Formatting as macros

Decided

D-0080, settled by the language’s author: part of spec §18. Older text cites it as 18.80.

print!, println!, eprint!, eprintln!, format! and panic! are the compiler’s own macros, as the intrinsics are, chosen by the user once M10 had landed: invoked println!(…) like an expr macro, since they read like a call (18.9; r64 had spelled them #println). format! replaces String::format and takes allocator = … by name. Any Format is an argument, a type’s own included, so printing a Vec2 no longer goes through a String; an enum, an optional and a pointer have none. Their names are reserved; a bare println(…) is an unknown name whose help says to write println!. Arguments are evaluated as a call’s are — all before anything is written, temporaries living until the text is — because the invocation lowers to one call: the text between placeholders and each spec, split at compile time, and the arguments as &Format, written in turn by std’s Formatter, straight to the stream with no intermediate String. Lowering to one statically-dispatched Format::format call per placeholder was tried and dropped: a sequence of statements ends each argument’s temporaries before the line is written, which a call does not. panic! formats without allocating, since the allocators panic too. error(…) stays a function: #error is a macro’s own rejection (§12.6), and an Error’s message keeps its static storage (§9.2) (§12.9)

Amended in r69 (T-0056, D-0082). An invocation no longer lowers to one call taking &Format: its arguments are evaluated and kept as a call’s are — so what their evaluation made still lives until the text is written — and then each placeholder is written by a direct call to its argument’s Format::format, the statically-dispatched shape dropped above, now without ending temporaries early. And a pointer is written as what it points at, its address by {:p} (D-0082), where this said a pointer has no Format.