tools/trim/trim.go - cue - Git at Google

 // Copyright 2018 The CUE Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 // Package trim removes defintiions that may be inferred from
 // templates.
 //
 // trim removes fields from structs that can be inferred from constraints

 // A field, struct, or list is removed if it is implied by a constraint, such
 // as from an optional field maching a required field, a list type value,
 // a comprehension or any other implied content. It will modify the files in place.

 // Limitations
 //
 // Removal is on a best effort basis. Some caveats:
 // - Fields in implied content may refer to fields within the struct in which
 //   they are included, but are only resolved on a best-effort basis.
 // - Disjunctions that contain structs in implied content cannot be used to
 //   remove fields.
 // - There is currently no verification step: manual verification is required.

 // Examples:
 //
 // 	light: [string]: {
 // 		room:          string
 // 		brightnessOff: *0.0 | >=0 & <=100.0
 // 		brightnessOn:  *100.0 | >=0 & <=100.0
 // 	}

 // 	light: ceiling50: {
 // 		room:          "MasterBedroom"
 // 		brightnessOff: 0.0    // this line
 // 		brightnessOn:  100.0  // and this line will be removed
 // 	}
 //
 // Results in:
 //
 // 	light: [string]: {
 // 		room:          string
 // 		brightnessOff: *0.0 | >=0 & <=100.0
 // 		brightnessOn:  *100.0 | >=0 & <=100.0
 // 	}
 //
 // 	light: ceiling50: {
 // 		room: "MasterBedroom"
 // 	}
 //
 package trim

 import (
 	"bytes"
 	"fmt"
 	"strconv"
 	"strings"

 	"cuelang.org/go/cue"
 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/token"
 	"cuelang.org/go/internal"
 )

 // TODO:
 // - remove the limitations mentioned in the documentation
 // - implement verification post-processing as extra safety

 // Config configures trim options.
 type Config struct {
 	Trace bool
 }

 // Files trims fields in the given files that can be implied from other fields,
 // as can be derived from the evaluated values in inst.
 // Trimming is done on a best-effort basis and only when the removed field
 // is clearly implied by another field, rather than equal sibling fields.
 func Files(files []*ast.File, inst *cue.Instance, cfg *Config) error {
 	gen := newTrimSet(cfg)
 	gen.files = files
 	for _, f := range files {
 		gen.markNodes(f)
 	}

 	root := inst.Lookup()
 	rm := gen.trim("root", root, cue.Value{}, root)

 	for _, f := range files {
 		f.Decls = gen.trimDecls(f.Decls, rm, cue.Value{}, true)
 	}
 	return nil
 }

 type trimSet struct {
 	cfg       Config
 	files     []*ast.File
 	stack     []string
 	exclude   map[ast.Node]bool // don't remove fields marked here
 	alwaysGen map[ast.Node]bool // node is always from a generated source
 	fromComp  map[ast.Node]bool // node originated from a comprehension
 }

 func newTrimSet(cfg *Config) *trimSet {
 	if cfg == nil {
 		cfg = &Config{}
 	}
 	return &trimSet{
 		cfg:       *cfg,
 		exclude:   map[ast.Node]bool{},
 		alwaysGen: map[ast.Node]bool{},
 		fromComp:  map[ast.Node]bool{},
 	}
 }

 func (t *trimSet) path() string {
 	return strings.Join(t.stack[1:], " ")
 }

 func (t *trimSet) traceMsg(msg string) {
 	if t.cfg.Trace {
 		fmt.Print(t.path())
 		msg = strings.TrimRight(msg, "\n")
 		msg = strings.Replace(msg, "\n", "\n    ", -1)
 		fmt.Printf(": %s\n", msg)
 	}
 }

 func (t *trimSet) markNodes(n ast.Node) {
 	ast.Walk(n, nil, func(n ast.Node) {
 		switch x := n.(type) {
 		case *ast.Ident:
 			if x.Node != nil {
 				t.exclude[x.Node] = true
 			}
 			if x.Scope != nil {
 				t.exclude[x.Scope] = true
 			}

