I checked why the unit test for the cli-args fails and the situation is somewhat complicated.

1. go build -mod=vendor is now checking that imports are in vendor/modules.txt.
2. The import that is missing in the build ( github.com/yuin/goldmark ) is in the go.mod of golang.org/x/tools but not in the vendor/modules.txt we generate.
3. It's not in our vendor/modules.txt because the test only builds a subpackage that doesn't use it. Go can detect this because we create our own temporary module and only reference the subpackage there. The resulting go.mod is

module gomod2nix/dummy/package

go 1.23.1

require golang.org/x/tools v0.1.11

require (
        golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4 // indirect
        golang.org/x/sys v0.0.0-20211019181941-9d821ace8654 // indirect
)

and so the resulting vendor/modules.txt is

# golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4
## explicit; go 1.17
golang.org/x/mod/semver
# golang.org/x/sys v0.0.0-20211019181941-9d821ace8654
## explicit; go 1.17
golang.org/x/sys/execabs
# golang.org/x/tools v0.1.11
## explicit; go 1.17
golang.org/x/tools/cmd/stringer
golang.org/x/tools/go/gcexportdata
golang.org/x/tools/go/internal/gcimporter
golang.org/x/tools/go/internal/packagesdriver
golang.org/x/tools/go/packages
golang.org/x/tools/internal/event
golang.org/x/tools/internal/event/core
golang.org/x/tools/internal/event/keys
golang.org/x/tools/internal/event/label
golang.org/x/tools/internal/gocommand
golang.org/x/tools/internal/packagesinternal
golang.org/x/tools/internal/typeparams
golang.org/x/tools/internal/typesinternal

This is absolutely correct. If you run go build -mod=vendor golang.org/x/tools/cmd/stringer on that, it works.

4. So why does go then later claim that it's missing? That's because we are not building our temporary go module but from the source of golang.org/x/tools which does include the dependency in its go.mod.

So, how to fix this? I think ultimately we would need to bring the environment that generates the gomod2nix.toml and the environment that consumes it closer together which is tedious :(

I tested this branch, and it seems it broke the --dir flag. Whatever is put in the --dir flag will be repeated in front of the downloaded vendor-gomod2nix, so inside the dir, you will have $DIR/$DIR/vendor-gomod2nix. Then you get a panic when it tries to read the modules.txt file which tries to read in the correct location.

panic: error generating pkgs: open $DIR/vendor-gomod2nix/modules.txt: no such file or directory