go.tools/oracle: implements: now shows whole-program implements relation for selected type.

(Previously it showed the implements relation for all types within the query package.)

R=crawshaw
CC=golang-dev
https://golang.org/cl/42000043

cmd/oracle/main.go

@@ -36,8 +36,8 @@ var ptalogFlag = flag.String("ptalog", "",
 
 var formatFlag = flag.String("format", "plain", "Output format.  One of {plain,json,xml}.")
 
-// TODO(adonovan): eliminate or flip this flag after PTA presolver is implemented.
-var reflectFlag = flag.Bool("reflect", true, "Analyze reflection soundly (slow).")
+// TODO(adonovan): flip this flag after PTA presolver is implemented.
+var reflectFlag = flag.Bool("reflect", false, "Analyze reflection soundly (slow).")
 
 const useHelp = "Run 'oracle -help' for more information.\n"
 

oracle/TODO

@@ -96,4 +96,9 @@ Emacs: use JSON to get the raw information from the oracle.  Don't
  open an editor buffer for simpler queries, just jump to the result
  and/or display it in the modeline.
 
+Emacs: go-root-and-paths depends on the current buffer, so be sure to
+   call it from within the source file, not the *go-oracle* buffer:
+   the user may have switched workspaces and the oracle should run in
+   the new one.
+
 Support other editors: vim, Eclipse, Sublime, etc.

oracle/implements.go

@@ -5,99 +5,197 @@
 package oracle
 
 import (
+	"fmt"
+	"go/ast"
 	"go/token"
+	"reflect"
+	"sort"
+	"strings"
 
 	"code.google.com/p/go.tools/go/types"
 	"code.google.com/p/go.tools/oracle/serial"
 )
 
-// Implements displays the "implements" relation among all
-// package-level named types in the package containing the query
-// position.
-//
-// TODO(adonovan): more features:
-// - should we include pairs of types belonging to
-//   different packages in the 'implements' relation?
-// - should we restrict the query to the type declaration identified
-//   by the query position, if any, and use all types in the package
-//   otherwise?
-// - should we show types that are local to functions?
-//   They can only have methods via promotion.
-// - abbreviate the set of concrete types implementing the empty
-//   interface.
-// - should we scan the instruction stream for MakeInterface
-//   instructions and report which concrete->interface conversions
-//   actually occur, with examples?  (NB: this is not a conservative
-//   answer due to ChangeInterface, i.e. subtyping among interfaces.)
+// Implements displays the "implements" relation as it pertains to the
+// selected type.
 //
 func implements(o *Oracle, qpos *QueryPos) (queryResult, error) {
-	pkg := qpos.info.Pkg
+	// Find the selected type.
+	// TODO(adonovan): fix: make it work on qualified Idents too.
+	path, action := findInterestingNode(qpos.info, qpos.path)
+	if action != actionType {
+		return nil, fmt.Errorf("no type here")
+	}
+	T := qpos.info.TypeOf(path[0].(ast.Expr))
+	if T == nil {
+		return nil, fmt.Errorf("no type here")
+	}
 
-	// Compute set of named interface/concrete types at package level.
-	var interfaces, concretes []*types.Named
-	scope := pkg.Scope()
-	for _, name := range scope.Names() {
-		mem := scope.Lookup(name)
-		if t, ok := mem.(*types.TypeName); ok {
-			nt := t.Type().(*types.Named)
-			if _, ok := nt.Underlying().(*types.Interface); ok {
-				interfaces = append(interfaces, nt)
+	// Find all named types, even local types (which can have
+	// methods via promotion) and the built-in "error".
+	//
+	// TODO(adonovan): include all packages in PTA scope too?
+	// i.e. don't reduceScope?
+	//
+	var allNamed []types.Type
+	for _, info := range o.typeInfo {
+		for id, obj := range info.Objects {
+			if obj, ok := obj.(*types.TypeName); ok && obj.Pos() == id.Pos() {
+				allNamed = append(allNamed, obj.Type())
+			}
+		}
+	}
+	allNamed = append(allNamed, types.Universe.Lookup("error").Type())
+
+	// Test each named type.
+	var to, from, fromPtr []types.Type
+	for _, U := range allNamed {
+		if isInterface(T) {
+			if T.MethodSet().Len() == 0 {
+				continue // empty interface
+			}
+			if isInterface(U) {
+				if U.MethodSet().Len() == 0 {
+					continue // empty interface
+				}
+
+				// T interface, U interface
+				if !types.IsIdentical(T, U) {
+					if types.IsAssignableTo(U, T) {
+						to = append(to, U)
+					}
+					if types.IsAssignableTo(T, U) {
+						from = append(from, U)
+					}
+				}
 			} else {
-				concretes = append(concretes, nt)
+				// T interface, U concrete
+				if types.IsAssignableTo(U, T) {
+					to = append(to, U)
+				} else if pU := types.NewPointer(U); types.IsAssignableTo(pU, T) {
+					to = append(to, pU)
+				}
+			}
+		} else if isInterface(U) {
+			if U.MethodSet().Len() == 0 {
+				continue // empty interface
+			}
+
+			// T concrete, U interface
+			if types.IsAssignableTo(T, U) {
+				from = append(from, U)
+			} else if pT := types.NewPointer(T); types.IsAssignableTo(pT, U) {
+				fromPtr = append(fromPtr, U)
 			}
 		}
 	}
 