 		case *ast.ListLit:
 			_, e := internal.ListEllipsis(x)
 			if e != nil && e.Type != nil {
 				t.markAlwaysGen(e.Type, false)
 			}

 		case *ast.Field:
 			switch x.Label.(type) {
 			case *ast.TemplateLabel, *ast.ListLit:
 				t.markAlwaysGen(x.Value, false)
 			}

 		case *ast.ListComprehension, *ast.Comprehension:
 			t.markAlwaysGen(x, true)
 		}
 	})
 }

 func (t *trimSet) markAlwaysGen(first ast.Node, isComp bool) {
 	ast.Walk(first, func(n ast.Node) bool {
 		if t.alwaysGen[n] {
 			return false
 		}
 		t.alwaysGen[n] = true
 		if isComp {
 			t.fromComp[n] = true
 		}
 		if x, ok := n.(*ast.Ident); ok && n != first {
 			// Also mark any value used within a constraint from an optional field.
 			if x.Node != nil {
 				// fmt.Println("MARKED", internal.DebugStr(x.Node),
 				// "by", internal.DebugStr(first))
 				// t.markAlwaysGen(x.Node)
 			}
 		}
 		return true
 	}, nil)
 }

 func (t *trimSet) canRemove(n ast.Node) bool {
 	return !t.exclude[n] && !t.alwaysGen[n]
 }

 func isDisjunctionOfStruct(n ast.Node) bool {
 	switch x := n.(type) {
 	case *ast.BinaryExpr:
 		if x.Op == token.OR {
 			return hasStruct(x.X) || hasStruct(x.Y)
 		}
 	}
 	return false
 }

 func hasStruct(n ast.Node) bool {
 	hasStruct := false
 	ast.Walk(n, func(n ast.Node) bool {
 		if _, ok := n.(*ast.StructLit); ok {
 			hasStruct = true
 		}
 		return !hasStruct
 	}, nil)
 	return hasStruct
 }

 // trim strips fields from structs that would otherwise be generated by implied
 // content, such as optional fields turned required, comprehensions, and list types.
 //
 // The algorithm walks the tree with two values in parallel: one for the full
 // configuration, and one for implied content. For each node in the tree it
 // determines the value of the implied content and that of the full value
 // and strips any of the non-implied fields if it subsumes the implied ones.
 //
 // There are a few gotchas:
 // - Fields in the implied content may refer to fields in the complete config.
 //   To support this, incomplete fields are detected and evaluated within the
 //   configuration.
 // - Values of optional fields are instantiated as a result of the declaration
 //   of concrete sibling fields. Such fields should not be removed even if the
 //   instantiated constraint completely subsumes such fields as the reason to
 //   apply the optional constraint will disappear with it.
 // - As the parallel structure is different, it may resolve to different
 //   default values. There is no support yet for selecting defaults of a value
 //   based on another value without doing a full unification. So for now we
 //   skip any disjunction containing structs.
 //
 //		v      the current value
 //		m      the "mixed-in" values
 //		scope  in which to evaluate expressions
 //		rmSet  nodes in v that may be removed by the caller
 func (t *trimSet) trim(label string, v, m, scope cue.Value) (rmSet []ast.Node) {
 	saved := t.stack
 	t.stack = append(t.stack, label)
 	defer func() { t.stack = saved }()

 	vSplit := v.Split()

 	// At the moment disjunctions of structs are not supported. Detect them and
 	// punt.
 	// TODO: support disjunctions.
 	mSplit := m.Split()
 	for _, v := range mSplit {
 		if isDisjunctionOfStruct(v.Source()) {
 			return
 		}
 	}

 	// Collect generated nodes.
 	// Only keep the good parts of the optional constraint.
 	// Incoming structs may be incomplete resulting in errors. It is safe
 	// to ignore these. If there is an actual error, it will manifest in
 	// the evaluation of v.
 	in := cue.Value{}
 	gen := []ast.Node{}
 	for _, v := range mSplit {
 		// TODO: consider resolving incomplete values within the current
 		// scope, as we do for fields.
 		if v.Exists() {
 			in = in.Unify(v)
 		}
 		gen = append(gen, v.Source())
 	}

