Build a go project with a different go.mod file

I would like to know how can I build a go project using a different go.mod file. Suppose I want to build project A inside project B module using project B go.mod file without copying the files around. That means I want to use dependencies in Project B to build Project A.

Manual option

"Module files" refers to both go.mod and go.sum

1. Rename or move project A's module files to some temporary names / location
2. Copy project B's module files into project A
3. Edit the newly copied go.mod file in project A, and change the module name:

module github.com/x/b changes to module github.com/x/a

4. Build whatever you need to build in project A
5. Delete the active module files in project A
6. Restore the proper module files for project A that you renamed or moved in step 1

These steps could be automated with a shell script or batch file if you need to do it often.

With build command

Using the go help build command, we can see the build flag -modfile

 -modfile file
                in module aware mode, read (and possibly write) an alternate go.mod
                file instead of the one in the module root directory. A file named
                "go.mod" must still be present in order to determine the module root
                directory, but it is not accessed. When -modfile is specified, an
                alternate go.sum file is also used: its path is derived from the
                -modfile flag by trimming the ".mod" extension and appending ".sum".

Using this, we can directly use an alternative set of module files to build things in project A.

First, the flag description indicates that it may write to the go.mod file, so it's probably still a good idea to create a copy of project B's module files to do this.

Second, using project B's module file is going to be a problem if: 1. project A and project B have a different module name declared in their module file, and 2. packages in project A import other packages in project A. The module name determines what the import path of packages in the module will be, so changing it could break imports.

So the best practice should still be to:

1. Make a copy of project B's module files
2. Change the module name in the copy

Then you can run the build command like this to build in project A:

go build -modfile path/to/projectb/go.mod

First, make a folder b somewhere. Then make a folder a inside b. Then make
b/b.go:

package b

const Something = 1

Then make b/a/a.go:

package a
import "b"

func something() int {
   return b.Something
}

Then go back to b folder, and do go mod init b. Done.