-	// For each interface, show the concrete types that implement it.
-	var facts []implementsFact
-	for _, iface := range interfaces {
-		fact := implementsFact{iface: iface}
-		for _, conc := range concretes {
-			if types.IsAssignableTo(conc, iface) {
-				fact.conc = conc
-			} else if ptr := types.NewPointer(conc); types.IsAssignableTo(ptr, iface) {
-				fact.conc = ptr
-			} else {
-				continue
-			}
-			facts = append(facts, fact)
-		}
+	var pos interface{} = qpos
+	if nt, ok := deref(T).(*types.Named); ok {
+		pos = nt.Obj()
 	}
-	// TODO(adonovan): sort facts to ensure test nondeterminism.
 
-	return &implementsResult{o.fset, facts}, nil
-}
+	// Sort types (arbitrarily) to ensure test nondeterminism.
+	sort.Sort(typesByString(to))
+	sort.Sort(typesByString(from))
+	sort.Sort(typesByString(fromPtr))
 
-type implementsFact struct {
-	iface *types.Named
-	conc  types.Type // Named or Pointer(Named)
+	return &implementsResult{T, pos, to, from, fromPtr}, nil
 }
 
 type implementsResult struct {
-	fset  *token.FileSet
-	facts []implementsFact // facts are grouped by interface
+	t       types.Type   // queried type (not necessarily named)
+	pos     interface{}  // pos of t (*types.Name or *QueryPos)
+	to      []types.Type // named or ptr-to-named types assignable to interface T
+	from    []types.Type // named interfaces assignable from T
+	fromPtr []types.Type // named interfaces assignable only from *T
 }
 
 func (r *implementsResult) display(printf printfFunc) {
-	var prevIface *types.Named
-	for _, fact := range r.facts {
-		if fact.iface != prevIface {
-			printf(fact.iface.Obj(), "\tInterface %s:", fact.iface)
-			prevIface = fact.iface
+	if isInterface(r.t) {
+		if r.t.MethodSet().Len() == 0 {
+			printf(r.pos, "empty interface type %s", r.t)
+			return
+		}
+
+		printf(r.pos, "interface type %s", r.t)
+		// Show concrete types first; use two passes.
+		for _, sub := range r.to {
+			if !isInterface(sub) {
+				printf(deref(sub).(*types.Named).Obj(), "\tis implemented by %s type %s",
+					typeKind(sub), sub)
+			}
+		}
+		for _, sub := range r.to {
+			if isInterface(sub) {
+				printf(deref(sub).(*types.Named).Obj(), "\tis implemented by %s type %s", typeKind(sub), sub)
+			}
+		}
+
+		for _, super := range r.from {
+			printf(super.(*types.Named).Obj(), "\timplements %s", super)
+		}
+	} else {
+		if r.from != nil {
+			printf(r.pos, "%s type %s", typeKind(r.t), r.t)
+			for _, super := range r.from {
+				printf(super.(*types.Named).Obj(), "\timplements %s", super)
+			}
+		}
+		if r.fromPtr != nil {
+			printf(r.pos, "pointer type *%s", r.t)
+			for _, psuper := range r.fromPtr {
+				printf(psuper.(*types.Named).Obj(), "\timplements %s", psuper)
+			}
+		} else if r.from == nil {
+			printf(r.pos, "%s type %s implements only interface{}", typeKind(r.t), r.t)
 		}
-		printf(deref(fact.conc).(*types.Named).Obj(), "\t\t%s", fact.conc)
 	}
 }
 