 	// Identify generated components and unify them with the mixin value.
 	exists := false
 	for _, v := range vSplit {
 		if src := v.Source(); t.alwaysGen[src] || inNodes(gen, src) {
 			if !v.IsConcrete() {
 				// The template has an expression that cannot be fully
 				// resolved. Attempt to complete the expression by
 				// evaluting it within the struct to which the template
 				// is applied.
 				expr := internal.ToExpr(v.Syntax())

 				// TODO: this only resolves references contained in scope.
 				v = internal.EvalExpr(scope, expr).(cue.Value)
 			}

 			if w := in.Unify(v); w.Err() == nil {
 				in = w
 			}
 			// One of the sources of this struct is generated. That means
 			// we can safely delete a non-generated version.
 			exists = true
 			gen = append(gen, src)
 		}
 	}

 	switch v.Kind() {
 	case cue.StructKind:
 		// Build map of mixin fields.
 		valueMap := map[key]cue.Value{}
 		for mIter, _ := in.Fields(cue.All(), cue.Optional(false)); mIter.Next(); {
 			valueMap[iterKey(mIter)] = mIter.Value()
 		}

 		fn := v.Template()

 		// Process fields.
 		rm := []ast.Node{}
 		for iter, _ := v.Fields(cue.All()); iter.Next(); {
 			mSub := valueMap[iterKey(iter)]
 			if fn != nil {
 				mSub = mSub.Unify(fn(iter.Label()))
 			}

 			removed := t.trim(iter.Label(), iter.Value(), mSub, v)
 			rm = append(rm, removed...)
 		}

 		canRemove := fn == nil
 		for _, v := range vSplit {
 			src := v.Source()
 			if t.fromComp[src] {
 				canRemove = true
 			}
 		}

 		// Remove fields from source.
 		for _, v := range vSplit {
 			if src := v.Source(); !t.alwaysGen[src] {
 				switch x := src.(type) {
 				case *ast.File:
 					// TODO: use in instead?
 					x.Decls = t.trimDecls(x.Decls, rm, m, canRemove)

 				case *ast.StructLit:
 					x.Elts = t.trimDecls(x.Elts, rm, m, canRemove)
 					exists = exists || m.Exists()
 					if len(x.Elts) == 0 && exists && t.canRemove(src) && !inNodes(gen, src) {
 						rmSet = append(rmSet, src)
 					}

 				default:
 					// Currently the ast.Files are not associated with the
 					// nodes. We detect this case here and iterate over all
 					// files.
 					// TODO: fix this. See cue/instance.go.
 					if len(t.stack) == 1 {
 						for _, x := range t.files {
 							x.Decls = t.trimDecls(x.Decls, rm, m, canRemove)
 						}
 					}
 				}
 			}
 		}

 		if t.cfg.Trace {
 			w := &bytes.Buffer{}
 			fmt.Fprintln(w)
 			fmt.Fprintln(w, "value:    ", v)
 			if in.Exists() {
 				fmt.Fprintln(w, "mixed in: ", in)
 			}
 			for _, v := range vSplit {
 				status := "[]"
 				src := v.Source()
 				if inNodes(rmSet, src) {
 					status = "[re]"
 				} else if t.alwaysGen[src] {
 					status = "[i]"
 				}
 				fmt.Fprintf(w, "    %4s %v: %v %T\n", status, v.Pos(), internal.DebugStr(src), src)
 			}

 			t.traceMsg(w.String())
 		}

 	case cue.ListKind:
 		mIter, _ := m.List()
 		i := 0
 		rmElem := []ast.Node{}
 		for iter, _ := v.List(); iter.Next(); i++ {
 			mIter.Next()
 			rm := t.trim(strconv.Itoa(i), iter.Value(), mIter.Value(), scope)
 			rmElem = append(rmElem, rm...)
 		}

