go/internal/gcimporter/iexport_test.go - tools - Git at Google

 // Copyright 2019 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.

 // This is a copy of bexport_test.go for iexport.go.

 //go:build go1.11
 // +build go1.11

 package gcimporter_test

 import (
 	"bufio"
 	"bytes"
 	"fmt"
 	"go/ast"
 	"go/build"
 	"go/constant"
 	"go/parser"
 	"go/token"
 	"go/types"
 	"io/ioutil"
 	"math/big"
 	"os"
 	"reflect"
 	"runtime"
 	"sort"
 	"strings"
 	"testing"

 	"golang.org/x/tools/go/buildutil"
 	"golang.org/x/tools/go/internal/gcimporter"
 	"golang.org/x/tools/go/loader"
 	"golang.org/x/tools/internal/testenv"
 )

 func readExportFile(filename string) ([]byte, error) {
 	f, err := os.Open(filename)
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()

 	buf := bufio.NewReader(f)
 	if _, err := gcimporter.FindExportData(buf); err != nil {
 		return nil, err
 	}

 	if ch, err := buf.ReadByte(); err != nil {
 		return nil, err
 	} else if ch != 'i' {
 		return nil, fmt.Errorf("unexpected byte: %v", ch)
 	}

 	return ioutil.ReadAll(buf)
 }

 func iexport(fset *token.FileSet, pkg *types.Package) ([]byte, error) {
 	var buf bytes.Buffer
 	if err := gcimporter.IExportData(&buf, fset, pkg); err != nil {
 		return nil, err
 	}
 	return buf.Bytes(), nil
 }

 // isUnifiedBuilder reports whether we are executing on a go builder that uses
 // unified export data.
 func isUnifiedBuilder() bool {
 	return os.Getenv("GO_BUILDER_NAME") == "linux-amd64-unified"
 }

 func TestIExportData_stdlib(t *testing.T) {
 	if runtime.Compiler == "gccgo" {
 		t.Skip("gccgo standard library is inaccessible")
 	}
 	if runtime.GOOS == "android" {
 		t.Skipf("incomplete std lib on %s", runtime.GOOS)
 	}
 	if isRace {
 		t.Skipf("stdlib tests take too long in race mode and flake on builders")
 	}

 	// Load, parse and type-check the program.
 	ctxt := build.Default // copy
 	ctxt.GOPATH = ""      // disable GOPATH
 	conf := loader.Config{
 		Build:       &ctxt,
 		AllowErrors: true,
 		TypeChecker: types.Config{
 			Sizes: types.SizesFor(ctxt.Compiler, ctxt.GOARCH),
 		},
 	}
 	// Temporarily skip packages that use generics on the unified builder, to fix
 	// TryBots.
 	//
 	// TODO(#48595): fix this test with GOEXPERIMENT=unified.
 	isUnified := isUnifiedBuilder()
 	for _, path := range buildutil.AllPackages(conf.Build) {
 		if !(isUnified && testenv.UsesGenerics(path)) {
 			conf.Import(path)
 		}
 	}

 	// Create a package containing type and value errors to ensure
 	// they are properly encoded/decoded.
 	f, err := conf.ParseFile("haserrors/haserrors.go", `package haserrors
 const UnknownValue = "" + 0
 type UnknownType undefined
 `)
 	if err != nil {
 		t.Fatal(err)
 	}
 	conf.CreateFromFiles("haserrors", f)

 	prog, err := conf.Load()
 	if err != nil {
 		t.Fatalf("Load failed: %v", err)
 	}

 	numPkgs := len(prog.AllPackages)
 	if want := 248; numPkgs < want {
 		t.Errorf("Loaded only %d packages, want at least %d", numPkgs, want)
 	}

 	var sorted []*types.Package
 	for pkg, info := range prog.AllPackages {
 		if info.Files != nil { // non-empty directory
 			sorted = append(sorted, pkg)
 		}
 	}
 	sort.Slice(sorted, func(i, j int) bool {
 		return sorted[i].Path() < sorted[j].Path()
 	})

 	for _, pkg := range sorted {
 		if exportdata, err := iexport(conf.Fset, pkg); err != nil {
 			t.Error(err)
 		} else {
 			testPkgData(t, conf.Fset, pkg, exportdata)
 		}

 		if pkg.Name() == "main" || pkg.Name() == "haserrors" {
 			// skip; no export data
 		} else if bp, err := ctxt.Import(pkg.Path(), "", build.FindOnly); err != nil {
 			t.Log("warning:", err)
 		} else if exportdata, err := readExportFile(bp.PkgObj); err != nil {
 			t.Log("warning:", err)
 		} else {
 			testPkgData(t, conf.Fset, pkg, exportdata)
 		}
 	}

