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cmd/gofmt: rewrite 'a -> a' transforms 'default:' into 'case :' #33103

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rillig opened this issue Jul 14, 2019 · 5 comments
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cmd/gofmt: rewrite 'a -> a' transforms 'default:' into 'case :' #33103

rillig opened this issue Jul 14, 2019 · 5 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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Go1.14

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@rillig
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rillig commented Jul 14, 2019

What version of Go are you using (go version)?

go version go1.13beta1 windows/amd64

Does this issue reproduce with the latest release?

yes

What did you do?

package main

func main() {
	func() {
		switch {
		default: // This is changed to `case :`.
		}
	}()
}
gofmt -r 'a->a' -d .

What did you expect to see?

After rewriting, the code still compiles.

What did you see instead?

The default: is rewritten to case :, which is syntactically invalid.
This only happens inside a function literal, but not in main itself.
@dmitshur
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Thanks for the report.

I checked, this affects Go 1.12.7 as well as Go 1.10.5, so it's not a new regression in Go 1.13 beta 1.

Given this and the other 2 issues you reported (#33104, #33105), it seems there are some bugs in the rewrite rules when doing an identity replacement (a -> a). This issue doesn't come up when doing something else like (a) -> a, but that's because default: doesn't match (a).

@rillig Have you seen this issue come up in more practical replace rules other than the no-op a -> a replacement, or are you reporting this just because it's a valid problem you discovered even though it's unlikely to come up in real-world usage?

I checked https://goissues.org/cmd/gofmt and it doesn't seem that this was reported in the past, so thanks again.

/cc @griesemer

@dmitshur dmitshur added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jul 15, 2019
@dmitshur dmitshur added this to the Go1.14 milestone Jul 15, 2019
@rillig
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rillig commented Jul 15, 2019

@dmitshur The reason I tried out the a->a transformation was that gofmt is known to sometimes discard comments and add additional empty lines, which makes it an unreliable tool that needs manual inspection afterwards. I experienced this myself when I did a large code transformation.

To make sure that gofmt doesn't damage my code, my idea was to first try the transformation by essentially doing nothing, before actually transforming something. So instead of running gofmt -r 'call(a, b, c) -> call(b)' directly, I would first run gofmt -r 'call(a, b, c) -> call(a, b, c)' to see whether anything is damaged.

The simplest such call is gofmt -r 'a->a', therefore I used that for playing around with gofmt. And since that command found so many inconsistencies, I thought I'd better report them immediately instead of letting gofmt break a whole lot of real life code.

@gopherbot
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Change https://golang.org/cl/187879 mentions this issue: cmd/gofmt: fix a->a rewrite rule for switch with default only

@gopherbot
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Change https://golang.org/cl/187880 mentions this issue: cmd/gofmt: fix a->a rewrite rule for switch with default only

@rillig rillig mentioned this issue Jul 28, 2019
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@gopherbot
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Change https://golang.org/cl/187917 mentions this issue: cmd/gofmt: don't turn nil slices into empty slices during rewriting

@golang golang locked and limited conversation to collaborators Sep 10, 2020
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