 		// Signal the removal of lists of which all elements have been marked
 		// for removal.
 		for _, v := range vSplit {
 			if src := v.Source(); !t.alwaysGen[src] {
 				l, ok := src.(*ast.ListLit)
 				if !ok {
 					break
 				}
 				rmList := true
 				iter, _ := v.List()
 				for i := 0; i < len(l.Elts) && iter.Next(); i++ {
 					if !inNodes(rmElem, l.Elts[i]) {
 						rmList = false
 						break
 					}
 				}
 				if rmList && m.Exists() && t.canRemove(src) && !inNodes(gen, src) {
 					rmSet = append(rmSet, src)
 				}
 			}
 		}
 		fallthrough

 	default:
 		// Mark any subsumed part that is covered by generated config.
 		if in.Err() == nil && v.Subsume(in) == nil {
 			for _, v := range vSplit {
 				src := v.Source()
 				if t.canRemove(src) && !inNodes(gen, src) &&
 					exists {
 					rmSet = append(rmSet, src)
 				}
 			}
 		}

 		if t.cfg.Trace {
 			w := &bytes.Buffer{}
 			if len(rmSet) > 0 {
 				fmt.Fprint(w, "field: SUBSUMED\n")
 			} else {
 				fmt.Fprint(w, "field: \n")
 			}
 			fmt.Fprintln(w, "value:    ", v)
 			if in.Exists() {
 				fmt.Fprintln(w, "mixed in: ", in)
 			}
 			for _, v := range vSplit {
 				status := "["
 				if inNodes(gen, v.Source()) {
 					status += "i"
 				}
 				if inNodes(rmSet, v.Source()) {
 					status += "r"
 				}
 				status += "]"
 				src := v.Source()
 				fmt.Fprintf(w, "   %4s %v: %v\n", status, v.Pos(), internal.DebugStr(src))
 			}

 			t.traceMsg(w.String())
 		}
 	}
 	return rmSet
 }

 func (t *trimSet) trimDecls(decls []ast.Decl, rm []ast.Node, m cue.Value, allow bool) []ast.Decl {
 	a := make([]ast.Decl, 0, len(decls))

 	for _, d := range decls {
 		if f, ok := d.(*ast.Field); ok {
 			label, _, err := ast.LabelName(f.Label)
 			v := m.Lookup(label)
 			if err == nil && inNodes(rm, f.Value) && (allow || v.Exists()) {
 				continue
 			}
 		}
 		a = append(a, d)
 	}
 	return a
 }

 func inNodes(a []ast.Node, n ast.Node) bool {
 	for _, e := range a {
 		if e == n {
 			return true
 		}
 	}
 	return false
 }

 type key struct {
 	label  string
 	hidden bool
 }

 func iterKey(v cue.Iterator) key {
 	return key{v.Label(), v.IsHidden()}
 }
	// Copyright 2018 The CUE Authors
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	// Package trim removes defintiions that may be inferred from
	// templates.
	//
	// trim removes fields from structs that can be inferred from constraints

	// A field, struct, or list is removed if it is implied by a constraint, such
	// as from an optional field maching a required field, a list type value,
	// a comprehension or any other implied content. It will modify the files in place.

	// Limitations
	//
	// Removal is on a best effort basis. Some caveats:
	// - Fields in implied content may refer to fields within the struct in which
	// they are included, but are only resolved on a best-effort basis.
	// - Disjunctions that contain structs in implied content cannot be used to
	// remove fields.
	// - There is currently no verification step: manual verification is required.

	// Examples:
	//
	// light: [string]: {
	// room: string
	// brightnessOff: *0.0 \| >=0 & <=100.0
	// brightnessOn: *100.0 \| >=0 & <=100.0
	// }

	// light: ceiling50: {
	// room: "MasterBedroom"
	// brightnessOff: 0.0 // this line
	// brightnessOn: 100.0 // and this line will be removed
	// }
	//
	// Results in:
	//
	// light: [string]: {
	// room: string
	// brightnessOff: *0.0 \| >=0 & <=100.0
	// brightnessOn: *100.0 \| >=0 & <=100.0
	// }
	//
	// light: ceiling50: {
	// room: "MasterBedroom"
	// }
	//
	package trim

	import (
	"bytes"
	"fmt"
	"strconv"
	"strings"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/ast"
	"cuelang.org/go/cue/token"
	"cuelang.org/go/internal"
	)

