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cmd/gofmt: doesn't simplify slices if there is a dot import #15153

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dominikh opened this issue Apr 6, 2016 · 7 comments
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cmd/gofmt: doesn't simplify slices if there is a dot import #15153

dominikh opened this issue Apr 6, 2016 · 7 comments
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Go1.7

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@dominikh
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dominikh commented Apr 6, 2016 •
edited

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?
go version go1.6 linux/amd64

What operating system and processor architecture are you using (go env)?
/

What did you do?
Run gofmt -s on http://play.golang.org/p/9NhvjR24S1

What did you expect to see?
I expected s[:len(s)] to be simplified to s{;}

What did you see instead?
It wasn't simplified.

gofmt has a special check for dot imports and refuses to simplify slices in that case, claiming that it's impossible to know whether len is referring to the built-in function or not. I don't see how that could be true. Even a dot import will only pull in exported identifiers from a package, and no exported identifier can be called "len". Am I missing something here?
@bradfitz
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bradfitz commented Apr 6, 2016

/cc @griesemer

@bradfitz bradfitz added this to the Unplanned milestone Apr 6, 2016
@griesemer
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@dominikh You're absolutely correct. In fact, even w/o the dot-import we actually can't be certain whether len is the builtin len since it might be declared in another file of the package, and gofmt is by design only working on a file-by-file basis and thus cannot type-check the package and resolve an identifier for certain.

So in some sense this was always wrong, but in practice, nobody in his/her right mind is going to declare a len function (as opposed to a len method which might be quite common).

I'm not sure how I got to this code - for sure it's pretty old code. Would need to track down the history of that line to figure it out. Let's blame it on lack of coffee. Fix coming shortly.

@griesemer griesemer self-assigned this Apr 6, 2016
@griesemer griesemer modified the milestones: Go1.7, Unplanned Apr 6, 2016
@dominikh
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dominikh commented Apr 6, 2016

Great, thanks.

P.S.: Anyone know where the description of the issue went? It certainly wasn't empty earlier :-)

@griesemer
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@dominikh No idea what happened here - I certainly didn't touch it. Hmm...

@gopherbot
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CL https://golang.org/cl/21613 mentions this issue.

@dominikh
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dominikh commented Apr 6, 2016

I've restored the description… let's hope GitHub doesn't eat it again.

@martisch
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martisch commented Apr 6, 2016

btw origin of the code:
0cbca26

@golang golang locked and limited conversation to collaborators Apr 6, 2017
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