diff --git a/refactor/satisfy/find.go b/refactor/satisfy/find.go
new file mode 100644
index 00000000..897fbc1c
--- /dev/null
+++ b/refactor/satisfy/find.go
@@ -0,0 +1,725 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package satisfy inspects the type-checked ASTs of Go packages and
+// reports the set of discovered type constraints of the form (lhs, rhs
+// Type) where lhs is a non-trivial interface, rhs satisfies this
+// interface, and this fact is necessary for the package to be
+// well-typed.
+//
+// THIS PACKAGE IS EXPERIMENTAL AND MAY CHANGE AT ANY TIME.
+//
+// It is provided only for the gorename tool.  Ideally this
+// functionality will become part of the type-checker in due course,
+// since it is computing it anyway, and it is robust for ill-typed
+// inputs, which this package is not.
+//
+package satisfy
+
+// NOTES:
+//
+// We don't care about numeric conversions, so we don't descend into
+// types or constant expressions.  This is unsound because
+// constant expressions can contain arbitrary statements, e.g.
+//   const x = len([1]func(){func() {
+//     ...
+//   }})
+//
+// TODO(adonovan): make this robust against ill-typed input.
+// Or move it into the type-checker.
+//
+// Assignability conversions are possible in the following places:
+// - in assignments y = x, y := x, var y = x.
+// - from call argument types to formal parameter types
+// - in append and delete calls
+// - from return operands to result parameter types
+// - in composite literal T{k:v}, from k and v to T's field/element/key type
+// - in map[key] from key to the map's key type
+// - in comparisons x==y and switch x { case y: }.
+// - in explicit conversions T(x)
+// - in sends ch <- x, from x to the channel element type
+// - in type assertions x.(T) and switch x.(type) { case T: }
+//
+// The results of this pass provide information equivalent to the
+// ssa.MakeInterface and ssa.ChangeInterface instructions.
+
+import (
+	"fmt"
+	"go/ast"
+	"go/token"
+
+	"code.google.com/p/go.tools/go/types"
+	"code.google.com/p/go.tools/go/types/typeutil"
+)
+
+// A Constraint records the fact that the RHS type does and must
+// satisify the LHS type, which is an interface.
+// The names are suggestive of an assignment statement LHS = RHS.
+type Constraint struct {
+	LHS, RHS types.Type
+}
+
+// A Finder inspects the type-checked ASTs of Go packages and
+// accumulates the set of type constraints (x, y) such that x is
+// assignable to y, y is an interface, and both x and y have methods.
+//
+// In other words, it returns the subset of the "implements" relation
+// that is checked during compilation of a package.  Refactoring tools
+// will need to preserve at least this part of the relation to ensure
+// continued compilation.
+//
+type Finder struct {
+	Result    map[Constraint]bool
+	msetcache types.MethodSetCache
+	canon     typeutil.Map // maps types to canonical type
+
+	// per-Find state
+	info *types.Info
+	sig  *types.Signature
+}
+
+// Find inspects a single package, populating Result with its pairs of
+// constrained types.
+//
+// The package must be free of type errors, and
+// info.{Defs,Uses,Selections,Types} must have been populated by the
+// type-checker.
+//
+func (f *Finder) Find(info *types.Info, files []*ast.File) {
+	if f.Result == nil {
+		f.Result = make(map[Constraint]bool)
+	}
+
+	f.info = info
+	for _, file := range files {
+		for _, d := range file.Decls {
+			switch d := d.(type) {
+			case *ast.GenDecl:
+				if d.Tok == token.VAR { // ignore consts
+					for _, spec := range d.Specs {
+						f.valueSpec(spec.(*ast.ValueSpec))
+					}
+				}
+
+			case *ast.FuncDecl:
+				if d.Body != nil {
+					f.sig = f.info.Defs[d.Name].Type().(*types.Signature)
+					f.stmt(d.Body)
+					f.sig = nil
+				}
+			}
+		}
+	}
+	f.info = nil
+}
+
+var (
+	tInvalid     = types.Typ[types.Invalid]
+	tUntypedBool = types.Typ[types.UntypedBool]
+	tUntypedNil  = types.Typ[types.UntypedNil]
+)
+
+// exprN visits an expression in a multi-value context.