 	var bundle bytes.Buffer
 	if err := gcimporter.IExportBundle(&bundle, conf.Fset, sorted); err != nil {
 		t.Fatal(err)
 	}
 	fset2 := token.NewFileSet()
 	imports := make(map[string]*types.Package)
 	pkgs2, err := gcimporter.IImportBundle(fset2, imports, bundle.Bytes())
 	if err != nil {
 		t.Fatal(err)
 	}

 	for i, pkg := range sorted {
 		testPkg(t, conf.Fset, pkg, fset2, pkgs2[i])
 	}
 }

 func testPkgData(t *testing.T, fset *token.FileSet, pkg *types.Package, exportdata []byte) {
 	imports := make(map[string]*types.Package)
 	fset2 := token.NewFileSet()
 	_, pkg2, err := gcimporter.IImportData(fset2, imports, exportdata, pkg.Path())
 	if err != nil {
 		t.Errorf("IImportData(%s): %v", pkg.Path(), err)
 	}

 	testPkg(t, fset, pkg, fset2, pkg2)
 }

 func testPkg(t *testing.T, fset *token.FileSet, pkg *types.Package, fset2 *token.FileSet, pkg2 *types.Package) {
 	if _, err := iexport(fset2, pkg2); err != nil {
 		t.Errorf("reexport %q: %v", pkg.Path(), err)
 	}

 	// Compare the packages' corresponding members.
 	for _, name := range pkg.Scope().Names() {
 		if !ast.IsExported(name) {
 			continue
 		}
 		obj1 := pkg.Scope().Lookup(name)
 		obj2 := pkg2.Scope().Lookup(name)
 		if obj2 == nil {
 			t.Errorf("%s.%s not found, want %s", pkg.Path(), name, obj1)
 			continue
 		}

 		fl1 := fileLine(fset, obj1)
 		fl2 := fileLine(fset2, obj2)
 		if fl1 != fl2 {
 			t.Errorf("%s.%s: got posn %s, want %s",
 				pkg.Path(), name, fl2, fl1)
 		}

 		if err := cmpObj(obj1, obj2); err != nil {
 			t.Errorf("%s.%s: %s\ngot:  %s\nwant: %s",
 				pkg.Path(), name, err, obj2, obj1)
 		}
 	}
 }

 // TestVeryLongFile tests the position of an import object declared in
 // a very long input file.  Line numbers greater than maxlines are
 // reported as line 1, not garbage or token.NoPos.
 func TestIExportData_long(t *testing.T) {
 	// parse and typecheck
 	longFile := "package foo" + strings.Repeat("\n", 123456) + "var X int"
 	fset1 := token.NewFileSet()
 	f, err := parser.ParseFile(fset1, "foo.go", longFile, 0)
 	if err != nil {
 		t.Fatal(err)
 	}
 	var conf types.Config
 	pkg, err := conf.Check("foo", fset1, []*ast.File{f}, nil)
 	if err != nil {
 		t.Fatal(err)
 	}

 	// export
 	exportdata, err := iexport(fset1, pkg)
 	if err != nil {
 		t.Fatal(err)
 	}

 	// import
 	imports := make(map[string]*types.Package)
 	fset2 := token.NewFileSet()
 	_, pkg2, err := gcimporter.IImportData(fset2, imports, exportdata, pkg.Path())
 	if err != nil {
 		t.Fatalf("IImportData(%s): %v", pkg.Path(), err)
 	}

 	// compare
 	posn1 := fset1.Position(pkg.Scope().Lookup("X").Pos())
 	posn2 := fset2.Position(pkg2.Scope().Lookup("X").Pos())
 	if want := "foo.go:1:1"; posn2.String() != want {
 		t.Errorf("X position = %s, want %s (orig was %s)",
 			posn2, want, posn1)
 	}
 }

 func TestIExportData_typealiases(t *testing.T) {
 	// parse and typecheck
 	fset1 := token.NewFileSet()
 	f, err := parser.ParseFile(fset1, "p.go", src, 0)
 	if err != nil {
 		t.Fatal(err)
 	}
 	var conf types.Config
 	pkg1, err := conf.Check("p", fset1, []*ast.File{f}, nil)
 	if err == nil {
 		// foo in undeclared in src; we should see an error
 		t.Fatal("invalid source type-checked without error")
 	}
 	if pkg1 == nil {
 		// despite incorrect src we should see a (partially) type-checked package
 		t.Fatal("nil package returned")
 	}
 	checkPkg(t, pkg1, "export")

