Add public facade value wrappers

internal/facade/conversion.go

@@ -0,0 +1,91 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+
+func fromEngineLocation(value engine.Location) Location {
+	return Location{SourceID: value.SourceID, Line: value.Line, Column: value.Column}
+}
+func toEngineLocation(value Location) engine.Location {
+	return engine.Location{SourceID: value.SourceID, Line: value.Line, Column: value.Column}
+}
+func fromEngineLocationPointer(value *engine.Location) *Location {
+	if value == nil {
+		return nil
+	}
+	result := fromEngineLocation(*value)
+	return &result
+}
+func toEngineLocationPointer(value *Location) *engine.Location {
+	if value == nil {
+		return nil
+	}
+	result := toEngineLocation(*value)
+	return &result
+}
+func fromEngineLocations(values []engine.Location) []Location {
+	if values == nil {
+		return nil
+	}
+	result := make([]Location, len(values))
+	for i := range values {
+		result[i] = fromEngineLocation(values[i])
+	}
+	return result
+}
+func toEngineLocations(values []Location) []engine.Location {
+	if values == nil {
+		return nil
+	}
+	result := make([]engine.Location, len(values))
+	for i := range values {
+		result[i] = toEngineLocation(values[i])
+	}
+	return result
+}
+func fromEngineSourceInfo(value engine.SourceInfo) SourceInfo {
+	return SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: Layer(value.Layer), ImportedFrom: fromEngineLocationPointer(value.ImportedFrom)}
+}
+func toEngineSourceInfo(value SourceInfo) engine.SourceInfo {
+	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: engine.Layer(value.Layer), ImportedFrom: toEngineLocationPointer(value.ImportedFrom)}
+}
+func fromEngineOrigin(value engine.Origin) Origin {
+	return Origin{Layer: Layer(value.Layer), Locations: fromEngineLocations(value.Locations), Operation: ChangeKind(value.Operation)}
+}
+func toEngineOrigin(value Origin) engine.Origin {
+	return engine.Origin{Layer: engine.Layer(value.Layer), Locations: toEngineLocations(value.Locations), Operation: engine.ChangeKind(value.Operation)}
+}
+func fromEngineDiagnostic(value engine.Diagnostic) Diagnostic {
+	return Diagnostic{Code: Code(value.Code), Severity: Severity(value.Severity), Path: fromEnginePath(value.Path), Message: value.Message, Primary: fromEngineLocationPointer(value.Primary), Related: fromEngineLocations(value.Related)}
+}
+func toEngineDiagnostic(value Diagnostic) engine.Diagnostic {
+	return engine.Diagnostic{Code: engine.Code(value.Code), Severity: engine.Severity(value.Severity), Path: toEnginePath(value.Path), Message: value.Message, Primary: toEngineLocationPointer(value.Primary), Related: toEngineLocations(value.Related)}
+}
+func toEngineDiagnostics(values []Diagnostic) []engine.Diagnostic {
+	if values == nil {
+		return nil
+	}
+	result := make([]engine.Diagnostic, len(values))
+	for i := range values {
+		result[i] = toEngineDiagnostic(values[i])
+	}
+	return result
+}
+func fromEngineFieldInfo(value engine.FieldInfo) FieldInfo {
+	return FieldInfo{Explicit: value.Explicit, Null: value.Null, Origin: fromEngineOrigin(value.Origin)}
+}
+func fromEngineChange(value engine.Change) Change {
+	result := Change{Path: fromEnginePath(value.Path), Origin: fromEngineOrigin(value.Origin), Reasons: append([]string(nil), value.Reasons...)}
+	if value.Previous != nil {
+		result.Previous = make([]Origin, len(value.Previous))
+		for i := range value.Previous {
+			result.Previous[i] = fromEngineOrigin(value.Previous[i])
+		}
+	}
+	if value.Dependencies != nil {
+		result.Dependencies = make([]Path, len(value.Dependencies))
+		for i := range value.Dependencies {
+			result.Dependencies[i] = fromEnginePath(value.Dependencies[i])
+		}
+	}
+	return result
+}

