is the one to fix, and how a single missing brace generates eleven downstream complaints
• cargo check versus cargo build: the same front end, stopping before codegen, so an error list arrives sooner
• Errors that are not rustc at all — a linker's, a build script's, a proc macro's panic — and how to tell each from a real compile error at a glance
• The one that fools everyone: a trait bound reported at the call site when the missing impl is somewhere else entirely

The trap it exists for

A borrow error and a type error look alike on the page and are nothing alike in the head. One says the shape is wrong; the other says the shape is fine and the timing is not, and applying a type-error habit to a lifetime problem produces the .clone() that makes the message go away without teaching anything.

See also

• What a warning is asking — the diagnostics that let the build succeed, and why they are still worth reading
• What a compiler does before your program runs — which stage can raise which error
• The linker — the failure that is not the compiler's, in someone else's words

Po polsku

Pierwsze pytanie przy czerwonym komunikacie nie brzmi „co on znaczy”, tylko który etap go wypisał — expected one of pochodzi od parsera, E0308 mismatched types od kontroli typów, E0502 cannot borrow od borrow checkera, a undefined symbols od konsolidatora (linker), czyli od programu, który kompilatorem już nie jest. Cztery słowniki, cztery zupełnie różne rodzaje poprawki. Najkosztowniejsza pomyłka to wzięcie błędu pożyczania za błąd typów: błąd typów mówi „kształt się nie zgadza”, a błąd pożyczania — „kształt jest dobry, zły jest moment”, więc odruchowe .clone() wycisza komunikat, nie ucząc niczego. Komunikaty rustc są po angielsku i polskiej wersji nie ma, ale zanim cokolwiek wkleisz w wyszukiwarkę, uruchom rustc --explain E0308 — ten sam błąd z dłuższym opisem i przykładem, lokalnie; i poprawiaj zawsze pierwszy błąd z listy, bo jeden brakujący nawias klamrowy potrafi wygenerować kilkanaście kolejnych, które znikną same.

Szukaj po polsku: komunikaty błędów kompilatora Rust · błąd pożyczania · rust --explain E0308 · rust E0502 cannot borrow · rust undefined symbols linker