 func (r *implementsResult) toSerial(res *serial.Result, fset *token.FileSet) {
-	var facts []*serial.Implements
-	for _, fact := range r.facts {
-		facts = append(facts, &serial.Implements{
-			I:    fact.iface.String(),
-			IPos: fset.Position(fact.iface.Obj().Pos()).String(),
-			C:    fact.conc.String(),
-			CPos: fset.Position(deref(fact.conc).(*types.Named).Obj().Pos()).String(),
-		})
+	res.Implements = &serial.Implements{
+		T:                 makeImplementsType(r.t, fset),
+		AssignableTo:      makeImplementsTypes(r.to, fset),
+		AssignableFrom:    makeImplementsTypes(r.from, fset),
+		AssignableFromPtr: makeImplementsTypes(r.fromPtr, fset),
 	}
-	res.Implements = facts
 }
+
+func makeImplementsTypes(tt []types.Type, fset *token.FileSet) []serial.ImplementsType {
+	var r []serial.ImplementsType
+	for _, t := range tt {
+		r = append(r, makeImplementsType(t, fset))
+	}
+	return r
+}
+
+func makeImplementsType(T types.Type, fset *token.FileSet) serial.ImplementsType {
+	var pos token.Pos
+	if nt, ok := deref(T).(*types.Named); ok { // implementsResult.t may be non-named
+		pos = nt.Obj().Pos()
+	}
+	return serial.ImplementsType{
+		Name: T.String(),
+		Pos:  fset.Position(pos).String(),
+		Kind: typeKind(T),
+	}
+}
+
+// typeKind returns a string describing the underlying kind of type,
+// e.g. "slice", "array", "struct".
+func typeKind(T types.Type) string {
+	s := reflect.TypeOf(T.Underlying()).String()
+	return strings.ToLower(strings.TrimPrefix(s, "*types."))
+}
+
+func isInterface(T types.Type) bool {
+	_, isI := T.Underlying().(*types.Interface)
+	return isI
+}
+
+type typesByString []types.Type
+
+func (p typesByString) Len() int           { return len(p) }
+func (p typesByString) Less(i, j int) bool { return p[i].String() < p[j].String() }
+func (p typesByString) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }

oracle/oracle.go

@@ -94,7 +94,7 @@ type modeInfo struct {
 }
 
 var modes = []*modeInfo{
-	// Pointer analyses: (whole program)
+	// Pointer analyses, whole program:
 	{"callees", needPTA | needExactPos, callees},
 	{"callers", needPTA | needPos, callers},
 	{"callgraph", needPTA, callgraph},
@@ -102,12 +102,14 @@ var modes = []*modeInfo{
 	{"peers", needPTA | needSSADebug | needPos, peers},
 	{"pointsto", needPTA | needSSADebug | needExactPos, pointsto},
 
-	// Type-based analyses: (modular, mostly)
+	// Type-based, modular analyses:
 	{"definition", needPos, definition},
 	{"describe", needExactPos, describe},
 	{"freevars", needPos, freevars},
-	{"implements", needPos, implements},
-	{"referrers", needRetainTypeInfo | needPos, referrers}, // (whole-program)
+
+	// Type-based, whole-program analyses:
+	{"implements", needRetainTypeInfo | needPos, implements},
+	{"referrers", needRetainTypeInfo | needPos, referrers},
 }
 
 func findMode(mode string) *modeInfo {

oracle/oracle_test.go

@@ -210,10 +210,12 @@ func TestOracle(t *testing.T) {
 		"testdata/src/main/reflection.go",
 		"testdata/src/main/what.go",
 		// JSON:
+		// TODO(adonovan): most of these are very similar; combine them.
 		"testdata/src/main/callgraph-json.go",
 		"testdata/src/main/calls-json.go",
 		"testdata/src/main/peers-json.go",
 		"testdata/src/main/describe-json.go",
+		"testdata/src/main/implements-json.go",
 		"testdata/src/main/pointsto-json.go",
 		"testdata/src/main/referrers-json.go",
 		"testdata/src/main/what-json.go",

oracle/serial/serial.go

@@ -99,15 +99,24 @@ type FreeVar struct {
 	Type string `json:"type"` // type of the expression
 }
 