 	// export
 	// use a nil fileset here to confirm that it doesn't panic
 	exportdata, err := iexport(nil, pkg1)
 	if err != nil {
 		t.Fatal(err)
 	}

 	// import
 	imports := make(map[string]*types.Package)
 	fset2 := token.NewFileSet()
 	_, pkg2, err := gcimporter.IImportData(fset2, imports, exportdata, pkg1.Path())
 	if err != nil {
 		t.Fatalf("IImportData(%s): %v", pkg1.Path(), err)
 	}
 	checkPkg(t, pkg2, "import")
 }

 // cmpObj reports how x and y differ. They are assumed to belong to different
 // universes so cannot be compared directly. It is an adapted version of
 // equalObj in bexport_test.go.
 func cmpObj(x, y types.Object) error {
 	if reflect.TypeOf(x) != reflect.TypeOf(y) {
 		return fmt.Errorf("%T vs %T", x, y)
 	}
 	xt := x.Type()
 	yt := y.Type()
 	switch x.(type) {
 	case *types.Var, *types.Func:
 		// ok
 	case *types.Const:
 		xval := x.(*types.Const).Val()
 		yval := y.(*types.Const).Val()
 		equal := constant.Compare(xval, token.EQL, yval)
 		if !equal {
 			// try approx. comparison
 			xkind := xval.Kind()
 			ykind := yval.Kind()
 			if xkind == constant.Complex || ykind == constant.Complex {
 				equal = same(constant.Real(xval), constant.Real(yval)) &&
 					same(constant.Imag(xval), constant.Imag(yval))
 			} else if xkind == constant.Float || ykind == constant.Float {
 				equal = same(xval, yval)
 			} else if xkind == constant.Unknown && ykind == constant.Unknown {
 				equal = true
 			}
 		}
 		if !equal {
 			return fmt.Errorf("unequal constants %s vs %s", xval, yval)
 		}
 	case *types.TypeName:
 		xt = xt.Underlying()
 		yt = yt.Underlying()
 	default:
 		return fmt.Errorf("unexpected %T", x)
 	}
 	return equalType(xt, yt)
 }

 // Use the same floating-point precision (512) as cmd/compile
 // (see Mpprec in cmd/compile/internal/gc/mpfloat.go).
 const mpprec = 512

 // same compares non-complex numeric values and reports if they are approximately equal.
 func same(x, y constant.Value) bool {
 	xf := constantToFloat(x)
 	yf := constantToFloat(y)
 	d := new(big.Float).Sub(xf, yf)
 	d.Abs(d)
 	eps := big.NewFloat(1.0 / (1 << (mpprec - 1))) // allow for 1 bit of error
 	return d.Cmp(eps) < 0
 }

 // copy of the function with the same name in iexport.go.
 func constantToFloat(x constant.Value) *big.Float {
 	var f big.Float
 	f.SetPrec(mpprec)
 	if v, exact := constant.Float64Val(x); exact {
 		// float64
 		f.SetFloat64(v)
 	} else if num, denom := constant.Num(x), constant.Denom(x); num.Kind() == constant.Int {
 		// TODO(gri): add big.Rat accessor to constant.Value.
 		n := valueToRat(num)
 		d := valueToRat(denom)
 		f.SetRat(n.Quo(n, d))
 	} else {
 		// Value too large to represent as a fraction => inaccessible.
 		// TODO(gri): add big.Float accessor to constant.Value.
 		_, ok := f.SetString(x.ExactString())
 		if !ok {
 			panic("should not reach here")
 		}
 	}
 	return &f
 }

 // copy of the function with the same name in iexport.go.
 func valueToRat(x constant.Value) *big.Rat {
 	// Convert little-endian to big-endian.
 	// I can't believe this is necessary.
 	bytes := constant.Bytes(x)
 	for i := 0; i < len(bytes)/2; i++ {
 		bytes[i], bytes[len(bytes)-1-i] = bytes[len(bytes)-1-i], bytes[i]
 	}
 	return new(big.Rat).SetInt(new(big.Int).SetBytes(bytes))
 }
	// Copyright 2019 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.

	// This is a copy of bexport_test.go for iexport.go.

