[Proposal] cmd/go: when mod=vendor, go get should automatically update the vendor directory

[marwan-at-work]

Summary

Now that Go 1.14 recognizes the vendor folder automatically, go get operations should automatically populate the vendor folder as well.

This will save programmers from having to do go get <module> && go mod vendor to just doing go get <module> -- It reduces two steps into one and from personal experience, it is what people expect the behavior to be and are surprised when they run into a compilation error and realize they have to run a follow up command aka go mod vendor.

Reasoning

This provides both consistency and better developer experience. If go build automatically uses the vendor folder, then go get should also do the same and therefore go build should succeed.

Question

If this proposal is accepted, should it be scoped to the go get command only and not other commands such as go mod tidy and go list?

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

1.14

Does this issue reproduce with the latest release?

Yes

What did you do?

• mkdir tempdir && cd tempdir && go mod init tempdir

• go get github.com/pkg/errors

• printf 'package main\nimport _ "github.com/pkg/errors"\nfunc main() {}' > main.go

• go mod vendor

• go build

• go get github.com/sirupsen/logrus

• go build

What did you expect to see?

The vendor directory automatically populated, without having to run a follow up go mod vendor command.

What did you see instead?

go: inconsistent vendoring in /tempdir:
        github.com/sirupsen/[email protected]: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt

run 'go mod vendor' to sync, or use -mod=mod or -mod=readonly to ignore the vendor directory

[mvdan]

CC @FiloSottile @bcmills from #29058

[bcmills]

See #30240 (comment) for our previous rationale.

[bcmills]

Duplicate of #29058