Clarify error message for missing plugin versions

When using buf generate, provide specific error messages based on whether the
plugin is unknown, or if it is just the version that is unknown.

1. For a valid plugin with non-existent version (e.g., v0.4.1 instead of v0.4.0):
   "unknown version v0.4.1 for plugin community/neoeinstein-prost. The latest
   version is v0.4.0. Check https://buf.build/community/neoeinstein-prost for
   other available versions"

2. For a completely non-existent plugin (e.g., a typo like "proust" instead of "prost"):
   "unknown plugin community/neoeinstein-proust, check https://buf.build/plugins for
   the list of available plugins"

This provides more clarity to users about whether they need to fix a version
number or if they have a completely incorrect plugin reference.

Fixes #3650

🤖 Generated with [Claude Code](https://docs.anthropic.com/s/claude-code)
Co-Authored-By: Claude <[email protected]>

private/buf/bufgen/generator.go

@@ -20,12 +20,14 @@ import (
 	"fmt"
 	"log/slog"
 	"path/filepath"
+	"regexp"
 
 	connect "connectrpc.com/connect"
 	"github.com/bufbuild/buf/private/buf/bufprotopluginexec"
 	"github.com/bufbuild/buf/private/bufpkg/bufconfig"
 	"github.com/bufbuild/buf/private/bufpkg/bufimage"
 	"github.com/bufbuild/buf/private/bufpkg/bufimage/bufimagemodify"
+	"github.com/bufbuild/buf/private/bufpkg/bufparse"
 	"github.com/bufbuild/buf/private/bufpkg/bufprotoplugin"
 	"github.com/bufbuild/buf/private/bufpkg/bufprotoplugin/bufprotopluginos"
 	"github.com/bufbuild/buf/private/bufpkg/bufremoteplugin"
@@ -392,7 +394,7 @@ func (g *generator) execRemotePluginsV2(
 		),
 	)
 	if err != nil {
-		return nil, err
+		return nil, g.enrichPluginResolutionError(ctx, err, requests, remote)
 	}
 	responses := response.Msg.GetResponses()
 	if len(responses) != len(requests) {
@@ -412,6 +414,65 @@ func (g *generator) execRemotePluginsV2(
 	return result, nil
 }
 
+func (g *generator) enrichPluginResolutionError(ctx context.Context, err error, requests []*registryv1alpha1.PluginGenerationRequest, remote string) error {
+	var connectErr *connect.Error
+	if !errors.As(err, &connectErr) && connectErr.Code() != connect.CodeNotFound {
+		// we only attempt to enrich NotFound errors
+		return err
+	}
+
+	// the error message for NotFound errors is expected to contain the specific failing plugin reference
+	// unfortunately we have to parse this out, it is not provided in a more direct form
+	pluginNotFoundErrorRegexp := regexp.MustCompile(`^plugin "([^"]*)" was not found$`)
+	matched := pluginNotFoundErrorRegexp.FindStringSubmatch(connectErr.Message())
+	if len(matched) <= 1 {
+		return err
+	}
+
+	pluginRefStr := matched[1]
+	pluginRef, parseErr := bufparse.ParseRef(pluginRefStr)
+	if parseErr != nil {
+		// we can't parse the plugin ref, so just return the original error
+		return err
+	}
+
+	repository := pluginRef.FullName().Registry()
+	owner := pluginRef.FullName().Owner()
+	name := pluginRef.FullName().Name()
+
+	// find the matching plugin in the request to determine what version was used
+	var missingVersion string = ""
+	for _, x := range requests {
+		if x.GetPluginReference().GetOwner() == owner && x.GetPluginReference().GetName() == name {
+			missingVersion = x.GetPluginReference().GetVersion()
+		}
+	}
+
+	if missingVersion == "" {
+		// no match in the requests, so just return the original error
+		return err
+	}
+
+	curationClientService := connectclient.Make(g.clientConfig, remote, registryv1alpha1connect.NewPluginCurationServiceClient)
+
+	response, requestErr := curationClientService.GetLatestCuratedPlugin(ctx, connect.NewRequest(
+		registryv1alpha1.GetLatestCuratedPluginRequest_builder{
+			Owner: owner,
+			Name:  name,
+		}.Build(),
+	))
+
+	if requestErr != nil {
+		// failed to get plugin information from the plugin curation service, so this plugin must not exist
+		return fmt.Errorf("unknown plugin %s/%s, check https://%s/plugins for the list of available plugins", owner, name, repository)
+	}
+
+	plugin := response.Msg.GetPlugin()
+	latestVersion := plugin.GetVersion()
+
+	return fmt.Errorf("unknown version %[1]s for plugin %[2]s/%[3]s. The latest version is %[4]s. Check https://%[5]s/%[2]s/%[3]s for other available versions", missingVersion, owner, name, latestVersion, repository)
+}
+
 func getPluginGenerationRequest(
 	pluginConfig bufconfig.GeneratePluginConfig,
 	includeImports bool,