proposal: cmd/go: add go mod tidy -diff

[samthanawalla]

Proposal Details

Proposal:

Summarizing #27005 adding go mod tidy -diff would:

• cause tidy to exit with a non-zero code if updates to go.mod or go.sum are needed, instead of modifying the files.
• print the diff between the tidy go.mod and the current go.mod
• print the diff between the tidy go.sum and the current go.sum

Motivation:

• "integrated to tools like pre-commit hooks to make sure that we don't commit untidied go.mod and go.sum files into version control"
• more transparency around go mod tidy

Discussion:

go mod tidy -check was discussed which would:

• cause tidy to exit with a non-zero code if updates to go.mod or go.sum are needed, instead of modifying the files.
• no diff information
• @rsc had concerns that it would "encourage a mindset of "go.sum and go.mod are a black box that no one understands" without the diff context

Example:

go.mod

module example.com

main.go

package main

import "rsc.io/quote"

func main() {
	println(quote.Hello())
}

$ go mod tidy -diff
diff old go.mod new go.mod
        --- old go.mod
        +++ new go.mod
        @@ -1,1 +1,11 @@
         module example.com
        +
        +go 1.23
        +
        +require rsc.io/quote v1.5.2
        +
        +require (
        +       golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
        +       rsc.io/sampler v1.3.0 // indirect
        +       rsc.io/testonly v1.0.0 // indirect
        +)

$ echo $?
1

Implementation pointers:

• A new flag would need to be added to src/cmd/go/internal/modcmd/tidy.go
• src/cmd/go/internal/modload/load.go
• src/cmd/go/internal/modload/init.go
• Use the internal diff package: src/internal/diff/diff.go

[rsc]

Duplicate of #27005, -diff is fine.