Bury Error::description() #2230

kornelski · Nov 29, 2017

Rendered

Thread on internals

nagisa · Nov 29, 2017

cf. rust-lang/rust#44842

durka · Nov 29, 2017

durka reviewed Nov 29, 2017

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durka · Nov 29, 2017

0000-bury-description.md

+# Drawbacks
+[drawbacks]: #drawbacks
+
+When users start omitting bespoke `description()` implementations, code that still uses this method will get machine-generated rather than human-written descriptions, so success of this change depends also on steering authors away from using this method as well.


Why not also formally deprecate the method?

It could create too many warnings. Perhaps it would be OK to do it after a while?

Some projects may still like using a static string for description. I'm concerned about saving time users who don't want to use it, but don't want to get in the way of users who still want it.

I would like to see the method formally deprecated at some point, but I agree that we should give it several releases before doing so, and make sure it has minimal impact on the ecosystem.

+1 for eventual formal deprecation

jnicholls · Nov 30, 2017

Totally agree on making this method optional/transparent/forgotten/deprecated/dismissed/booted/chopped/kicked/etc.

aturon · Feb 14, 2018

@withoutboats can you shepherd this RFC?

withoutboats · Feb 14, 2018

Why not deprecate it also?

Centril · Feb 14, 2018

@withoutboats It might be just me... but .description() seems like a cheap way to describe error enums with unit-only-variants... There's always Debug, but a &str is cheaper..

To make .description() a bit more useful you could perhaps make it derivable?

sfackler · Feb 14, 2018

@Centril can you point to some code that uses description in that way?

Centril · Feb 14, 2018

@sfackler Not really; If you have such a unit-variant enum, then I guess you can use an inherent method instead - the bound isn't important wrt. .description() here, and neither as a trait object.

Mark-Simulacrum · Apr 20, 2018

Mark-Simulacrum reviewed Apr 20, 2018

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Mark-Simulacrum · Apr 20, 2018

0000-bury-description.md

+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The description method can have default implementation returning `core::intrinsics::type_name::<Self>()`. This preserves basic functionality of this method for backwards compatibility.


Are we comfortable somewhat stabilizing type_name? This makes it at least somewhat easy to (abuse) this to get it's functionality on stable for non-error types in at least binaries where the impl of Error doesn't matter too much from an exposed API perspective.

Either way, we should explicitly state that this method's default return value is not stable and should not be relied on from version to version.

Simon's current PR just returns ""

U007D · Apr 20, 2018

Overall, I like this. Although I'm not sure why we'd steer people away from implementing .description() in documentation and not officially deprecate it. The rationale given:

... this RFC does not propose formal deprecation at this time to avoid unnecessary warnings during the transition.

would seem to apply any time any feature is deprecated, no? The above-mentioned warning would also serve to reinforce the updated documentation, essentially ensuring discovery.

Overall, things are currently a bit messier than I think we'd all like around errors and error handling--moving as swiftly as we can (but not recklessly) seems to me to be a good way to realizing the full potential of Rust's error handling--to that end, I'd (also) support official deprecation of .description().

Procedural question: Should I review with "change requested", or just leave this feedback as it is?

repax · Apr 20, 2018

I would like description() to be deprecated, or at least just return "". I think it's undesirable to expose identifier names (which are just source-code level entities).

The identifier name of a type should not have any consequence on its public output/effect - especially not by default.

kornelski · Apr 21, 2018

What should the default message be? Type name is out. Empty string is out.

repax · Apr 21, 2018

Is is out of the question to have different outputs depending on whether dbg_print is enabled? If enabled, a more detailed description could be provided by the compiler, and if not a minimal default would be used. The developer can always add his/her own implementation, if necessary.

The default output for both the dbg_print and the non-dbg_print case could simply be left unspecified. That should be compatible with the current situation.

kornelski · Apr 21, 2018

I'm thinking about making the message push authors towards switching to Display instead, so "description is deprecated" or "use Display instead" or "https://rust-lang.org/how-to-fix-descriptions"

U007D · Apr 21, 2018

Hmm.. this is tricky! :)

".description() is deprecated--see https://rust-lang.org/how-to-fix-description" (along with explanatory content at that address) seems to be a reasonable default message to me, given the excellent points made by @repax and @Mark-Simulacrum. (Suggest singular 'description' in URL)

kornelski · Apr 30, 2018

the implementation is merged!

U007D · Apr 30, 2018

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U007D · Apr 30, 2018

0000-bury-description.md

+# Drawbacks
+[drawbacks]: #drawbacks
+
+When users start omitting bespoke `description()` implementations, code that still uses this method will start getting empty strings instead of human-written description. If this becomes a problem, the `description()` method can also be formally deprecated (with the `#[deprecated]` attribute). However, there's no urgency to remove existing implementations of `description()`, so this RFC does not propose formal deprecation at this time to avoid unnecessary warnings during the transition.


...code that still uses this method will start getting empty strings instead of human-written description

No longer--the implementation merged into master now returns "description() is deprecated; use Display" as a default message.

Suggest: s/empty strings/.description() deprecated message

U007D · Apr 30, 2018

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U007D · Apr 30, 2018

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U007D · Apr 30, 2018

I have no power... :)

alexcrichton · May 3, 2018

It turns out that the implementation for this RFC has already merged! In light of that I'm going to go ahead and merge the RFC

Bury Error::description()

98a5691

dtolnay added the T-libs label Nov 29, 2017

kornelski referenced this pull request Nov 29, 2017
Closed
Provide better guidance for `Error::description` #45249

Note possibility of later deprecation

d340931

CAD97 referenced this pull request Dec 18, 2017
Open
nom4: the trait `std::error::Error` is not implemented for `nom::internal::Err<&[u8]>` #591

aturon assigned withoutboats Feb 14, 2018

SimonSapin referenced this pull request Apr 15, 2018
Closed
Soft-deprecate the description() method of the Error trait #49536

kornelski added a commit to kornelski/book that referenced this pull request Apr 20, 2018

Remove mentions of Error::description()
rust-lang/rfcs#2230

e0352f1

Make deprecated description return empty string

a15b80f

kornelski referenced this pull request Apr 22, 2018
Merged
Bury Error::description() #50163

Update RFC to match implementation

01434ea

alexcrichton merged commit 01434ea into rust-lang:master May 3, 2018

Centril added A-traits-libstd A-error-handling labels Nov 23, 2018

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Bury Error::description() #2230

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kornelski commented Nov 29, 2017 • edited by Centril

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