	// TODO:
	// - remove the limitations mentioned in the documentation
	// - implement verification post-processing as extra safety

	// Config configures trim options.
	type Config struct {
	Trace bool
	}

	// Files trims fields in the given files that can be implied from other fields,
	// as can be derived from the evaluated values in inst.
	// Trimming is done on a best-effort basis and only when the removed field
	// is clearly implied by another field, rather than equal sibling fields.
	func Files(files []ast.File, inst cue.Instance, cfg *Config) error {
	gen := newTrimSet(cfg)
	gen.files = files
	for _, f := range files {
	gen.markNodes(f)
	}

	root := inst.Lookup()
	rm := gen.trim("root", root, cue.Value{}, root)

	for _, f := range files {
	f.Decls = gen.trimDecls(f.Decls, rm, cue.Value{}, true)
	}
	return nil
	}

	type trimSet struct {
	cfg Config
	files []*ast.File
	stack []string
	exclude map[ast.Node]bool // don't remove fields marked here
	alwaysGen map[ast.Node]bool // node is always from a generated source
	fromComp map[ast.Node]bool // node originated from a comprehension
	}

	func newTrimSet(cfg Config) trimSet {
	if cfg == nil {
	cfg = &Config{}
	}
	return &trimSet{
	cfg: *cfg,
	exclude: map[ast.Node]bool{},
	alwaysGen: map[ast.Node]bool{},
	fromComp: map[ast.Node]bool{},
	}
	}

	func (t *trimSet) path() string {
	return strings.Join(t.stack[1:], " ")
	}

	func (t *trimSet) traceMsg(msg string) {
	if t.cfg.Trace {
	fmt.Print(t.path())
	msg = strings.TrimRight(msg, "\n")
	msg = strings.Replace(msg, "\n", "\n ", -1)
	fmt.Printf(": %s\n", msg)
	}
	}

	func (t *trimSet) markNodes(n ast.Node) {
	ast.Walk(n, nil, func(n ast.Node) {
	switch x := n.(type) {
	case *ast.Ident:
	if x.Node != nil {
	t.exclude[x.Node] = true
	}
	if x.Scope != nil {
	t.exclude[x.Scope] = true
	}

	case *ast.ListLit:
	_, e := internal.ListEllipsis(x)
	if e != nil && e.Type != nil {
	t.markAlwaysGen(e.Type, false)
	}

	case *ast.Field:
	switch x.Label.(type) {
	case ast.TemplateLabel, ast.ListLit:
	t.markAlwaysGen(x.Value, false)
	}

	case ast.ListComprehension, ast.Comprehension:
	t.markAlwaysGen(x, true)
	}
	})
	}

	func (t *trimSet) markAlwaysGen(first ast.Node, isComp bool) {
	ast.Walk(first, func(n ast.Node) bool {
	if t.alwaysGen[n] {
	return false
	}
	t.alwaysGen[n] = true
	if isComp {
	t.fromComp[n] = true
	}
	if x, ok := n.(*ast.Ident); ok && n != first {
	// Also mark any value used within a constraint from an optional field.
	if x.Node != nil {
	// fmt.Println("MARKED", internal.DebugStr(x.Node),
	// "by", internal.DebugStr(first))
	// t.markAlwaysGen(x.Node)
	}
	}
	return true
	}, nil)
	}

	func (t *trimSet) canRemove(n ast.Node) bool {
	return !t.exclude[n] && !t.alwaysGen[n]
	}

	func isDisjunctionOfStruct(n ast.Node) bool {
	switch x := n.(type) {
	case *ast.BinaryExpr:
	if x.Op == token.OR {
	return hasStruct(x.X) \|\| hasStruct(x.Y)
	}
	}
	return false
	}

	func hasStruct(n ast.Node) bool {
	hasStruct := false
	ast.Walk(n, func(n ast.Node) bool {
	if _, ok := n.(*ast.StructLit); ok {
	hasStruct = true
	}
	return !hasStruct
	}, nil)
	return hasStruct
	}

