cmd/go: documentation doesn't mention GOENV=off

[mdempsky]

The documentation at https://golang.org/cmd/go/#hdr-Environment_variables doesn't mention that GOENV=off is a special case for disabling the use of a Go environment configuration file. It probably should, and maybe to mention GOENV=./off is a workaround if they actually want to use a file named "off" in the current directory.

[ianlancetaylor]

Perhaps also worth noting that

GOENV=off go env GOENV

just prints a newline. It does not print off. I can't tell whether this is a bug or not.

[tpaschalis]

I think the reason why GOENV=off go env GOENV prints a newline instead of 'off' is that in EnvFile we handle this case by returning an empty string and an error; the empty string is eventually printed out and the error is discarded during the go env invocation.

I'm not the best person to point out if this is a bug or not, but for my 2¢, a) the fact that it returns an empty string is utilized in updateEnvFile, and b) returning off instead of this empty string would be weird (returning both a valid value and an error), so to me, the current implementation feels okay.

I can drop a small documentation CL for this; hopefully it's innocuous enough to fit in this cycle.

[odeke-em]

Please go ahead @tpaschalis with the documentation CL, this issue is meant for Go1.18 :-)

[gopherbot]

Change https://golang.org/cl/381894 mentions this issue: cmd/go: document that GOENV=off disables the default config file