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>

5 files changed, 39 insertions, 8 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
 	}
 
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index 3f70d04145..445ebb262f 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -15,6 +15,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -27,6 +28,7 @@ import (
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/mvs"
 	"cmd/go/internal/search"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/modfile"
 	"golang.org/x/mod/module"
@@ -845,13 +847,15 @@ func AllowWriteGoMod() {
 // MinReqs returns a Reqs with minimal additional dependencies of Target,
 // as will be written to go.mod.
 func MinReqs() mvs.Reqs {
-	var retain []string
+	retain := append([]string{}, additionalExplicitRequirements...)
 	for _, m := range buildList[1:] {
 		_, explicit := index.require[m]
 		if explicit || loaded.direct[m.Path] {
 			retain = append(retain, m.Path)
 		}
 	}
+	sort.Strings(retain)
+	str.Uniq(&retain)
 	min, err := mvs.Req(Target, retain, &mvsReqs{buildList: buildList})
 	if err != nil {
 		base.Fatalf("go: %v", err)
diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
index e35e0fc16e..8affd179bb 100644
--- a/src/cmd/go/internal/modload/query.go
+++ b/src/cmd/go/internal/modload/query.go
@@ -21,6 +21,7 @@ import (
 	"cmd/go/internal/imports"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/search"
+	"cmd/go/internal/str"
 	"cmd/go/internal/trace"
 
 	"golang.org/x/mod/module"
@@ -1005,13 +1006,8 @@ func (rr *replacementRepo) Versions(prefix string) ([]string, error) {
 	sort.Slice(versions, func(i, j int) bool {
 		return semver.Compare(versions[i], versions[j]) < 0
 	})
-	uniq := versions[:1]
-	for _, v := range versions {
-		if v != uniq[len(uniq)-1] {
-			uniq = append(uniq, v)
-		}
-	}
-	return uniq, nil
+	str.Uniq(&versions)
+	return versions, nil
 }
 
 func (rr *replacementRepo) Stat(rev string) (*modfetch.RevInfo, error) {
diff --git a/src/cmd/go/internal/str/str.go b/src/cmd/go/internal/str/str.go
index 0413ed8e69..9106ebf74d 100644
--- a/src/cmd/go/internal/str/str.go
+++ b/src/cmd/go/internal/str/str.go
@@ -96,6 +96,20 @@ func Contains(x []string, s string) bool {
 	return false
 }
 
+// Uniq removes consecutive duplicate strings from ss.
+func Uniq(ss *[]string) {
+	if len(*ss) <= 1 {
+		return
+	}
+	uniq := (*ss)[:1]
+	for _, s := range *ss {
+		if s != uniq[len(uniq)-1] {
+			uniq = append(uniq, s)
+		}
+	}
+	*ss = uniq
+}
+
 func isSpaceByte(c byte) bool {
 	return c == ' ' || c == '\t' || c == '\n' || c == '\r'
 }
diff --git a/src/cmd/go/testdata/script/mod_get_promote_implicit.txt b/src/cmd/go/testdata/script/mod_get_promote_implicit.txt
index 33f6a299e2..c64e0c0f70 100644
--- a/src/cmd/go/testdata/script/mod_get_promote_implicit.txt
+++ b/src/cmd/go/testdata/script/mod_get_promote_implicit.txt
@@ -14,6 +14,12 @@ go get -d m/use-indirect
 cmp go.mod go.mod.use
 cp go.mod.orig go.mod
 
+# We can also promote implicit requirements using 'go get' on them, or their
+# packages. This gives us "// indirect" requirements, since 'go get' doesn't
+# know they're needed by the main module. See #43131 for the rationale.
+go get -d indirect-with-pkg indirect-without-pkg
+cmp go.mod go.mod.indirect
+
 -- go.mod.orig --
 module m