-// An Implements is one element of the result of an 'implements' query.
-// Each one indicates a row in the "implements" relation over
-// package-level named types defined by the package containing the
-// selection.
+// An Implements contains the result of an 'implements' query.
+
+// It describes the queried type, the set of named non-empty interface
+// types to which it is assignable, and the set of named/*named types
+// (concrete or non-empty interface) which may be assigned to it.
+//
 type Implements struct {
-	I    string `json:"i"`    // full name of the interface type
-	IPos string `json:"ipos"` // location of its definition
-	C    string `json:"c"`    // full name of the concrete type
-	CPos string `json:"cpos"` // location of its definition
+	T                 ImplementsType   `json:"type,omitempty"`    // the queried type
+	AssignableTo      []ImplementsType `json:"to,omitempty"`      // types assignable to T
+	AssignableFrom    []ImplementsType `json:"from,omitempty"`    // interface types assignable from T
+	AssignableFromPtr []ImplementsType `json:"fromptr,omitempty"` // interface types assignable only from *T
+}
+
+// An ImplementsType describes a single type as part of an 'implements' query.
+type ImplementsType struct {
+	Name string `json:"name"` // full name of the type
+	Pos  string `json:"pos"`  // location of its definition
+	Kind string `json:"kind"` // "basic", "array", etc
 }
 
 // A SyntaxNode is one element of a stack of enclosing syntax nodes in
@@ -222,23 +231,25 @@ type PTAWarning struct {
 // A Result is the common result of any oracle query.
 // It contains a query-specific result element.
 //
+// TODO(adonovan): perhaps include other info such as: analysis scope,
+// raw query position, stack of ast nodes, query package, etc.
 type Result struct {
 	Mode string `json:"mode"` // mode of the query
 
 	// Exactly one of the following fields is populated:
 	// the one specified by 'mode'.
-	Callees    *Callees      `json:"callees,omitempty"`
-	Callers    []Caller      `json:"callers,omitempty"`
-	Callgraph  []CallGraph   `json:"callgraph,omitempty"`
-	Callstack  *CallStack    `json:"callstack,omitempty"`
-	Definition *Definition   `json:"definition,omitempty"`
-	Describe   *Describe     `json:"describe,omitempty"`
-	Freevars   []*FreeVar    `json:"freevars,omitempty"`
-	Implements []*Implements `json:"implements,omitempty"`
-	Peers      *Peers        `json:"peers,omitempty"`
-	PointsTo   []PointsTo    `json:"pointsto,omitempty"`
-	Referrers  *Referrers    `json:"referrers,omitempty"`
-	What       *What         `json:"what,omitempty"`
+	Callees    *Callees    `json:"callees,omitempty"`
+	Callers    []Caller    `json:"callers,omitempty"`
+	Callgraph  []CallGraph `json:"callgraph,omitempty"`
+	Callstack  *CallStack  `json:"callstack,omitempty"`
+	Definition *Definition `json:"definition,omitempty"`
+	Describe   *Describe   `json:"describe,omitempty"`
+	Freevars   []*FreeVar  `json:"freevars,omitempty"`
+	Implements *Implements `json:"implements,omitempty"`
+	Peers      *Peers      `json:"peers,omitempty"`
+	PointsTo   []PointsTo  `json:"pointsto,omitempty"`
+	Referrers  *Referrers  `json:"referrers,omitempty"`
+	What       *What       `json:"what,omitempty"`
 
 	Warnings []PTAWarning `json:"warnings,omitempty"` // warnings from pointer analysis
 }

oracle/testdata/src/main/describe-json.go

@@ -1,8 +1,6 @@
 package describe // @describe pkgdecl "describe"
 
-// @implements implements "^"
-
-// Tests of 'describe' and 'implements' queries, -format=json.
+// Tests of 'describe' query, -format=json.
 // See go.tools/oracle/oracle_test.go for explanation.
 // See describe-json.golden for expected query results.
 

