Formatting

The compiler-checked std/fmt.Format declaration is the exact static interface contract. String Interpolation defines how the language selects it. Format requires one recoverable append operation and derives the aborting operation used by owned interpolation.

The distributed library conforms str, String, bool, and every built-in integer. A nominal project type may implement the interface and build its representation with canonical members: output.try_push_str for text and value.try_format_into(output) for nested formatted values. try_format_into reports recoverable destination growth failure and may leave a successfully appended prefix. The default format_into converts that failure to the ordinary allocation-abort policy used by interpolation. Formatting dispatch is static and does not require a runtime interface object.

std/fmt publishes the Format contract, not one append function for every built-in type. The closed scalar append operations used by the old surface are replaced by each scalar's one try_format_into implementation. Type-owned try_to_string and standard-library diagnostics call that same method. This keeps decimal spelling under one implementation authority without presenting a closed type matrix as a general formatting API.

Formatting and byte output deliberately remain distinct contracts. Format reports only mutation failure from its owned String destination; Writer may report an I/O failure after publishing a prefix outside the process. Their sole value boundary is well-formed &str. The standard library does not define a second direct-to-writer formatting protocol or reinterpret a writer error as allocation failure.