Summary

Removes the one-type-only restriction on format_args! arguments. Expressions like format_args!("{0:x} {0:o}", foo) now work as intended, where each argument is still evaluated only once, in order of appearance (i.e. left-to-right).

Motivation

The format_args! macro and its friends historically only allowed a single type per argument, such that trivial format strings like "{0:?} == {0:x}" or "rgb({r}, {g}, {b}) is #{r:02x}{g:02x}{b:02x}" are illegal. This is massively inconvenient and counter-intuitive, especially considering the formatting syntax is borrowed from Python where such things are perfectly valid.

Upon closer investigation, the restriction is in fact an artificial implementation detail. For mapping format placeholders to macro arguments the format_args! implementation did not bother to record type information for all the placeholders sequentially, but rather chose to remember only one type per argument. Also the formatting logic has not received significant attention since after its conception, but the uses have greatly expanded over the years, so the mechanism as a whole certainly needs more love.

Detailed design

Formatting is done during both compile-time (expansion-time to be pedantic) and runtime in Rust. As we are concerned with format string parsing, not outputting, this RFC only touches the compile-time side of the existing formatting mechanism which is libsyntax_ext and libfmt_macros.

Before continuing with the details, it is worth noting that the core flow of current Rust formatting is mapping arguments to placeholders to format specs. For clarity, we distinguish among placeholders, macro arguments and argument objects. They are all italicized to provide some visual hint for distinction.

To implement the proposed design, the following changes in behavior are made:

  • implicit references are resolved during parse of format string;
  • named macro arguments are resolved into positional ones;
  • placeholder types are remembered and de-duplicated for each macro argument,
  • the argument objects are emitted with information gathered in steps above.

As most of the details is best described in the code itself, we only illustrate some of the high-level changes below.

Implicit reference resolution

Currently two forms of implicit references exist: ArgumentNext and CountIsNextParam. Both take a positional macro argument and advance the same internal pointer, but format is parsed before position, as shown in format strings like "{foo:.*} {} {:.*}" which is in every way equivalent to "{foo:.0$} {1} {3:.2$}".

As the rule is already known even at compile-time, and does not require the whole format string to be known beforehand, the resolution can happen just inside the parser after a placeholder is successfully parsed. As a natural consequence, both forms can be removed from the rest of the compiler, simplifying work later.

Named argument resolution

Not seen elsewhere in Rust, named arguments in format macros are best seen as syntactic sugar, and we’d better actually treat them as such. Just after successfully parsing the macro arguments, we immediately rewrite every name to its respective position in the argument list, which again simplifies the process.

Processing and expansion

We only have absolute positional references to macro arguments at this point, and it’s straightforward to remember all unique placeholders encountered for each. The unique placeholders are emitted into argument objects in order, to preserve evaluation order, but no difference in behavior otherwise.

Drawbacks

Due to the added data structures and processing, time and memory costs of compilations may slightly increase. However this is mere speculation without actual profiling and benchmarks. Also the ergonomical benefits alone justifies the additional costs.

Alternatives

Do nothing

One can always write a little more code to simulate the proposed behavior, and this is what people have most likely been doing under today’s constraints. As in:

fn main() {
	let r = 0x66;
	let g = 0xcc;
	let b = 0xff;

	// rgb(102, 204, 255) == #66ccff
	// println!("rgb({r}, {g}, {b}) == #{r:02x}{g:02x}{b:02x}", r=r, g=g, b=b);
	println!("rgb({}, {}, {}) == #{:02x}{:02x}{:02x}", r, g, b, r, g, b);
}

Or slightly more verbose when side effects are in play:

fn do_something(i: &mut usize) -> usize {
	let result = *i;
	*i += 1;
	result
}

fn main() {
	let mut i = 0x1234usize;

	// 0b1001000110100 0o11064 0x1234
	// 0x1235
	// println!("{0:#b} {0:#o} {0:#x}", do_something(&mut i));
	// println!("{:#x}", i);

	// need to consider side effects, hence a temp var
	{
		let r = do_something(&mut i);
		println!("{:#b} {:#o} {:#x}", r, r, r);
		println!("{:#x}", i);
	}
}

While the effects are the same and nothing requires modification, the ergonomics is simply bad and the code becomes unnecessarily convoluted.

Unresolved questions

None.