x/playground: goimports removes a needed import

[harrybarrer]

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

Latest.

The playground uses the latest stable release of Go.
The current version is go1.16.4.

Does this issue reproduce with the latest release?

Yes

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

Playground

What did you do?

Tested in go playground https://play.golang.org/

1. Select multiple files
2. Change folder name from -- foo/foo.go -- to -- foo_test/foo.go --
3. Change import in main from "play.ground/foo" to "play.ground/foo_test"
4. Run. It works. Result: "This function lives in an another file!"
5. Check imports button to update imports
6. Run Format (this will remove import)
7. Run again. You will see the error ./prog.go:4:2: undefined: foo

Playground link: https://play.golang.org/p/Fq83n0Gc2YC

What did you expect to see?

goimports ideally should add the package name before the import as an alias. That was the previous behaviour. Or at least, not remove the import because it will break the code.

What did you see instead?

goimports removes the needed import and breaks the code.

error ./prog.go:4:2: undefined: foo

Related discussion

#29557

[heschi]

I believe this is a bug specific to the playground. Without investigating, I'd guess that it's only passing the main file to goimports, so there's no way to account for surprisingly-named packages. Per the bug you linked, it's intended that unknown imports are removed when they don't obviously satisfy a requirement.

[heschi]

cc @toothrot