cmd/go: in 'go get', promote named implicit dependencies to explicit

'go get pkg@vers' will now add an explicit requirement for the module
providing pkg if that version was already indirectly required.

'go get mod@vers' will do the same if mod is a module path but not a
package.

Requirements promoted this way will be marked "// indirect" because
'go get' doesn't know whether they're needed to build packages in the
main module. So users should prefer to run 'go get ./pkg' (where ./pkg
is a package in the main module) to promote requirements.

Fixes #43131

Change-Id: Ifbb65b71274b3cc752a7a593d6ddd875f7de23b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/278812
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modload/buildlist.go b/src/cmd/go/internal/modload/buildlist.go
index 896adebbb1..45f220a6ee 100644
--- a/src/cmd/go/internal/modload/buildlist.go
+++ b/src/cmd/go/internal/modload/buildlist.go
@@ -28,6 +28,11 @@ import (
 //
 var buildList []module.Version
 
+// additionalExplicitRequirements is a list of modules paths for which
+// WriteGoMod should record explicit requirements, even if they would be
+// selected without those requirements. Each path must also appear in buildList.
+var additionalExplicitRequirements []string
+
 // capVersionSlice returns s with its cap reduced to its length.
 func capVersionSlice(s []module.Version) []module.Version {
 	return s[:len(s):len(s)]
@@ -121,6 +126,12 @@ func EditBuildList(ctx context.Context, add, mustSelect []module.Version) error
 
 	if !inconsistent {
 		buildList = final
+		additionalExplicitRequirements = make([]string, 0, len(mustSelect))
+		for _, m := range mustSelect {
+			if m.Version != "none" {
+				additionalExplicitRequirements = append(additionalExplicitRequirements, m.Path)
+			}
+		}
 		return nil
 	}