cmd/go: clean sub command should interop with generate a bit better

[mmlb]

We currently have a file that is generated via go generate to which the .go suffix is added to the filename. When packaging we run go clean which proceeds to delete the source file, this was unexpected. Here is a minimal example:

~/t  
❯ tail -n +1 main.go print.dummy
==> main.go <==
package main

//go:generate cp print.dummy print.dummy.go

func main() {
    printIt("hi")
}

==> print.dummy <==
package main

import "fmt"

func printIt(s string) {
    fmt.Println(s)
}

~/t  
❯ go generate && ls
main.go  print.dummy  print.dummy.go

~/t  
❯ go clean -x
cd /home/mmlb/t
rm -f t t.exe t.test t.test.exe main main.exe print.dummy print.dummy.exe

I expect that the $source.file -> $source.file.go pattern is probably common enough that go clean should special case it. I was very surprised to see go clean -x purposefully deleting the file even though the match is documented/specified.

A couple of options are possible to handle this case:

1. Do not delete MAINFILE if it is not executable.
   It seems MAINFILE is deleted due to it being an expected binary generated via go build *.go or similar.
2. Special case a file that shares the root file name of a .go source file as long as the root contains a . in the name.

[robpike]

Working as intended, and has nothing to do with go generate.

Go clean removes executables, which include files named foo when there is a foo.go source file. Here, you have a file print.dummy.go and a file print.dummy, which go clean is documented to remove.

Rename print.dummy in your example and the problem will go away.