	// trim strips fields from structs that would otherwise be generated by implied
	// content, such as optional fields turned required, comprehensions, and list types.
	//
	// The algorithm walks the tree with two values in parallel: one for the full
	// configuration, and one for implied content. For each node in the tree it
	// determines the value of the implied content and that of the full value
	// and strips any of the non-implied fields if it subsumes the implied ones.
	//
	// There are a few gotchas:
	// - Fields in the implied content may refer to fields in the complete config.
	// To support this, incomplete fields are detected and evaluated within the
	// configuration.
	// - Values of optional fields are instantiated as a result of the declaration
	// of concrete sibling fields. Such fields should not be removed even if the
	// instantiated constraint completely subsumes such fields as the reason to
	// apply the optional constraint will disappear with it.
	// - As the parallel structure is different, it may resolve to different
	// default values. There is no support yet for selecting defaults of a value
	// based on another value without doing a full unification. So for now we
	// skip any disjunction containing structs.
	//
	// v the current value
	// m the "mixed-in" values
	// scope in which to evaluate expressions
	// rmSet nodes in v that may be removed by the caller
	func (t *trimSet) trim(label string, v, m, scope cue.Value) (rmSet []ast.Node) {
	saved := t.stack
	t.stack = append(t.stack, label)
	defer func() { t.stack = saved }()

	vSplit := v.Split()

	// At the moment disjunctions of structs are not supported. Detect them and
	// punt.
	// TODO: support disjunctions.
	mSplit := m.Split()
	for _, v := range mSplit {
	if isDisjunctionOfStruct(v.Source()) {
	return
	}
	}

	// Collect generated nodes.
	// Only keep the good parts of the optional constraint.
	// Incoming structs may be incomplete resulting in errors. It is safe
	// to ignore these. If there is an actual error, it will manifest in
	// the evaluation of v.
	in := cue.Value{}
	gen := []ast.Node{}
	for _, v := range mSplit {
	// TODO: consider resolving incomplete values within the current
	// scope, as we do for fields.
	if v.Exists() {
	in = in.Unify(v)
	}
	gen = append(gen, v.Source())
	}

	// Identify generated components and unify them with the mixin value.
	exists := false
	for _, v := range vSplit {
	if src := v.Source(); t.alwaysGen[src] \|\| inNodes(gen, src) {
	if !v.IsConcrete() {
	// The template has an expression that cannot be fully
	// resolved. Attempt to complete the expression by
	// evaluting it within the struct to which the template
	// is applied.
	expr := internal.ToExpr(v.Syntax())

	// TODO: this only resolves references contained in scope.
	v = internal.EvalExpr(scope, expr).(cue.Value)
	}

	if w := in.Unify(v); w.Err() == nil {
	in = w
	}
	// One of the sources of this struct is generated. That means
	// we can safely delete a non-generated version.
	exists = true
	gen = append(gen, src)
	}
	}

	switch v.Kind() {
	case cue.StructKind:
	// Build map of mixin fields.
	valueMap := map[key]cue.Value{}
	for mIter, _ := in.Fields(cue.All(), cue.Optional(false)); mIter.Next(); {
	valueMap[iterKey(mIter)] = mIter.Value()
	}

	fn := v.Template()

	// Process fields.
	rm := []ast.Node{}
	for iter, _ := v.Fields(cue.All()); iter.Next(); {
	mSub := valueMap[iterKey(iter)]
	if fn != nil {
	mSub = mSub.Unify(fn(iter.Label()))
	}

	removed := t.trim(iter.Label(), iter.Value(), mSub, v)
	rm = append(rm, removed...)
	}

	canRemove := fn == nil
	for _, v := range vSplit {
	src := v.Source()
	if t.fromComp[src] {
	canRemove = true
	}
	}

