add warning with what to when calling init on an implicitly required plugin

command/init.go

@@ -125,9 +125,37 @@ func (c *InitCommand) RunContext(buildCtx context.Context, cla *InitArgs) int {
 			ret = 1
 		}
 		if newInstall != nil {
-			msg := fmt.Sprintf("Installed plugin %s %s in %q", pluginRequirement.Identifier, newInstall.Version, newInstall.BinaryPath)
+			if pluginRequirement.Implicit {
-			ui.Say(msg)
+				msg := fmt.Sprintf("Installed implicitly required plugin %s %s in %q", pluginRequirement.Identifier, newInstall.Version, newInstall.BinaryPath)
+				ui.Say(msg)
+
+				warn := fmt.Sprintf(`
+Warning, init with implicitly required plugin is always going to install the 
+latest possible plugin, if a latest version is backward incompatible with your
+config file or your version of Packer, a build will fail. To avoid this, lock
+the plugin version by pasting the following to your configuration:
+
+packer {
+  required_plugins {
+    %s = {
+      source  = "%s"
+      version = "~> %s"
+    }
+  }
+}
+`,
+					pluginRequirement.Identifier.Type,
+					pluginRequirement.Identifier,
+					newInstall.Version,
+				)
+				ui.Error(warn)
+			} else {
+				msg := fmt.Sprintf("Installed plugin %s %s in %q", pluginRequirement.Identifier, newInstall.Version, newInstall.BinaryPath)
+				ui.Say(msg)
+			}
+
 		}
+
 	}
 	return ret
 }

hcl2template/plugin.go

@@ -41,6 +41,7 @@ func (cfg *PackerConfig) PluginRequirements() (plugingetter.Requirements, hcl.Di
 				Accessor:           name,
 				Identifier:         block.Type,
 				VersionConstraints: block.Requirement.Required,
+				Implicit:           block.PluginDependencyReason == PluginDependencyImplicit,
 			})
 			uniq[name] = block
 		}

packer/plugin-getter/plugins.go

@@ -38,6 +38,9 @@ type Requirement struct {
 	// VersionConstraints as defined by user. Empty ( to be avoided ) means
 	// highest found version.
 	VersionConstraints version.Constraints
+
+	// was this require implicitly guessed ?
+	Implicit bool
 }
 
 type BinaryInstallationOptions struct {