Application exit and error reports

The native wrapper calls the application’s main once. Unit and Ok(()) produce status zero, i32 and Ok(i32) supply the status, and Err produces one. The operating system may truncate statuses; small nonnegative values are portable. Entrypoint signatures are defined in the language contract.

A returned Err produces one report, identically for compile-and-run and a compiled executable. The wrapper flushes standard output, writes one line to standard error, destroys the error payload once, and returns one. Intermediate ? propagation prints nothing.

The line begins error: main returned . When the concrete error type satisfies std.fmt.Formattable, its borrowed format method supplies the message. An anonymous sum takes this path only when every member satisfies the protocol. Otherwise the payload is rendered as str.repr would render it. Errors needing an explanation add one after a colon: IoError.System(code) uses the OS description, payload-free IoError variants use fixed descriptions, and Failure states that it retains no original details. Choose a concrete error type when the report should identify the cause.

Reporting is best effort. On the structural path, if allocation fails or the type cannot be rendered (for example it contains a generator or recursive data), the wrapper writes the allocation-free fallback error: main returned Err. Standard-error write failure, including a closed descriptor or a pipe without a reader, is ignored. Every path still destroys the payload and returns one.

Human-readable reporting allows 16,384 bytes for the prefix, message and final newline, with a maximum rendering depth of 64. A formatting or budget failure appends [report unavailable] and a newline, outside that byte budget. The already-written prefix remains; the formatter is never called again and its error is not reported recursively. A terminal stderr failure stops output without attempting the suffix. These writes suppress SIGPIPE and need no heap allocation, though a user formatter may allocate. Reports stream in pieces and can interleave with other writers; they are not atomic records. User-supplied text is emitted unchanged, so it may itself contain newlines.

Context[E] uses human-readable reporting when its cause supports it, printing the operation/path followed by the cause once. Other contexts retain structural reporting. Reporting does not change which types may be returned in Err or the behavior of intermediate propagation.

Compilation/checking errors execute no program code. Runtime errors can leave effects already performed. Traps and stack overflow do not unwind.