Also see rust-lang/rfcs#1402 and #33170

I know it's been discussed a bunch before. Having the impl exist by itself doesn't mean the APIs need to use it.

Indeed, thanks for the cc @jonas-schievink! @seanmonstar this has been discussed and turned down before, do you have new information which has changed since the decision was last made? If not we can probably just close this.

@alexcrichton a difference to the previous PR is that this only adds the From impl for Option, which is a legitimate implementation currently missing.

The choice to alter APIs to use that is not made in this PR, and so the standard library does not need to worry about stabilizing "optional arguments" in itself that could conflict with lang features.

Here's a longer rationale:

I don't see these as "optional arguments" in the usual sense. I have several arguments that are definitely not optional, they must have an answer, but whether that answer has a value, or a null, is the reason to use Option<T>.

Optional arguments look like this:

fn action(&mut self, action: Action, dur: Option<Duration> = None) {

}

thing.action(Action::Read); // same as thing.action(Action::Read, None)

Instead, I have methods that do not make sense to have this form of "optional arguments":

fn timeout(&mut self, dur: Option<Duration>) {

}

thing.timeout(); // this would not make sense
thing.timeout(None); // you must give an answer
thing.timeout(Some(10));

The point with this method is to specify if there should be a timeout, and if so, for how long. It does not make sense for this to have a signature of fn timeout(&mut self, dur: Option<Duration> = None).

So, what to do. Seems there's 4 options.

1. Accept a T in the argument.

   fn timeout(&mut self, dur: Duration);
   
   // elsewhere
   if let Some(dur) = config.action_timeout {
     thing.timeout(dur)
   }

   Passes the Option noise on to the user. This also means that if the timeout cannot be default to Some, because there is no way to pass the intent "please no timeout (None)".

2. Accept an Option<T> in the argument.

   fn timeout(&mut self, dur: Option<Duration>);
   
   thing.timeout(Some(10)); // thing.timeout(10) would be less noisy

   This removes the Option matching, and now allows thing to have a default of Some(1000), because you can pass None. However, requires that all instances wrap the value in Some, which becomes noisy.

3. Accept Into<Option<T>> in the argument.

   fn timeout<T: Into<Option<Duration>>>(&mut self, dur: T);
   
   thing.timeout(10);
   thing.timeout(None);

   Requires a patch to libstd, such as this PR.

4. Create Maybe<T>, with a bunch of Into impls, and now I have the API ergonomics I need.

   enum Maybe<T> { Yes(T), No }
   impl<T> From<T> for Maybe<T> { ... }
   impl From<Option<T>> for Maybe<T> { ... }
   impl From<Maybe<T>> for Option<T> { ... }
   
   fn timeout<T: Into<Maybe<Duration>>>(&mut self, dur: T) {
     let opt_dur = dur.into().into(); // -> Maybe<T> -> Option<T>
   }
   
   thing.timeout(10);
   thing.timeout(Some(5));
   thing.timeout(None);

   Downside is I now have this public Maybe enum, whose sole purpose is so that I can present an ergonomic API to users.

The fact that we have 2 issues and 2 PRs for exactly the same thing (and without any of the optional arguments stuff, although I see it being as a primary motivation), proves that it is a very desired feature, even without some implementation of optional arguments. I know I wanted it for something that’s not optional arguments in some generic code.

@alexcrichton

this has been discussed and turned down before

Turned down without any motivation except for vague concerns about possible future language features.
From<T> for Option<T> meets all possible criterions for From/Into impls, it's faultless, lossless, unambigous, don't change value domain, it doesn't even do extra allocations. It's better than large part of the existing impls in libstd from this point of view.

From my recollection, the concerns were over modifications to the timeout APIs in particular. The From impl seems to match with my intuition of what makes sense.

I think the concern here is that library authors will start using this feature for optional arguments. Then, if rust ever adds built-in optional/default argument support, there'll be two different ways to do it.

(Note: I'm still in favor of this feature).

Yes it sounds like this is different than #33170 in that it's not proposing the standard library uses it for optional arguments, but the motivation is still optional arguments in external libraries.

Adding the From impl standalone somewhat makes sense to me, but I disagree with the motivation to add it for optional arguments externally.

We discussed this during libs triage today and the decision was to merge given that this PR is only adding a From impl and it's the most reasonable impl that could be done. The standard library won't be using this for optional arguments and conventionally it won't "be a thing", but it's up to library authors externally if they'd like to do this themselves.

@bors: r+

Thanks again for the PR @seanmonstar!

📌 Commit 501c615 has been approved by alexcrichton

⌛ Testing commit 501c615 with merge 18e4090...

💔 Test failed - auto-linux-64-opt

@bors: retry

• Cascading failure on bots when interrupted when performing git operations #34595

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💔 Test failed - auto-linux-64-opt
https://buildbot.rust-lang.org/builders/auto-linux-64-opt/builds/9812

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⌛ Testing commit 501c615 with merge db889f0...

💔 Test failed - auto-mac-64-opt-rustbuild

@bors: retry

• Cascading failure on bots when interrupted when performing git operations #34595

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💔 Test failed - auto-mac-64-opt-rustbuild
https://buildbot.rust-lang.org/builders/auto-mac-64-opt-rustbuild/builds/1808

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@bors: r-

Looks like travis failure is legitimate

Added use convert::From;.

@bors: r+ fbfee42 rollup

⌛ Testing commit fbfee42 with merge 13700cf...

💔 Test failed - auto-linux-64-cargotest

@bors: retry

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