	// Remove fields from source.
	for _, v := range vSplit {
	if src := v.Source(); !t.alwaysGen[src] {
	switch x := src.(type) {
	case *ast.File:
	// TODO: use in instead?
	x.Decls = t.trimDecls(x.Decls, rm, m, canRemove)

	case *ast.StructLit:
	x.Elts = t.trimDecls(x.Elts, rm, m, canRemove)
	exists = exists \|\| m.Exists()
	if len(x.Elts) == 0 && exists && t.canRemove(src) && !inNodes(gen, src) {
	rmSet = append(rmSet, src)
	}

	default:
	// Currently the ast.Files are not associated with the
	// nodes. We detect this case here and iterate over all
	// files.
	// TODO: fix this. See cue/instance.go.
	if len(t.stack) == 1 {
	for _, x := range t.files {
	x.Decls = t.trimDecls(x.Decls, rm, m, canRemove)
	}
	}
	}
	}
	}

	if t.cfg.Trace {
	w := &bytes.Buffer{}
	fmt.Fprintln(w)
	fmt.Fprintln(w, "value: ", v)
	if in.Exists() {
	fmt.Fprintln(w, "mixed in: ", in)
	}
	for _, v := range vSplit {
	status := "[]"
	src := v.Source()
	if inNodes(rmSet, src) {
	status = "[re]"
	} else if t.alwaysGen[src] {
	status = "[i]"
	}
	fmt.Fprintf(w, " %4s %v: %v %T\n", status, v.Pos(), internal.DebugStr(src), src)
	}

	t.traceMsg(w.String())
	}

	case cue.ListKind:
	mIter, _ := m.List()
	i := 0
	rmElem := []ast.Node{}
	for iter, _ := v.List(); iter.Next(); i++ {
	mIter.Next()
	rm := t.trim(strconv.Itoa(i), iter.Value(), mIter.Value(), scope)
	rmElem = append(rmElem, rm...)
	}

	// Signal the removal of lists of which all elements have been marked
	// for removal.
	for _, v := range vSplit {
	if src := v.Source(); !t.alwaysGen[src] {
	l, ok := src.(*ast.ListLit)
	if !ok {
	break
	}
	rmList := true
	iter, _ := v.List()
	for i := 0; i < len(l.Elts) && iter.Next(); i++ {
	if !inNodes(rmElem, l.Elts[i]) {
	rmList = false
	break
	}
	}
	if rmList && m.Exists() && t.canRemove(src) && !inNodes(gen, src) {
	rmSet = append(rmSet, src)
	}
	}
	}
	fallthrough

	default:
	// Mark any subsumed part that is covered by generated config.
	if in.Err() == nil && v.Subsume(in) == nil {
	for _, v := range vSplit {
	src := v.Source()
	if t.canRemove(src) && !inNodes(gen, src) &&
	exists {
	rmSet = append(rmSet, src)
	}
	}
	}

	if t.cfg.Trace {
	w := &bytes.Buffer{}
	if len(rmSet) > 0 {
	fmt.Fprint(w, "field: SUBSUMED\n")
	} else {
	fmt.Fprint(w, "field: \n")
	}
	fmt.Fprintln(w, "value: ", v)
	if in.Exists() {
	fmt.Fprintln(w, "mixed in: ", in)
	}
	for _, v := range vSplit {
	status := "["
	if inNodes(gen, v.Source()) {
	status += "i"
	}
	if inNodes(rmSet, v.Source()) {
	status += "r"
	}
	status += "]"
	src := v.Source()
	fmt.Fprintf(w, " %4s %v: %v\n", status, v.Pos(), internal.DebugStr(src))
	}

	t.traceMsg(w.String())
	}
	}
	return rmSet
	}

	func (t *trimSet) trimDecls(decls []ast.Decl, rm []ast.Node, m cue.Value, allow bool) []ast.Decl {
	a := make([]ast.Decl, 0, len(decls))

	for _, d := range decls {
	if f, ok := d.(*ast.Field); ok {
	label, _, err := ast.LabelName(f.Label)
	v := m.Lookup(label)
	if err == nil && inNodes(rm, f.Value) && (allow \|\| v.Exists()) {
	continue
	}
	}
	a = append(a, d)
	}
	return a
	}

	func inNodes(a []ast.Node, n ast.Node) bool {
	for _, e := range a {
	if e == n {
	return true
	}
	}
	return false
	}

	type key struct {
	label string
	hidden bool
	}

	func iterKey(v cue.Iterator) key {
	return key{v.Label(), v.IsHidden()}
	}