+func (f *Finder) exprN(e ast.Expr) types.Type {
+	typ := f.info.Types[e].Type.(*types.Tuple)
+	switch e := e.(type) {
+	case *ast.ParenExpr:
+		return f.exprN(e.X)
+
+	case *ast.CallExpr:
+		// x, err := f(args)
+		sig := f.expr(e.Fun).Underlying().(*types.Signature)
+		f.call(sig, e.Args)
+
+	case *ast.IndexExpr:
+		// y, ok := x[i]
+		x := f.expr(e.X)
+		f.assign(f.expr(e.Index), x.Underlying().(*types.Map).Key())
+
+	case *ast.TypeAssertExpr:
+		// y, ok := x.(T)
+		f.typeAssert(f.expr(e.X), typ.At(0).Type())
+
+	case *ast.UnaryExpr: // must be receive <-
+		// y, ok := <-x
+		f.expr(e.X)
+
+	default:
+		panic(e)
+	}
+	return typ
+}
+
+func (f *Finder) call(sig *types.Signature, args []ast.Expr) {
+	if len(args) == 0 {
+		return
+	}
+
+	// Ellipsis call?  e.g. f(x, y, z...)
+	if _, ok := args[len(args)-1].(*ast.Ellipsis); ok {
+		for i, arg := range args {
+			// The final arg is a slice, and so is the final param.
+			f.assign(sig.Params().At(i).Type(), f.expr(arg))
+		}
+		return
+	}
+
+	var argtypes []types.Type
+
+	// Gather the effective actual parameter types.
+	if tuple, ok := f.info.Types[args[0]].Type.(*types.Tuple); ok {
+		// f(g()) call where g has multiple results?
+		f.expr(args[0])
+		// unpack the tuple
+		for i := 0; i < tuple.Len(); i++ {
+			argtypes = append(argtypes, tuple.At(i).Type())
+		}
+	} else {
+		for _, arg := range args {
+			argtypes = append(argtypes, f.expr(arg))
+		}
+	}
+
+	// Assign the actuals to the formals.
+	if !sig.Variadic() {
+		for i, argtype := range argtypes {
+			f.assign(sig.Params().At(i).Type(), argtype)
+		}
+	} else {
+		// The first n-1 parameters are assigned normally.
+		nnormals := sig.Params().Len() - 1
+		for i, argtype := range argtypes[:nnormals] {
+			f.assign(sig.Params().At(i).Type(), argtype)
+		}
+		// Remaining args are assigned to elements of varargs slice.
+		tElem := sig.Params().At(nnormals).Type().(*types.Slice).Elem()
+		for i := nnormals; i < len(argtypes); i++ {
+			f.assign(tElem, argtypes[i])
+		}
+	}
+}
+
+func (f *Finder) builtin(obj *types.Builtin, sig *types.Signature, args []ast.Expr, T types.Type) types.Type {
+	switch obj.Name() {
+	case "make", "new":
+		// skip the type operand
+		for _, arg := range args[1:] {
+			f.expr(arg)
+		}
+
+	case "append":
+		s := f.expr(args[0])
+		if _, ok := args[len(args)-1].(*ast.Ellipsis); ok && len(args) == 2 {
+			// append(x, y...)   including append([]byte, "foo"...)
+			f.expr(args[1])
+		} else {
+			// append(x, y, z)
+			tElem := s.Underlying().(*types.Slice).Elem()
+			for _, arg := range args[1:] {
+				f.assign(tElem, f.expr(arg))
+			}
+		}
+
+	case "delete":
+		m := f.expr(args[0])
+		k := f.expr(args[1])
+		f.assign(m.Underlying().(*types.Map).Key(), k)
+
+	default:
+		// ordinary call
+		f.call(sig, args)
+	}
+
+	return T
+}
+
+func (f *Finder) extract(tuple types.Type, i int) types.Type {
+	if tuple, ok := tuple.(*types.Tuple); ok && i < tuple.Len() {
+		return tuple.At(i).Type()
+	}
+	return tInvalid
+}
+
+func (f *Finder) valueSpec(spec *ast.ValueSpec) {
+	var T types.Type
+	if spec.Type != nil {
+		T = f.info.Types[spec.Type].Type
+	}
+	switch len(spec.Values) {
+	case len(spec.Names): // e.g. var x, y = f(), g()
+		for _, value := range spec.Values {
+			v := f.expr(value)
+			if T != nil {
+				f.assign(T, v)
+			}
+		}
+
+	case 1: // e.g. var x, y = f()
+		tuple := f.exprN(spec.Values[0])
+		for i := range spec.Names {
+			if T != nil {
+				f.assign(T, f.extract(tuple, i))
+			}
+		}
+	}
+}
+
+// assign records pairs of distinct types that are related by
+// assignability, where the left-hand side is an interface and both
+// sides have methods.
+//
+// It should be called for all assignability checks, type assertions,
+// explicit conversions and comparisons between two types, unless the
+// types are uninteresting (e.g. lhs is a concrete type, or the empty
+// interface; rhs has no methods).