internal/facade/diagnostic.go

@@ -0,0 +1,44 @@
+package configkit
+
+import (
+	"io"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+)
+
+// LoadError reports safe structured diagnostics for a failed configuration load.
+type LoadError struct{ value *engine.LoadError }
+
+func (e *LoadError) Error() string {
+	if e == nil || e.value == nil {
+		return "configkit: configuration load failed"
+	}
+	return e.value.Error()
+}
+func (e *LoadError) Diagnostics() []Diagnostic {
+	if e == nil || e.value == nil {
+		return nil
+	}
+	values := e.value.Diagnostics()
+	result := make([]Diagnostic, len(values))
+	for i := range values {
+		result[i] = fromEngineDiagnostic(values[i])
+	}
+	return result
+}
+func (e *LoadError) Sources() []SourceInfo {
+	if e == nil || e.value == nil {
+		return nil
+	}
+	values := e.value.Sources()
+	result := make([]SourceInfo, len(values))
+	for i := range values {
+		result[i] = fromEngineSourceInfo(values[i])
+	}
+	return result
+}
+
+// WriteDiagnostics writes diagnostics in canonical order.
+func WriteDiagnostics(output io.Writer, diagnostics []Diagnostic) error {
+	return engine.WriteDiagnostics(output, toEngineDiagnostics(diagnostics))
+}

internal/facade/facade_external_test.go

@@ -0,0 +1,43 @@
+package configkit_test
+
+import (
+	"encoding/json"
+	"fmt"
+	"reflect"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit/internal/facade"
+)
+
+var (
+	_ error          = (*configkit.LoadError)(nil)
+	_ fmt.Formatter  = configkit.Secret{}
+	_ fmt.Stringer   = configkit.Path{}
+	_ json.Marshaler = configkit.Secret{}
+)
+
+func TestFacadeValuesRemainRootOwned(t *testing.T) {
+	t.Parallel()
+	const packagePath = "gitea.maximumdirect.net/eric/ConfigKit/internal/facade"
+	for _, candidate := range []reflect.Type{
+		reflect.TypeFor[configkit.Path](), reflect.TypeFor[configkit.Source](),
+		reflect.TypeFor[configkit.Secret](), reflect.TypeFor[configkit.Diagnostic](),
+		reflect.TypeFor[configkit.LoadError](),
+	} {
+		if candidate.PkgPath() != packagePath {
+			t.Fatalf("type %s belongs to %q", candidate, candidate.PkgPath())
+		}
+	}
+	secret := configkit.SecretEnv("SAFE_NAME")
+	formatted := fmt.Sprintf("%#v", secret)
+	encoded, err := json.Marshal(secret)
+	if formatted == "" || err != nil || string(encoded) != `"SAFE_NAME"` {
+		t.Fatalf("secret rendering = %q, %s, %v", formatted, encoded, err)
+	}
+	if got := configkit.Keys("a/b~").Index(0).String(); got != "/a~1b~0/0" {
+		t.Fatalf("path = %q", got)
+	}
+	if got := (&configkit.LoadError{}).Error(); got != "configkit: configuration load failed" {
+		t.Fatalf("zero LoadError = %q", got)
+	}
+}

internal/facade/facade_test.go

@@ -0,0 +1,52 @@
+package configkit
+
+import (
+	"errors"
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+)
+
+func TestWrappedSourcesPreserveEngineOwnership(t *testing.T) {
+	t.Parallel()
+
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	data := []byte("value: retained\n")
+	source := Bytes("snapshot.yml", data)
+	data[7] = 'X'
+	result, err := engine.Load[config](source.toEngine(), engine.SecretBinding{})
+	if err != nil || result.Config.Value != "retained" {
+		t.Fatalf("snapshotted source = %#v, %v", result, err)
+	}
+
+	readerSource := Reader("reader.yml", strings.NewReader("value: once\n"))
+	first := readerSource
+	if _, err := engine.Load[config](first.toEngine(), engine.SecretBinding{}); err != nil {
+		t.Fatal(err)
+	}
+	_, err = engine.Load[config](readerSource.toEngine(), engine.SecretBinding{})
+	var loadError *engine.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) != 1 || loadError.Diagnostics()[0].Code != engine.CodeSourceReaderUsed {
+		t.Fatalf("copied reader source error = %#v", err)
+	}
+}
+
+func TestWrappedPathsRetainEngineSegmentIntent(t *testing.T) {
+	t.Parallel()
+
+	parsed, err := ParsePath("/0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	keyed, indexed := Keys("0"), Path{}.Index(0)
+	if parsed.String() != "/0" || keyed.String() != "/0" || indexed.String() != "/0" {
+		t.Fatalf("path spellings = %q, %q, %q", parsed, keyed, indexed)
+	}
+	if reflect.DeepEqual(parsed.value, keyed.value) || reflect.DeepEqual(parsed.value, indexed.value) || reflect.DeepEqual(keyed.value, indexed.value) {
+		t.Fatal("path wrappers collapsed parsed, mapping-key, and index segments")
+	}
+}

