Go: Add diagnostic for 1.21 toolchain error

Author: mbg

go/extractor/diagnostics/diagnostics.go

@@ -482,3 +482,14 @@ func EmitGoModVersionSupportedLowerEqualGoEnv(msg string) {
 		noLocation,
 	)
 }
+
+func EmitNewerSystemGoRequired(requiredVersion string) {
+	emitDiagnostic(
+		"go/autobuilder/newer-system-go-version-required",
+		"The Go version installed on the system is too old to support this project",
+		"At least Go version `"+requiredVersion+"` is required to build this project, but the version installed on the system is older. [Install a newer version](https://github.com/actions/setup-go#basic).",
+		severityError,
+		fullVisibility,
+		noLocation,
+	)
+}

go/extractor/extractor.go

@@ -91,6 +91,10 @@ func ExtractWithFlags(buildFlags []string, patterns []string) error {
 	}
 	pkgs, err := packages.Load(cfg, patterns...)
 	if err != nil {
+		// the toolchain directive is only supported in Go 1.21 and above
+		if strings.Contains(err.Error(), "unknown directive: toolchain") {
+			diagnostics.EmitNewerSystemGoRequired("1.21.0")
+		}
 		return err
 	}
 	log.Println("Done running packages.Load.")