core and std: Optimize write*!() and print*!() for a single argument

[erickt]

Before this patch, write!(wr, "foo") was not as fast as wr.write_all("foo".as_bytes()) because the underlying write_fmt has some overhead in order to work with multiple arguments. This PR speeds that up by optimizing this specific case.

[rust-highfive]

r? @huonw

(rust_highfive has picked a reviewer for you, use r? to override)

[cmr]

@bors r+ bc7dd05 rollup

[erickt]

@bors r-

Ugh. This is actually a breaking change. Doing write!(wr, "{{") would now produce "{{" instead of "{". I'll have to tweak this

[erickt]

Turns out panic! right now also has this issue. panic!("{") produces a single "{", panic!("{{") produces "{{", panic!("{ {}", 5) produces a compile time error, and panic!("{{ {}", 5) produces "{ 5".

[pczarn]

format_args! could expand to a single string literal.

[erickt]

@pczarn: Unfortunately then format_args!("foo") would be then returning a different type than format_args!("{}", 5), which would cause all sorts of havoc. I think we instead need a format_arg!(fmt) that uses the same syntax as format_args! but only allows for one argument. I have that implemented and I'll push it up in a few moments after I make sure it passes the tests locally first.

[huonw]

I suspect the two cases need to be in the same macro definition to be rendered,

macro_rules! format_args {
     ($fmt: expr) => ({ /* compiler built-in */ });
     ($fmt: expr, $($args: tt)*) => ({ /* compiler built-in */ });
}

[huonw]

@erickt pointed out my mistake on IRC (interpreted this just a variation of format_args rather than a new format_arg macro).

In any case, I think the doc string for this needs updating, since AFAICT, it doesn't return Arguments any more.

[erickt]

@huonw: I also have a branch that merges format_arg!() and format_args!() if we want to go that direction.

[alexcrichton]

If this is adding a new format_arg! macro then I think this needs an RFC behind it (as it's adding a new macro to the prelude).

[pczarn]

How is this different from format_args!($fmt, $($arg)*).to_string()?

[pczarn]

@erickt: you can add a method for extracting the only string piece out of fmt::Arguments and avoid adding a new macro.

[erickt]

@pczarn: doing that would be doing roughly the same amount of work Writer::write_fmt does, so at that point it'd be better to stick with what we've got right now. If we don't want this optimization, it's easy enough to call Writer::write_str. I'll close this for now in deference for an RFC, since I don't think I'd be able to do this without adding a new macro, or changing format_args to work differently for a single format string.