oracle/testdata/src/main/describe-json.golden

@@ -11,75 +11,58 @@
 				{
 					"name": "C",
 					"type": "int",
-					"pos": "testdata/src/main/describe-json.go:27:6",
+					"pos": "testdata/src/main/describe-json.go:25:6",
 					"kind": "type",
 					"methods": [
 						{
 							"name": "method (C) f()",
-							"pos": "testdata/src/main/describe-json.go:30:12"
+							"pos": "testdata/src/main/describe-json.go:28:12"
 						}
 					]
 				},
 				{
 					"name": "D",
 					"type": "struct{}",
-					"pos": "testdata/src/main/describe-json.go:28:6",
+					"pos": "testdata/src/main/describe-json.go:26:6",
 					"kind": "type",
 					"methods": [
 						{
 							"name": "method (*D) f()",
-							"pos": "testdata/src/main/describe-json.go:31:13"
+							"pos": "testdata/src/main/describe-json.go:29:13"
 						}
 					]
 				},
 				{
 					"name": "I",
 					"type": "interface{f()}",
-					"pos": "testdata/src/main/describe-json.go:23:6",
+					"pos": "testdata/src/main/describe-json.go:21:6",
 					"kind": "type",
 					"methods": [
 						{
 							"name": "method (I) f()",
-							"pos": "testdata/src/main/describe-json.go:24:2"
+							"pos": "testdata/src/main/describe-json.go:22:2"
 						}
 					]
 				},
 				{
 					"name": "main",
 					"type": "func()",
-					"pos": "testdata/src/main/describe-json.go:9:6",
+					"pos": "testdata/src/main/describe-json.go:7:6",
 					"kind": "func"
 				}
 			]
 		}
 	}
-}-------- @implements implements --------
-{
-	"mode": "implements",
-	"implements": [
-		{
-			"i": "describe.I",
-			"ipos": "testdata/src/main/describe-json.go:23:6",
-			"c": "describe.C",
-			"cpos": "testdata/src/main/describe-json.go:27:6"
-		},
-		{
-			"i": "describe.I",
-			"ipos": "testdata/src/main/describe-json.go:23:6",
-			"c": "*describe.D",
-			"cpos": "testdata/src/main/describe-json.go:28:6"
-		}
-	]
 }-------- @describe desc-val-p --------
 {
 	"mode": "describe",
 	"describe": {
 		"desc": "identifier",
-		"pos": "testdata/src/main/describe-json.go:11:2",
+		"pos": "testdata/src/main/describe-json.go:9:2",
 		"detail": "value",
 		"value": {
 			"type": "*int",
-			"objpos": "testdata/src/main/describe-json.go:11:2"
+			"objpos": "testdata/src/main/describe-json.go:9:2"
 		}
 	}
 }-------- @describe desc-val-i --------
@@ -87,11 +70,11 @@
 	"mode": "describe",
 	"describe": {
 		"desc": "identifier",
-		"pos": "testdata/src/main/describe-json.go:18:8",
+		"pos": "testdata/src/main/describe-json.go:16:8",
 		"detail": "value",
 		"value": {
 			"type": "I",
-			"objpos": "testdata/src/main/describe-json.go:14:6"
+			"objpos": "testdata/src/main/describe-json.go:12:6"
 		}
 	}
 }-------- @describe desc-stmt --------
@@ -99,7 +82,7 @@
 	"mode": "describe",
 	"describe": {
 		"desc": "go statement",
-		"pos": "testdata/src/main/describe-json.go:20:2",
+		"pos": "testdata/src/main/describe-json.go:18:2",
 		"detail": "unknown"
 	}
 }-------- @describe desc-type-C --------
@@ -107,16 +90,16 @@
 	"mode": "describe",
 	"describe": {
 		"desc": "definition of type C (size 8, align 8)",
-		"pos": "testdata/src/main/describe-json.go:27:6",
+		"pos": "testdata/src/main/describe-json.go:25:6",
 		"detail": "type",
 		"type": {
 			"type": "C",
-			"namepos": "testdata/src/main/describe-json.go:27:6",
+			"namepos": "testdata/src/main/describe-json.go:25:6",
 			"namedef": "int",
 			"methods": [
 				{
 					"name": "method (C) f()",
-					"pos": "testdata/src/main/describe-json.go:30:12"
+					"pos": "testdata/src/main/describe-json.go:28:12"
 				}
 			]
 		}

oracle/testdata/src/main/implements-json.go

@@ -0,0 +1,27 @@
+package main
+
+// Tests of 'implements' query, -output=json.
+// See go.tools/oracle/oracle_test.go for explanation.
+// See implements.golden for expected query results.
+
+func main() {
+}
+
+type E interface{} // @implements E "E"
+
+type F interface { // @implements F "F"
+	f()
+}
+
+type FG interface { // @implements FG "FG"
+	f()
+	g() []int // @implements slice "..int"
+}
+
+type C int // @implements C "C"
+type D struct{}
+
+func (c *C) f() {} // @implements starC ".C"
+func (d D) f()  {} // @implements D "D"
+
+func (d *D) g() []int { return nil } // @implements starD ".D"

