add --replace to change dependency version

builder.go

@@ -179,6 +179,13 @@ type Dependency struct {
 	Version string `json:"version,omitempty"`
 }
 
+func (d Dependency) String() string {
+	if d.Version != "" {
+		return d.PackagePath + "@" + d.Version
+	}
+	return d.PackagePath
+}
+
 // ReplacementPath represents an old or new path component
 // within a Go module replacement directive.
 type ReplacementPath string

cmd/main.go

@@ -97,9 +97,30 @@ func runBuild(ctx context.Context, args []string) error {
 					}
 					log.Printf("[INFO] Resolved relative replacement %s to %s", args[i], repl)
 				}
-				replacements = append(replacements, xcaddy.NewReplace(mod, repl))
+				replacements = append(replacements, xcaddy.NewReplace(xcaddy.Dependency{PackagePath: mod, Version: ver}.String(), repl))
 			}
-
+		case "--replace":
+			if i == len(args)-1 {
+				return fmt.Errorf("expected value after --replace flag")
+			}
+			i++
+			mod, ver, repl, err := splitWith(args[i])
+			if err != nil {
+				return err
+			}
+			mod = strings.TrimSuffix(mod, "/") // easy to accidentally leave a trailing slash if pasting from a URL, but is invalid for Go modules
+			if repl == "" {
+				return fmt.Errorf("expected value after --replace flag")
+			}
+			// adjust relative replacements in current working directory since our temporary module is in a different directory
+			if strings.HasPrefix(repl, ".") {
+				repl, err = filepath.Abs(repl)
+				if err != nil {
+					log.Fatalf("[FATAL] %v", err)
+				}
+				log.Printf("[INFO] Resolved relative replacement %s to %s", args[i], repl)
+			}
+			replacements = append(replacements, xcaddy.NewReplace(xcaddy.Dependency{PackagePath: mod, Version: ver}.String(), repl))
 		case "--output":
 			if i == len(args)-1 {
 				return fmt.Errorf("expected value after --output flag")