+//
+func (f *Finder) assign(lhs, rhs types.Type) {
+	if types.Identical(lhs, rhs) {
+		return
+	}
+	if !isInterface(lhs) {
+		return
+	}
+	if f.msetcache.MethodSet(lhs).Len() == 0 {
+		return
+	}
+	if f.msetcache.MethodSet(rhs).Len() == 0 {
+		return
+	}
+	// canonicalize types
+	lhsc, ok := f.canon.At(lhs).(types.Type)
+	if !ok {
+		lhsc = lhs
+		f.canon.Set(lhs, lhsc)
+	}
+	rhsc, ok := f.canon.At(rhs).(types.Type)
+	if !ok {
+		rhsc = rhs
+		f.canon.Set(rhs, rhsc)
+	}
+	// record the pair
+	f.Result[Constraint{lhsc, rhsc}] = true
+}
+
+// typeAssert must be called for each type assertion x.(T) where x has
+// interface type I.
+func (f *Finder) typeAssert(I, T types.Type) {
+	// Type assertions are slightly subtle, because they are allowed
+	// to be "impossible", e.g.
+	//
+	// 	var x interface{f()}
+	//	_ = x.(interface{f()int}) // legal
+	//
+	// (In hindsight, the language spec should probably not have
+	// allowed this, but it's too late to fix now.)
+	//
+	// This means that a type assert from I to T isn't exactly a
+	// constraint that T is assignable to I, but for a refactoring
+	// tool it is a conditional constraint that, if T is assignable
+	// to I before a refactoring, it should remain so after.
+
+	if types.AssignableTo(T, I) {
+		f.assign(I, T)
+	}
+}
+
+// compare must be called for each comparison x==y.
+func (f *Finder) compare(x, y types.Type) {
+	if types.AssignableTo(x, y) {
+		f.assign(y, x)
+	} else if types.AssignableTo(y, x) {
+		f.assign(x, y)
+	}
+}
+
+// expr visits a true expression (not a type or defining ident)
+// and returns its type.
+func (f *Finder) expr(e ast.Expr) types.Type {
+	tv := f.info.Types[e]
+	if tv.Value != nil {
+		return tv.Type // prune the descent for constants
+	}
+
+	// tv.Type may be nil for an ast.Ident.
+
+	switch e := e.(type) {
+	case *ast.BadExpr, *ast.BasicLit:
+		// no-op
+
+	case *ast.Ident:
+		// (referring idents only)
+		if obj, ok := f.info.Uses[e]; ok {
+			return obj.Type()
+		}
+		if e.Name == "_" { // e.g. "for _ = range x"
+			return tInvalid
+		}
+		panic("undefined ident: " + e.Name)
+
+	case *ast.Ellipsis:
+		if e.Elt != nil {
+			f.expr(e.Elt)
+		}
+
+	case *ast.FuncLit:
+		saved := f.sig
+		f.sig = tv.Type.(*types.Signature)
+		f.stmt(e.Body)
+		f.sig = saved
+
+	case *ast.CompositeLit:
+		switch T := deref(tv.Type).Underlying().(type) {
+		case *types.Struct:
+			for i, elem := range e.Elts {
+				if kv, ok := elem.(*ast.KeyValueExpr); ok {
+					f.assign(f.info.Uses[kv.Key.(*ast.Ident)].Type(), f.expr(kv.Value))
+				} else {
+					f.assign(T.Field(i).Type(), f.expr(elem))
+				}
+			}
+
+		case *types.Map:
+			for _, elem := range e.Elts {
+				elem := elem.(*ast.KeyValueExpr)
+				f.assign(T.Key(), f.expr(elem.Key))
+				f.assign(T.Elem(), f.expr(elem.Value))
+			}
+
+		case *types.Array, *types.Slice:
+			tElem := T.(interface {
+				Elem() types.Type
+			}).Elem()
+			for _, elem := range e.Elts {
+				if kv, ok := elem.(*ast.KeyValueExpr); ok {
+					// ignore the key
+					f.assign(tElem, f.expr(kv.Value))
+				} else {
+					f.assign(tElem, f.expr(elem))
+				}
+			}
+
+		default:
+			panic("unexpected composite literal type: " + tv.Type.String())
+		}
+
+	case *ast.ParenExpr:
+		f.expr(e.X)
+
+	case *ast.SelectorExpr:
+		if _, ok := f.info.Selections[e]; ok {
+			f.expr(e.X) // selection
+		} else {
+			return f.info.Uses[e.Sel].Type() // qualified identifier
+		}
+
+	case *ast.IndexExpr:
+		x := f.expr(e.X)
+		i := f.expr(e.Index)
+		if ux, ok := x.Underlying().(*types.Map); ok {
+			f.assign(ux.Elem(), i)
+		}
+
+	case *ast.SliceExpr:
+		f.expr(e.X)
+		if e.Low != nil {
+			f.expr(e.Low)
+		}
+		if e.High != nil {
+			f.expr(e.High)
+		}
+		if e.Max != nil {
+			f.expr(e.Max)
+		}
+
+	case *ast.TypeAssertExpr:
+		x := f.expr(e.X)
+		f.typeAssert(x, f.info.Types[e.Type].Type)
+
+	case *ast.CallExpr:
+		if tvFun := f.info.Types[e.Fun]; tvFun.IsType() {
+			// conversion
+			arg0 := f.expr(e.Args[0])
+			f.assign(tvFun.Type, arg0)
+		} else {
+			// function call
+			if id, ok := unparen(e.Fun).(*ast.Ident); ok {
+				if obj, ok := f.info.Uses[id].(*types.Builtin); ok {
+					sig := f.info.Types[id].Type.(*types.Signature)
+					return f.builtin(obj, sig, e.Args, tv.Type)
+				}
+			}
+			// ordinary call
+			f.call(f.expr(e.Fun).Underlying().(*types.Signature), e.Args)
+		}
+
+	case *ast.StarExpr:
+		f.expr(e.X)
+
+	case *ast.UnaryExpr:
+		f.expr(e.X)
+
+	case *ast.BinaryExpr:
+		x := f.expr(e.X)
+		y := f.expr(e.Y)
+		if e.Op == token.EQL || e.Op == token.NEQ {
+			f.compare(x, y)
+		}
+
+	case *ast.KeyValueExpr:
+		f.expr(e.Key)
+		f.expr(e.Value)
+
+	case *ast.ArrayType,
+		*ast.StructType,
+		*ast.FuncType,
+		*ast.InterfaceType,
+		*ast.MapType,
+		*ast.ChanType:
+		panic(e)
+	}
+
+	if tv.Type == nil {
+		panic(fmt.Sprintf("no type for %T", e))
+	}
+
+	return tv.Type
+}
+
+func (f *Finder) stmt(s ast.Stmt) {
+	switch s := s.(type) {
+	case *ast.BadStmt,
+		*ast.EmptyStmt,
+		*ast.BranchStmt:
+		// no-op
+
+	case *ast.DeclStmt:
+		d := s.Decl.(*ast.GenDecl)
+		if d.Tok == token.VAR { // ignore consts
+			for _, spec := range d.Specs {
+				f.valueSpec(spec.(*ast.ValueSpec))
+			}
+		}
+
+	case *ast.LabeledStmt:
+		f.stmt(s.Stmt)
+
+	case *ast.ExprStmt:
+		f.expr(s.X)
+
+	case *ast.SendStmt:
+		ch := f.expr(s.Chan)
+		val := f.expr(s.Value)
+		f.assign(ch.Underlying().(*types.Chan).Elem(), val)
+
+	case *ast.IncDecStmt:
+		f.expr(s.X)
+
+	case *ast.AssignStmt:
+		switch s.Tok {
+		case token.ASSIGN, token.DEFINE:
+			// y := x   or   y = x
+			var rhsTuple types.Type
+			if len(s.Lhs) != len(s.Rhs) {
+				rhsTuple = f.exprN(s.Rhs[0])
+			}
+			for i := range s.Lhs {
+				var lhs, rhs types.Type
+				if rhsTuple == nil {
+					rhs = f.expr(s.Rhs[i]) // 1:1 assignment
+				} else {
+					rhs = f.extract(rhsTuple, i) // n:1 assignment
+				}
+
+				if id, ok := s.Lhs[i].(*ast.Ident); ok {
+					if id.Name != "_" {
+						if obj, ok := f.info.Defs[id]; ok {
+							lhs = obj.Type() // definition
+						}
+					}
+				}
+				if lhs == nil {
+					lhs = f.expr(s.Lhs[i]) // assignment
+				}
+				f.assign(lhs, rhs)
+			}
+
+		default:
+			// y op= x
+			f.expr(s.Lhs[0])
+			f.expr(s.Rhs[0])
+		}
+
+	case *ast.GoStmt:
+		f.expr(s.Call)
+
+	case *ast.DeferStmt:
+		f.expr(s.Call)
+
+	case *ast.ReturnStmt:
+		formals := f.sig.Results()
+		switch len(s.Results) {
+		case formals.Len(): // 1:1
+			for i, result := range s.Results {
+				f.assign(formals.At(i).Type(), f.expr(result))
+			}
+
+		case 1: // n:1
+			tuple := f.exprN(s.Results[0])
