Add QuirksCannotAddRemove

internal/api/types.go

@@ -130,6 +130,10 @@ const (
 	// This constant indicates that remove cannot be performed
 	// without a lockfile.
 	QuirkRemoveNeedsLockfile
+
+	// This constant indicates that the packager is unable to
+	// add or remove packages.
+	QuirksCannotAddRemove
 )
 
 // LanguageBackend is the core abstraction of UPM. It represents an
@@ -364,8 +368,8 @@ func (b *LanguageBackend) Setup() {
 		"missing package dir":              b.GetPackageDir == nil,
 		"missing Search":                   b.Search == nil,
 		"missing Info":                     b.Info == nil,
-		"missing Add":                      b.Add == nil,
-		"missing Remove":                   b.Remove == nil,
+		"missing Add":                      b.QuirksCanAddRemove() && b.Add == nil,
+		"missing Remove":                   b.QuirksCanAddRemove() && b.Remove == nil,
 		"missing IsAvailable":              b.IsAvailable == nil,
 		// The lock method should be unimplemented if
 		// and only if builds are not reproducible.

internal/api/util.go

@@ -62,3 +62,9 @@ func (b *LanguageBackend) QuirksDoesLockNotAlsoInstall() bool {
 func (b *LanguageBackend) QuirkRemoveNeedsLockfile() bool {
 	return (b.Quirks & QuirkRemoveNeedsLockfile) != 0
 }
+
+// QuirksCanAddRemove returns true if the language backend is
+// able to add and remove packages
+func (b *LanguageBackend) QuirksCanAddRemove() bool {
+	return (b.Quirks & QuirksCannotAddRemove) == 0
+}

internal/cli/cmds.go

@@ -267,6 +267,11 @@ func runAdd(
 	defer span.Finish()
 	b := backends.GetBackend(ctx, language)
 
+	if !b.QuirksCanAddRemove() {
+		fmt.Println("This backend does not suppport dependency additions")
+		os.Exit(1)
+	}
+
 	normPkgs := b.NormalizePackageArgs(args)
 
 	if guess {
@@ -357,6 +362,11 @@ func runRemove(language string, args []string, upgrade bool,
 	defer span.Finish()
 	b := backends.GetBackend(ctx, language)
 
+	if !b.QuirksCanAddRemove() {
+		fmt.Println("This backend does not suppport dependency removal")
+		os.Exit(1)
+	}
+
 	if !util.Exists(b.Specfile) {
 		return
 	}