cmd/go: better error for directory outside modules

[switchupcb]

directory . outside modules listed in go.work or their selected dependencies

It's not clear what's going on with this error due to the period alongside the lack of slashes indicating a file location. For this very specific situation, "directory ." could be replaced with "current directory" and save Google millions of engineer hours.

proposal: replace directory . outside modules listed in go.work or their selected dependencies with current directory is outside modules listed in go.work or their selected dependencies or directory ./ outside modules listed in go.work or their selected dependencies
proposal2: add go work toggle feature
proposal3: add way to exclude folder from go workspace

[switchupcb]

This occurred when I attempted to use go build from ./_gen/ in this repository with the ./ in go.work removed. This happened because something (VSCODE?) set the GOWORK variable to the opened folder (./disgo), which toggles workspace mode (https://go.dev/ref/mod#workspaces). The fix was to set workspace mode off by setting the environment variable: GOWORK=off.

[switchupcb]

Either I'm using workspaces incorrectly, or this is a good opportunity to suggest a toggle feature for the go work tool. Hopefully someone can show me that I'm wrong on the internet and how to fix that issue. Specifically, how to exclude directories from the workspace (cause I need to include ./ in go.work) and how to not have to use the GOWORK environment variable to build in that folder?

[ianlancetaylor]

CC @bcmills @matloob

Changing an error messages doesn't require a proposal, so taking this out of the proposal process. If you want to make one of the proposals you mention, I think it needs more explanation. Thanks.

[switchupcb]

Here is my eventual end goal (use case) for that repository.

disgo.go (developers import this)
- `disgo` go module
- bundled code (fieldaligned)
- used in ./tools, ./cache, ./shard, ./_examples
- filepath is ./

wrapper (for development purposes only)
- `wrapper` go module
- human readable code
- not used anywhere
- filepath is ./wrapper

generator (used during development)
- not used as a go module (`go build`)
- generator code execution
- not used anywhere
- filepath is ./_gen

Currently, I can't do this because I must use ./ in the go workspace, but cannot exclude _,/gen or ./wrapper from the go.work file.

[gopherbot]

Change https://go.dev/cl/451295 mentions this issue: cmd/go: replace 'directory .' with 'current directory' in some errors