internal/facade/path.go

@@ -0,0 +1,33 @@
+package configkit
+
+import (
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+)
+
+// Path identifies a configuration value using YAML field names. Its textual form
+// is a JSON Pointer. Paths are immutable; the zero value is the root path.
+type Path struct {
+	value engine.Path
+}
+
+// Keys constructs a path of mapping keys. Keys with no arguments returns the root path.
+func Keys(keys ...string) Path { return Path{value: engine.Keys(keys...)} }
+
+// ParsePath parses a JSON Pointer while preserving context-dependent numeric tokens.
+func ParsePath(pointer string) (Path, error) {
+	value, err := engine.ParsePath(pointer)
+	return Path{value: value}, err
+}
+
+// Key returns a new path with key appended as a mapping-key segment.
+func (p Path) Key(key string) Path { return Path{value: p.value.Key(key)} }
+
+// Index returns a new path with index appended as a sequence-index segment.
+func (p Path) Index(index int) Path { return Path{value: p.value.Index(index)} }
+
+// String returns the path's canonical JSON Pointer spelling.
+func (p Path) String() string { return p.value.String() }
+
+func (p Path) clone() Path                 { return Path{value: p.value} }
+func toEnginePath(path Path) engine.Path   { return path.value }
+func fromEnginePath(path engine.Path) Path { return Path{value: path} }

