As the title says: if I change the way a struct is formatted via fmt::Display, is this considered a breaking change? This would seem to be a "Behavioral change" as defined in RFC 1105, but the rule-of-thumb stated in that section raises another question: to what extent should the format used by fmt:…

[image] H2CO3: The Display trait is supposed to losslessly represent the implementing type as a string, and it should be the inverse of FromStr , i.e. types are generally expected to implement either both traits or neither of them. While I agree this is usefully true for types that implement F…

IMHO, changes to Display shouldn't be breaking unless you've explicitly documented any special guarantees. The std docs say Display is for "user-facing output" which I'd interpret to mean it's not really meant to be parsed by programs, or otherwise relied on. That said, if it's the only way to get …

Practically, changing anything delivered to the user breaks someone's workflow. You may say it should be ok but they'll not agree on it. As always there's some xkcd for such problem. Workflow [image] There are probably children out there holding down spacebar to stay warm in the winter…

[image] chrisd: That said, if it's the only way to get at certain information, programmers may end up "abusing" it to get what they want. So to them changing it would break their program. And for the record, the correct way to deal with this is to provide proper debugging and observability ho…

[image] rlee287: (I'm guessing the answer depends on whether the major version is 0 or not, and would appreciate answers for both cases.) In the variation of SemVer that Rust uses , changing a or b in: 0.a.b 0.0.a Is the same as changing a or b in: a.b.c (a > 0)

Since nobody directly answered the question yet: To put it simply and bluntly: Yes, they (generally) are. The Display trait is supposed to losslessly represent the implementing type as a string, and it should be the inverse of FromStr, i.e. types are generally expected to implement either both tr…

[image] CAD97: I disagree that this applies to all types, though, because Display is part of the interface for Error Yes, you are right, I missed this detail. (I was careful enough to include a "(generally)" clause above, though :sweat_smile:)

[image] H2CO3: The Display trait is supposed to losslessly represent the implementing type as a string, and it should be the inverse of FromStr , i.e. types are generally expected to implement either both traits or neither of them. Where's the source of that? I don't remember neither the loss…

The first constraint is asserted in the fmt module-level docs, here : fmt::Display implementations assert that the type can be faithfully represented as a UTF-8 string at all times. I don't remember where I read the invertibility one, maybe in the Rust API Guidlines? Anyway, the standard librar…

With OsString and Paths being notable exceptions.

Backing up what @H2CO3 said, the libs team explicitly reserves the right to change Debug output but as far as I know we make no such guarantees about Display, so my understanding is that we currently treat Display output as part of our stable API. That said, we already have a history of making small…

Are changes to fmt::Display considered breaking?

matklad May 14, 2021, 12:20pm 9

Where's the source of that? I don't remember neither the losslessness nor invertability constraints for Display in Rust. I know those are the rules for __repr__ / eval in Python and Read / Show in Haskell, but I don't think those are relevant for Rust?

Why is there no whitespace between the punctuator `$` and the identifier after transforming by the macro?

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Are changes to fmt::Display considered breaking?

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