proposal: cmd/go: add '// test' after mod like '// indirect' in go.mod

[ftcopilot106]

now go mod tidy command will write go.mod like this, if I import github.com/package/a and a import github.com/package/b, and b import github.com/package/c

module package_name

go 1.21.3

require (
	github.com/package/a v1.0.0 
)


require (
	github.com/package/b v1.0.0 // indirect
	github.com/package/c v1.0.0 // indirect
)

and then I import github.com/package/b by my test code, then run go mod tidy, go.mod file will like this

module package_name

go 1.21.3

require (
	github.com/package/a v1.0.0 
	github.com/package/b v1.0.0 
)

require (
	github.com/package/c v1.0.0 // indirect
)

github.com/package/b will move, but not mark it is test import

so I hope go.mod like this

module package_name

go 1.21.3

require (
	github.com/package/a v1.0.0 
)


require (
	github.com/package/b v1.0.0 // test
)

require (
	github.com/package/c v1.0.0 // indirect
)

it will show me packages I import just in test, and not need run go mod why -m ...

[mvdan]

Very closely related to #26913, #42766, and #26955 - I don't think more open issues on this are helpful. See the last one I linked, which is still open.