internal/facade/records.go

@@ -0,0 +1,286 @@
+package configkit
+
+// Layer identifies the precedence layer responsible for a configuration value.
+type Layer string
+
+const (
+	LayerDefault    Layer = "default"
+	LayerBase       Layer = "base"
+	LayerProfile    Layer = "profile"
+	LayerCLI        Layer = "cli"
+	LayerResolution Layer = "resolution"
+)
+
+// ChangeKind identifies a recorded operation. The zero value is
+// used for input and default origins.
+type ChangeKind string
+
+const (
+	// ChangeDecoding records the built-in transition to canonical typed values.
+	// It is inspection metadata, not an operation accepted by Resolution.Record.
+	ChangeDecoding       ChangeKind = "decoding"
+	ChangeDerivedDefault ChangeKind = "derived_default"
+	ChangeNormalization  ChangeKind = "normalization"
+)
+
+// Severity classifies a diagnostic.
+type Severity string
+
+const (
+	SeverityError   Severity = "error"
+	SeverityWarning Severity = "warning"
+)
+
+// Code is a stable, machine-readable diagnostic classification.
+type Code string
+
+const (
+	CodeSourceInvalid       Code = "source.invalid"
+	CodeSourceMissing       Code = "source.missing"
+	CodeSourcePermission    Code = "source.permission"
+	CodeSourceRead          Code = "source.read"
+	CodeSourceNotRegular    Code = "source.not_regular"
+	CodeSourceLimitExceeded Code = "source.limit_exceeded"
+	CodeSourceReaderUsed    Code = "source.reader_used"
+
+	CodeYAMLInvalid           Code = "yaml.invalid"
+	CodeYAMLEmpty             Code = "yaml.empty"
+	CodeYAMLMultipleDocuments Code = "yaml.multiple_documents"
+	CodeYAMLRootType          Code = "yaml.root_type"
+	CodeYAMLDuplicateKey      Code = "yaml.duplicate_key"
+	CodeYAMLTag               Code = "yaml.tag"
+	CodeYAMLAnchor            Code = "yaml.anchor"
+	CodeYAMLAlias             Code = "yaml.alias"
+	CodeYAMLMergeKey          Code = "yaml.merge_key"
+	CodeYAMLScalar            Code = "yaml.scalar"
+	CodeYAMLDepthLimit        Code = "yaml.depth_limit"
+	CodeYAMLNodeLimit         Code = "yaml.node_limit"
+
+	CodeSchemaRoot            Code = "schema.root"
+	CodeSchemaUnsupportedType Code = "schema.unsupported_type"
+	CodeSchemaAmbiguousField  Code = "schema.ambiguous_field"
+	CodeSchemaTag             Code = "schema.tag"
+	CodeSchemaTextCodec       Code = "schema.text_codec"
+	CodeSchemaInline          Code = "schema.inline"
+	CodeSchemaCycle           Code = "schema.cycle"
+	CodeSchemaProjectionLimit Code = "schema.projection_limit"
+	CodeSchemaCodec           Code = "schema.codec"
+	CodeSchemaNonFinite       Code = "schema.non_finite"
+	CodeSchemaDynamicType     Code = "schema.dynamic_type"
+
+	CodeDecodeType         Code = "decode.type"
+	CodeDecodeNull         Code = "decode.null"
+	CodeDecodeRange        Code = "decode.range"
+	CodeDecodeDuration     Code = "decode.duration"
+	CodeDecodeText         Code = "decode.text"
+	CodeDecodeUnknownField Code = "decode.unknown_field"
+	CodeDecodeArrayLength  Code = "decode.array_length"
+
+	CodeOptionsInvalid     Code = "options.invalid"
+	CodeOptionsUnavailable Code = "options.unavailable"
+
+	CodeImportDisabled      Code = "import.disabled"
+	CodeImportInvalid       Code = "import.invalid"
+	CodeImportMissing       Code = "import.missing"
+	CodeImportPermission    Code = "import.permission"
+	CodeImportRead          Code = "import.read"
+	CodeImportOutsideRoot   Code = "import.outside_root"
+	CodeImportCycle         Code = "import.cycle"
+	CodeImportRepeated      Code = "import.repeated"
+	CodeImportLimitExceeded Code = "import.limit_exceeded"
+	CodeImportVersion       Code = "import.version"
+
+	CodeProfileInvalid       Code = "profile.invalid"
+	CodeProfileMissing       Code = "profile.missing"
+	CodeProfileOutsideRoot   Code = "profile.outside_root"
+	CodeProfileRepeated      Code = "profile.repeated"
+	CodeProfileDirective     Code = "profile.directive"
+	CodeProfileVersion       Code = "profile.version"
+	CodeProfileLimitExceeded Code = "profile.limit_exceeded"
+
+	CodeComposeInvalid  Code = "compose.invalid"
+	CodeComposeConflict Code = "compose.conflict"
+
+	CodeVersionType       Code = "version.type"
+	CodeVersionError      Code = "version.error"
+	CodeVersionWarning    Code = "version.warning"
+	CodeVersionHookFailed Code = "version.hook_failed"
+
+	CodeResolveError            Code = "resolve.error"
+	CodeResolveWarning          Code = "resolve.warning"
+	CodeResolveHookFailed       Code = "resolve.hook_failed"
+	CodeResolveInvalidPath      Code = "resolve.invalid_path"
+	CodeResolveInvalidKind      Code = "resolve.invalid_kind"
+	CodeResolveInvalidRecord    Code = "resolve.invalid_record"
+	CodeResolveInvalidTarget    Code = "resolve.invalid_target"
+	CodeResolveInvalidValue     Code = "resolve.invalid_value"
+	CodeResolveUnrecordedChange Code = "resolve.unrecorded_change"
+
+	CodeValidateError       Code = "validate.error"
+	CodeValidateWarning     Code = "validate.warning"