+			for i := 0; i < formals.Len(); i++ {
+				f.assign(formals.At(i).Type(), f.extract(tuple, i))
+			}
+		}
+
+	case *ast.SelectStmt:
+		f.stmt(s.Body)
+
+	case *ast.BlockStmt:
+		for _, s := range s.List {
+			f.stmt(s)
+		}
+
+	case *ast.IfStmt:
+		if s.Init != nil {
+			f.stmt(s.Init)
+		}
+		f.expr(s.Cond)
+		f.stmt(s.Body)
+		if s.Else != nil {
+			f.stmt(s.Else)
+		}
+
+	case *ast.SwitchStmt:
+		if s.Init != nil {
+			f.stmt(s.Init)
+		}
+		var tag types.Type = tUntypedBool
+		if s.Tag != nil {
+			tag = f.expr(s.Tag)
+		}
+		for _, cc := range s.Body.List {
+			cc := cc.(*ast.CaseClause)
+			for _, cond := range cc.List {
+				f.compare(tag, f.info.Types[cond].Type)
+			}
+			for _, s := range cc.Body {
+				f.stmt(s)
+			}
+		}
+
+	case *ast.TypeSwitchStmt:
+		if s.Init != nil {
+			f.stmt(s.Init)
+		}
+		var I types.Type
+		switch ass := s.Assign.(type) {
+		case *ast.ExprStmt: // x.(type)
+			I = f.expr(unparen(ass.X).(*ast.TypeAssertExpr).X)
+		case *ast.AssignStmt: // y := x.(type)
+			I = f.expr(unparen(ass.Rhs[0]).(*ast.TypeAssertExpr).X)
+		}
+		for _, cc := range s.Body.List {
+			cc := cc.(*ast.CaseClause)
+			for _, cond := range cc.List {
+				tCase := f.info.Types[cond].Type
+				if tCase != tUntypedNil {
+					f.typeAssert(I, tCase)
+				}
+			}
+			for _, s := range cc.Body {
+				f.stmt(s)
+			}
+		}
+
+	case *ast.CommClause:
+		if s.Comm != nil {
+			f.stmt(s.Comm)
+		}
+		for _, s := range s.Body {
+			f.stmt(s)
+		}
+
+	case *ast.ForStmt:
+		if s.Init != nil {
+			f.stmt(s.Init)
+		}
+		if s.Cond != nil {
+			f.expr(s.Cond)
+		}
+		if s.Post != nil {
+			f.stmt(s.Post)
+		}
+		f.stmt(s.Body)
+
+	case *ast.RangeStmt:
+		x := f.expr(s.X)
+		// No conversions are involved when Tok==DEFINE.
+		if s.Tok == token.ASSIGN {
+			if s.Key != nil {
+				k := f.expr(s.Key)
+				var xelem types.Type
+				// keys of array, *array, slice, string aren't interesting
+				switch ux := x.Underlying().(type) {
+				case *types.Chan:
+					xelem = ux.Elem()
+				case *types.Map:
+					xelem = ux.Key()
+				}
+				if xelem != nil {
+					f.assign(xelem, k)
+				}
+			}
+			if s.Value != nil {
+				val := f.expr(s.Value)
+				var xelem types.Type
+				// values of strings aren't interesting
+				switch ux := x.Underlying().(type) {
+				case *types.Array:
+					xelem = ux.Elem()
+				case *types.Chan:
+					xelem = ux.Elem()
+				case *types.Map:
+					xelem = ux.Elem()
+				case *types.Pointer: // *array
+					xelem = deref(ux).(*types.Array).Elem()
+				case *types.Slice:
+					xelem = ux.Elem()
+				}
+				if xelem != nil {
+					f.assign(xelem, val)
+				}
+			}
+		}
+		f.stmt(s.Body)
+
+	default:
+		panic(s)
+	}
+}
+
+// -- Plundered from code.google.com/p/go.tools/go/ssa -----------------
+
+// deref returns a pointer's element type; otherwise it returns typ.
+func deref(typ types.Type) types.Type {
+	if p, ok := typ.Underlying().(*types.Pointer); ok {
+		return p.Elem()
+	}
+	return typ
+}
+
+// unparen returns e with any enclosing parentheses stripped.
+func unparen(e ast.Expr) ast.Expr {
+	for {
+		p, ok := e.(*ast.ParenExpr)
+		if !ok {
+			break
+		}
+		e = p.X
+	}
+	return e
+}
+
+func isInterface(T types.Type) bool {
+	_, ok := T.Underlying().(*types.Interface)
+	return ok
+}