language/go/package.go
func (pkg *goPackage) addFile(c *config.Config, info fileInfo, cgo bool) error {
	switch {
	case info.ext == unknownExt || !cgo && (info.ext == cExt || info.ext == csExt):
		return nil
	case info.ext == protoExt:
		if pcMode := getProtoMode(c); pcMode == proto.LegacyMode {
			// Only add files in legacy mode. This is used to generate a filegroup
			// that contains all protos. In order modes, we get the .proto files
			// from information emitted by the proto language extension.
			pkg.proto.addFile(c, info)
		}
	case info.isTest:
		if info.isCgo {
			return fmt.Errorf("%s: use of cgo in test not supported", info.path)
		}
		pkg.test.addFile(c, info)
	default:
		pkg.library.addFile(c, info)
	}

	return nil
}
func (pkg *goPackage) addFile(c *config.Config, info fileInfo, cgo bool) error {
	switch {
	case info.ext == unknownExt || !cgo && (info.ext == cExt || info.ext == csExt):
		return nil
	case info.ext == protoExt:
		if pcMode := getProtoMode(c); pcMode == proto.LegacyMode {
			// Only add files in legacy mode. This is used to generate a filegroup
			// that contains all protos. In order modes, we get the .proto files
			// from information emitted by the proto language extension.
			pkg.proto.addFile(c, info)
		}
	case info.isTest:
		if info.isCgo {
			return fmt.Errorf("%s: use of cgo in test not supported", info.path)
		}
		pkg.test.addFile(c, info)
	default:
		pkg.library.addFile(c, info)
	}

	return nil
}
// addFile adds the file described by "info" to a target in the package "p" if
// the file is buildable.
//
// "cgo" tells whether any ".go" file in the package contains cgo code. This
// affects whether C files are added to targets.
//
// An error is returned if a file is buildable but invalid (for example, a
// test .go file containing cgo code). Files that are not buildable will not
// be added to any target (for example, .txt files).