cmd/go: do not resolve missing imports in 'go mod why'

[FiloSottile]

Starting at https://github.com/FiloSottile/mkcert/tree/44af1b655699715a3468685d46b5ca530469a0c4

$ GO111MODULE=on go1.11rc1 mod init
go: creating new go.mod: module github.com/FiloSottile/mkcert
go: copying requirements from Gopkg.lock

$ GO111MODULE=on go1.11rc1 mod vendor
go: finding github.com/DHowett/go-plist v0.0.0-20180609054337-500bd5b9081b
go: finding software.sslmate.com/src/go-pkcs12 v0.0.0-20180114231543-2291e8f0f237
go: finding golang.org/x/net v0.0.0-20180627171509-e514e69ffb8b
go: finding golang.org/x/text v0.3.0
go: downloading software.sslmate.com/src/go-pkcs12 v0.0.0-20180114231543-2291e8f0f237
go: downloading golang.org/x/net v0.0.0-20180627171509-e514e69ffb8b
go: downloading github.com/DHowett/go-plist v0.0.0-20180609054337-500bd5b9081b
go: downloading golang.org/x/text v0.3.0

$ cat go.mod
module github.com/FiloSottile/mkcert

require (
	github.com/DHowett/go-plist v0.0.0-20180609054337-500bd5b9081b
	golang.org/x/net v0.0.0-20180627171509-e514e69ffb8b
	golang.org/x/text v0.3.0 // indirect
	software.sslmate.com/src/go-pkcs12 v0.0.0-20180114231543-2291e8f0f237
)

So far so good. It builds, and everything works.

Then I run go mod why (which does not answer me because I want #26968).

$ GO111MODULE=on go1.11rc1 mod why golang.org/x/text
go: finding howett.net/plist latest
go: downloading howett.net/plist v0.0.0-20180609054337-500bd5b9081b
# golang.org/x/text
(main module does not need package golang.org/x/text)

And now a new line appeared in the go.mod, which does not seem something I should expect?

diff --git a/go.mod b/go.mod
index 458640f..cd3792e 100644
--- a/go.mod
+++ b/go.mod
@@ -4,5 +4,6 @@ require (
        github.com/DHowett/go-plist v0.0.0-20180609054337-500bd5b9081b
        golang.org/x/net v0.0.0-20180627171509-e514e69ffb8b
        golang.org/x/text v0.3.0 // indirect
+       howett.net/plist v0.0.0-20180609054337-500bd5b9081b // indirect
        software.sslmate.com/src/go-pkcs12 v0.0.0-20180114231543-2291e8f0f237
 )
diff --git a/go.sum b/go.sum
index 68659a5..1f591aa 100644
--- a/go.sum
+++ b/go.sum
@@ -4,5 +4,7 @@ golang.org/x/net v0.0.0-20180627171509-e514e69ffb8b h1:oXs/nlnyk1ue6g+mFGEHIuIaQ
 golang.org/x/net v0.0.0-20180627171509-e514e69ffb8b/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+howett.net/plist v0.0.0-20180609054337-500bd5b9081b h1:r4LwkXZhdblHVSgAvfRjsFpQBorl6S9pAH+AOHVs+28=
+howett.net/plist v0.0.0-20180609054337-500bd5b9081b/go.mod h1:jInWmjR7JRkkon4jlLXDZGVEeY/wo3kOOJEWYhNE+9Y=
 software.sslmate.com/src/go-pkcs12 v0.0.0-20180114231543-2291e8f0f237 h1:iAEkCBPbRaflBgZ7o9gjVUuWuvWeV4sytFWg9o+Pj2k=
 software.sslmate.com/src/go-pkcs12 v0.0.0-20180114231543-2291e8f0f237/go.mod h1:/xvNRWUqm0+/ZMiF4EX00vrSCMsE4/NHb+Pt3freEeQ=

[rsc]

Maybe we can revisit in Go 1.12.

[dmitris]

seeing the same issue (with fo1.11.1), go mod why <pkg> functions as go mod tidy - even though there seems to be nothing in the go mod help tidy regarding that, and as a user I don't expect a "why" query to have silent side-effect modifications on go.mod

[bcmills]

I think the fix here is to make go mod why simply skip over imports that are not satisfied by any existing module dependency.

That would also address the problem in #34432, in which an error in an unresolved dependency prevents go mod why from producing useful output for an unrelated module.

CC @jayconrod

[gopherbot]

Change https://golang.org/cl/255938 mentions this issue: cmd/go/internal/modget: consolidate Load entrypoints