+	CodeValidateInvalidPath Code = "validate.invalid_path"
+	CodeValidateMutation    Code = "validate.mutation"
+
+	CodeSecretsInvalidPath   Code = "secrets.invalid_path"
+	CodeSecretsMissing       Code = "secrets.missing"
+	CodeSecretsPermission    Code = "secrets.permission"
+	CodeSecretsRead          Code = "secrets.read"
+	CodeSecretsNotRegular    Code = "secrets.not_regular"
+	CodeSecretsInvalidName   Code = "secrets.invalid_name"
+	CodeSecretsLimitExceeded Code = "secrets.limit_exceeded"
+	CodeSecretsInvalidValue  Code = "secrets.invalid_value"
+	CodeSecretsApplyFailed   Code = "secrets.apply_failed"
+
+	CodeSecretRequired    Code = "secret.required"
+	CodeSecretInvalidName Code = "secret.invalid_name"
+	CodeSecretUnset       Code = "secret.unset"
+	CodeSecretEmpty       Code = "secret.empty"
+
+	CodeOverrideInvalidPath   Code = "override.invalid_path"
+	CodeOverrideInvalidLabel  Code = "override.invalid_label"
+	CodeOverrideInvalidValue  Code = "override.invalid_value"
+	CodeOverrideDuplicate     Code = "override.duplicate"
+	CodeOverrideOverlap       Code = "override.overlap"
+	CodeOverrideDirective     Code = "override.directive"
+	CodeOverrideVersion       Code = "override.version"
+	CodeOverrideLimitExceeded Code = "override.limit_exceeded"
+
+	CodeExplainLimitExceeded Code = "explain.limit_exceeded"
+)
+
+// Location identifies a position in a configuration source. Zero line and column
+// values mean that coordinates are unavailable or synthetic.
+type Location struct {
+	SourceID string
+	Line     int
+	Column   int
+}
+
+// SourceInfo describes a source discovered during a load. Path is a canonical
+// local path when one exists. ImportedFrom identifies the declaration that
+// included this source.
+type SourceInfo struct {
+	ID           string
+	Label        string
+	Path         string
+	Layer        Layer
+	ImportedFrom *Location
+}
+
+// Diagnostic is a safe structured problem report. Message and source labels must
+// never contain configuration values or other secrets.
+type Diagnostic struct {
+	Code     Code
+	Severity Severity
+	Path     Path
+	Message  string
+	Primary  *Location
+	Related  []Location
+}
+
+// Origin describes where an effective definition came from. Multiple locations
+// represent a composite origin; resolution origins are synthetic and identify their
+// operation instead of a file location.
+type Origin struct {
+	Layer     Layer
+	Locations []Location
+	Operation ChangeKind
+}
+
+// FieldInfo describes an effective configuration path.
+type FieldInfo struct {
+	Explicit bool
+	Null     bool
+	Origin   Origin
+}
+
+// Change describes a definition or application-resolution operation. Path can name
+// an ancestor operation affecting the queried descendant. Historical values are
+// intentionally not exposed by this record.
+type Change struct {
+	Path         Path
+	Origin       Origin
+	Previous     []Origin
+	Reasons      []string
+	Dependencies []Path
+}
+
+func cloneLocation(location *Location) *Location {
+	if location == nil {
+		return nil
+	}
+	cloned := *location
+	return &cloned
+}
+
+func cloneLocations(locations []Location) []Location {
+	if locations == nil {
+		return nil
+	}
+	return append([]Location(nil), locations...)
+}
+
+func cloneSourceInfo(source SourceInfo) SourceInfo {
+	source.ImportedFrom = cloneLocation(source.ImportedFrom)
+	return source
+}
+
+func cloneSourceInfos(sources []SourceInfo) []SourceInfo {
+	if sources == nil {
+		return nil
+	}
+	cloned := make([]SourceInfo, len(sources))
+	for i, source := range sources {
+		cloned[i] = cloneSourceInfo(source)
+	}
+	return cloned
+}
+
+func cloneOrigin(origin Origin) Origin {
+	origin.Locations = cloneLocations(origin.Locations)
+	return origin
+}
+
+func cloneFieldInfo(info FieldInfo) FieldInfo {
+	info.Origin = cloneOrigin(info.Origin)
+	return info
+}
+
+func cloneChange(change Change) Change {
+	change.Path = change.Path.clone()
+	change.Origin = cloneOrigin(change.Origin)
+	if change.Previous != nil {
+		previous := make([]Origin, len(change.Previous))
+		for i, origin := range change.Previous {
+			previous[i] = cloneOrigin(origin)
+		}
+		change.Previous = previous
+	}
+	change.Reasons = append([]string(nil), change.Reasons...)
+	if change.Dependencies != nil {
+		dependencies := make([]Path, len(change.Dependencies))
+		for i, path := range change.Dependencies {
+			dependencies[i] = path.clone()
+		}
+		change.Dependencies = dependencies
+	}
+	return change
+}
+
+func cloneDiagnostic(diagnostic Diagnostic) Diagnostic {
+	diagnostic.Path = diagnostic.Path.clone()
+	diagnostic.Primary = cloneLocation(diagnostic.Primary)
+	diagnostic.Related = cloneLocations(diagnostic.Related)
+	return diagnostic
+}
+
+func cloneDiagnostics(diagnostics []Diagnostic) []Diagnostic {
+	if diagnostics == nil {
+		return nil
+	}
+	cloned := make([]Diagnostic, len(diagnostics))
+	for i, diagnostic := range diagnostics {
+		cloned[i] = cloneDiagnostic(diagnostic)
+	}
+	return cloned
+}