oracle/testdata/src/main/implements-json.golden

@@ -0,0 +1,152 @@
+-------- @implements E --------
+{
+	"mode": "implements",
+	"implements": {
+		"type": {
+			"name": "main.E",
+			"pos": "testdata/src/main/implements-json.go:10:6",
+			"kind": "interface"
+		}
+	}
+}-------- @implements F --------
+{
+	"mode": "implements",
+	"implements": {
+		"type": {
+			"name": "main.F",
+			"pos": "testdata/src/main/implements-json.go:12:6",
+			"kind": "interface"
+		},
+		"to": [
+			{
+				"name": "*main.C",
+				"pos": "testdata/src/main/implements-json.go:21:6",
+				"kind": "pointer"
+			},
+			{
+				"name": "main.D",
+				"pos": "testdata/src/main/implements-json.go:22:6",
+				"kind": "struct"
+			},
+			{
+				"name": "main.FG",
+				"pos": "testdata/src/main/implements-json.go:16:6",
+				"kind": "interface"
+			}
+		]
+	}
+}-------- @implements FG --------
+{
+	"mode": "implements",
+	"implements": {
+		"type": {
+			"name": "main.FG",
+			"pos": "testdata/src/main/implements-json.go:16:6",
+			"kind": "interface"
+		},
+		"to": [
+			{
+				"name": "*main.D",
+				"pos": "testdata/src/main/implements-json.go:22:6",
+				"kind": "pointer"
+			}
+		],
+		"from": [
+			{
+				"name": "main.F",
+				"pos": "testdata/src/main/implements-json.go:12:6",
+				"kind": "interface"
+			}
+		]
+	}
+}-------- @implements slice --------
+{
+	"mode": "implements",
+	"implements": {
+		"type": {
+			"name": "[]int",
+			"pos": "-",
+			"kind": "slice"
+		}
+	}
+}-------- @implements C --------
+{
+	"mode": "implements",
+	"implements": {
+		"type": {
+			"name": "main.C",
+			"pos": "testdata/src/main/implements-json.go:21:6",
+			"kind": "basic"
+		},
+		"fromptr": [
+			{
+				"name": "main.F",
+				"pos": "testdata/src/main/implements-json.go:12:6",
+				"kind": "interface"
+			}
+		]
+	}
+}-------- @implements starC --------
+{
+	"mode": "implements",
+	"implements": {
+		"type": {
+			"name": "*main.C",
+			"pos": "testdata/src/main/implements-json.go:21:6",
+			"kind": "pointer"
+		},
+		"from": [
+			{
+				"name": "main.F",
+				"pos": "testdata/src/main/implements-json.go:12:6",
+				"kind": "interface"
+			}
+		]
+	}
+}-------- @implements D --------
+{
+	"mode": "implements",
+	"implements": {
+		"type": {
+			"name": "main.D",
+			"pos": "testdata/src/main/implements-json.go:22:6",
+			"kind": "struct"
+		},
+		"from": [
+			{
+				"name": "main.F",
+				"pos": "testdata/src/main/implements-json.go:12:6",
+				"kind": "interface"
+			}
+		],
+		"fromptr": [
+			{
+				"name": "main.FG",
+				"pos": "testdata/src/main/implements-json.go:16:6",
+				"kind": "interface"
+			}
+		]
+	}
+}-------- @implements starD --------
+{
+	"mode": "implements",
+	"implements": {
+		"type": {
+			"name": "*main.D",
+			"pos": "testdata/src/main/implements-json.go:22:6",
+			"kind": "pointer"
+		},
+		"from": [
+			{
+				"name": "main.F",
+				"pos": "testdata/src/main/implements-json.go:12:6",
+				"kind": "interface"
+			},
+			{
+				"name": "main.FG",
+				"pos": "testdata/src/main/implements-json.go:16:6",
+				"kind": "interface"
+			}
+		]
+	}
+}

oracle/testdata/src/main/implements.go

@@ -4,26 +4,37 @@ package main
 // See go.tools/oracle/oracle_test.go for explanation.
 // See implements.golden for expected query results.
 
