I don't quite understand fmt internals, but Make fmt::Arguments::as_str() return a 'static str. seems unnecessarily restrictive. And it breaks runtime-fmt which depends on hidden apis e.g. Arguments::new_v1

lzutao commented on Aug 7, 2020

Arguments cannot be generated at runtime because it cannot safely be done. So as_str returns &'static str makes sense.

sfackler commented 6 days ago

I'd like to be able to use this in the log crate to correctly implement a codegen optimization that doesn't really work otherwise: rust-lang/log#446

@rfcbot fcp merge

rfcbot commented 6 days ago •

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BurntSushi commented 6 days ago

This SGTM. One question though. Are we sure that the 'static bound will always be correct here?

m-ou-se commented 6 days ago

fmt::Arguments is the result from parsing a formatting string at compile time. In the future, this might not always be a literal but might be a const too. But a non-static/constant format string will be a very different beast.

BurntSushi commented 6 days ago

That's what I figured, but wanted to make sure it was explicitly addressed.

rfcbot commented 6 days ago

This is now entering its final comment period, as per the review above.

jhpratt commented 5 days ago

Does this make any guarantees about when Some is returned? I've been interested in redoing the format_args! macro to be much more intelligent (and thus performant), but obviously don't know when it's possible to return something statically knowable.

m-ou-se commented 5 days ago

Currently it returns Some for format_args!("string literal") only. But my plan is to extend that also to format_args!("{}", "literal"), format_args!("{}", constant), and possibly also things like format_args!("a {} b {} c", const, format_args!("...")), etc. See #78356

jhpratt commented 4 days ago

Yeah, I see that from looking at the source code. I was more inquiring about whether it would be considered breaking to go from returning Some to returning None. I don't suspect anything would be out-of-reach when reimplementing the macro, but you never know.