internal/facade/secret.go

@@ -0,0 +1,52 @@
+package configkit
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"reflect"
+	"strconv"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+)
+
+// Secret is an opaque reference to an environment variable. Its name is public
+// configuration; its resolved value is available only through Value.
+type Secret struct {
+	name, value         string
+	specified, resolved bool
+}
+
+// SecretEnv constructs an unresolved environment-backed secret reference.
+func SecretEnv(name string) Secret { return Secret{name: name, specified: true} }
+
+// EnvName returns the public environment-variable name.
+func (s Secret) EnvName() string { return s.name }
+
+// Value returns the resolved credential, or an empty string when unresolved.
+func (s Secret) Value() string {
+	if !s.resolved {
+		return ""
+	}
+	return s.value
+}
+
+// IsResolved reports whether this reference resolved to a nonempty value during its configuration load.
+func (s Secret) IsResolved() bool { return s.resolved }
+
+// Format renders only the public reference name for every formatting verb.
+func (s Secret) Format(state fmt.State, _ rune) {
+	_, _ = io.WriteString(state, "configkit.SecretEnv("+strconv.Quote(s.name)+")")
+}
+
+// MarshalJSON emits only the public environment-variable name.
+func (s Secret) MarshalJSON() ([]byte, error) { return json.Marshal(s.name) }
+
+func secretBinding() engine.SecretBinding {
+	return engine.SecretBinding{Type: reflect.TypeFor[Secret](), Read: func(value reflect.Value) engine.SecretState {
+		secret := value.Interface().(Secret)
+		return engine.SecretState{Name: secret.name, Value: secret.value, Specified: secret.specified, Resolved: secret.resolved}
+	}, Make: func(state engine.SecretState) reflect.Value {
+		return reflect.ValueOf(Secret{name: state.Name, value: state.Value, specified: state.Specified, resolved: state.Resolved})
+	}}
+}

internal/facade/source.go

@@ -0,0 +1,25 @@
+package configkit
+
+import (
+	"io"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+)
+
+// Source is an opaque configuration input. Construct sources with File, Bytes,
+// Reader, or NoSource; the zero value is invalid.
+type Source struct{ value engine.Source }
+
+// File returns a source that reopens path for every load attempt.
+func File(path string) Source { return Source{value: engine.File(path)} }
+
+// Bytes snapshots data immediately.
+func Bytes(label string, data []byte) Source { return Source{value: engine.Bytes(label, data)} }
+
+// Reader returns a single-use source. Copies share the same single-use claim.
+func Reader(label string, input io.Reader) Source { return Source{value: engine.Reader(label, input)} }
+
+// NoSource explicitly selects defaults-only input.
+func NoSource() Source { return Source{value: engine.NoSource()} }
+
+func (s Source) toEngine() engine.Source { return s.value }