-// @implements impl ""
+import _ "lib"
+import _ "sort"
 
 func main() {
 }
 
-type E interface{}
+type E interface{} // @implements E "E"
 
-type F interface {
+type F interface { // @implements F "F"
 	f()
 }
 
-type FG interface {
+type FG interface { // @implements FG "FG"
 	f()
-	g() int
+	g() []int // @implements slice "..int"
 }
 
-type C int
+type C int // @implements C "C"
 type D struct{}
 
-func (c *C) f() {}
-func (d D) f()  {}
+func (c *C) f() {} // @implements starC ".C"
+func (d D) f()  {} // @implements D "D"
 
-func (d *D) g() int { return 0 }
+func (d *D) g() []int { return nil } // @implements starD ".D"
+
+type sorter []int // @implements sorter "sorter"
+
+func (sorter) Len() int           { return 0 }
+func (sorter) Less(i, j int) bool { return false }
+func (sorter) Swap(i, j int)      {}
+
+type I interface { // @implements I "I"
+	Method(*int) *int
+}

oracle/testdata/src/main/implements.golden

@@ -1,10 +1,44 @@
--------- @implements impl --------
-	Interface main.E:
-		main.C
-		main.D
-	Interface main.F:
-		*main.C
-		main.D
-	Interface main.FG:
-		*main.D
+-------- @implements E --------
+empty interface type main.E
+
+-------- @implements F --------
+interface type main.F
+	is implemented by pointer type *main.C
+	is implemented by struct type main.D
+	is implemented by interface type main.FG
+
+-------- @implements FG --------
+interface type main.FG
+	is implemented by pointer type *main.D
+	implements main.F
+
+-------- @implements slice --------
+slice type []int implements only interface{}
+
+-------- @implements C --------
+pointer type *main.C
+	implements main.F
+
+-------- @implements starC --------
+pointer type *main.C
+	implements main.F
+
+-------- @implements D --------
+struct type main.D
+	implements main.F
+pointer type *main.D
+	implements main.FG
+
+-------- @implements starD --------
+pointer type *main.D
+	implements main.F
+	implements main.FG
+
+-------- @implements sorter --------
+slice type main.sorter
+	implements sort.Interface
+
+-------- @implements I --------
+interface type main.I
+	is implemented by basic type lib.Type
 

oracle/testdata/src/main/what.golden

@@ -22,7 +22,7 @@ variable declaration statement
 block
 function declaration
 source file
-modes: [callers callgraph callstack describe freevars implements pointsto]
+modes: [callers callgraph callstack describe freevars pointsto]
 srcdir: testdata/src
 import path: main
 

oracle/what.go

@@ -34,17 +34,23 @@ func what(posFlag string, buildContext *build.Context) (*Result, error) {
 	srcdir, importPath, _ := guessImportPath(qpos.fset.File(qpos.start).Name(), buildContext)
 
 	// Determine which query modes are applicable to the selection.
+	// TODO(adonovan): refactor: make each minfo have an 'enable'
+	// predicate over qpos.
 	enable := map[string]bool{
-		"callgraph":  true,                  // whole program; always enabled
-		"implements": true,                  // whole package; always enabled
-		"freevars":   qpos.end > qpos.start, // nonempty selection?
-		"describe":   true,                  // any syntax; always enabled
+		"callgraph": true, // whole program; always enabled
+		"describe":  true, // any syntax; always enabled
 	}
+
+	if qpos.end > qpos.start {
+		enable["freevars"] = true // nonempty selection?
+	}
+
 	for _, n := range qpos.path {
 		switch n := n.(type) {
 		case *ast.Ident:
 			enable["definition"] = true
 			enable["referrers"] = true
+			enable["implements"] = true
 		case *ast.CallExpr:
 			enable["callees"] = true
 		case *ast.FuncDecl:
@@ -58,6 +64,19 @@ func what(posFlag string, buildContext *build.Context) (*Result, error) {
 			}
 		}
 
+		// For implements, we approximate findInterestingNode.
+		if _, ok := enable["implements"]; !ok {
+			switch n.(type) {
+			case *ast.ArrayType,
+				*ast.StructType,
+				*ast.FuncType,
+				*ast.InterfaceType,
+				*ast.MapType,
+				*ast.ChanType:
+				enable["implements"] = true
+			}
+		}
+
 		// For pointsto, we approximate findInterestingNode.
 		if _, ok := enable["pointsto"]; !ok {
 			switch n.(type) {