	//go:build go1.11
	// +build go1.11

	package gcimporter_test

	import (
	"bufio"
	"bytes"
	"fmt"
	"go/ast"
	"go/build"
	"go/constant"
	"go/parser"
	"go/token"
	"go/types"
	"io/ioutil"
	"math/big"
	"os"
	"reflect"
	"runtime"
	"sort"
	"strings"
	"testing"

	"golang.org/x/tools/go/buildutil"
	"golang.org/x/tools/go/internal/gcimporter"
	"golang.org/x/tools/go/loader"
	"golang.org/x/tools/internal/testenv"
	)

	func readExportFile(filename string) ([]byte, error) {
	f, err := os.Open(filename)
	if err != nil {
	return nil, err
	}
	defer f.Close()

	buf := bufio.NewReader(f)
	if _, err := gcimporter.FindExportData(buf); err != nil {
	return nil, err
	}

	if ch, err := buf.ReadByte(); err != nil {
	return nil, err
	} else if ch != 'i' {
	return nil, fmt.Errorf("unexpected byte: %v", ch)
	}

	return ioutil.ReadAll(buf)
	}

	func iexport(fset token.FileSet, pkg types.Package) ([]byte, error) {
	var buf bytes.Buffer
	if err := gcimporter.IExportData(&buf, fset, pkg); err != nil {
	return nil, err
	}
	return buf.Bytes(), nil
	}

	// isUnifiedBuilder reports whether we are executing on a go builder that uses
	// unified export data.
	func isUnifiedBuilder() bool {
	return os.Getenv("GO_BUILDER_NAME") == "linux-amd64-unified"
	}

	func TestIExportData_stdlib(t *testing.T) {
	if runtime.Compiler == "gccgo" {
	t.Skip("gccgo standard library is inaccessible")
	}
	if runtime.GOOS == "android" {
	t.Skipf("incomplete std lib on %s", runtime.GOOS)
	}
	if isRace {
	t.Skipf("stdlib tests take too long in race mode and flake on builders")
	}

	// Load, parse and type-check the program.
	ctxt := build.Default // copy
	ctxt.GOPATH = "" // disable GOPATH
	conf := loader.Config{
	Build: &ctxt,
	AllowErrors: true,
	TypeChecker: types.Config{
	Sizes: types.SizesFor(ctxt.Compiler, ctxt.GOARCH),
	},
	}
	// Temporarily skip packages that use generics on the unified builder, to fix
	// TryBots.
	//
	// TODO(#48595): fix this test with GOEXPERIMENT=unified.
	isUnified := isUnifiedBuilder()
	for _, path := range buildutil.AllPackages(conf.Build) {
	if !(isUnified && testenv.UsesGenerics(path)) {
	conf.Import(path)
	}
	}

	// Create a package containing type and value errors to ensure
	// they are properly encoded/decoded.
	f, err := conf.ParseFile("haserrors/haserrors.go", `package haserrors
	const UnknownValue = "" + 0
	type UnknownType undefined
	`)
	if err != nil {
	t.Fatal(err)
	}
	conf.CreateFromFiles("haserrors", f)

	prog, err := conf.Load()
	if err != nil {
	t.Fatalf("Load failed: %v", err)
	}

	numPkgs := len(prog.AllPackages)
	if want := 248; numPkgs < want {
	t.Errorf("Loaded only %d packages, want at least %d", numPkgs, want)
	}

	var sorted []*types.Package
	for pkg, info := range prog.AllPackages {
	if info.Files != nil { // non-empty directory
	sorted = append(sorted, pkg)
	}
	}
	sort.Slice(sorted, func(i, j int) bool {
	return sorted[i].Path() < sorted[j].Path()
	})

	for _, pkg := range sorted {
	if exportdata, err := iexport(conf.Fset, pkg); err != nil {
	t.Error(err)
	} else {
	testPkgData(t, conf.Fset, pkg, exportdata)
	}

	if pkg.Name() == "main" \|\| pkg.Name() == "haserrors" {
	// skip; no export data
	} else if bp, err := ctxt.Import(pkg.Path(), "", build.FindOnly); err != nil {
	t.Log("warning:", err)
	} else if exportdata, err := readExportFile(bp.PkgObj); err != nil {
	t.Log("warning:", err)
	} else {
	testPkgData(t, conf.Fset, pkg, exportdata)
	}
	}

	var bundle bytes.Buffer
	if err := gcimporter.IExportBundle(&bundle, conf.Fset, sorted); err != nil {
	t.Fatal(err)
	}
	fset2 := token.NewFileSet()
	imports := make(map[string]*types.Package)
	pkgs2, err := gcimporter.IImportBundle(fset2, imports, bundle.Bytes())
	if err != nil {
	t.Fatal(err)
	}

	for i, pkg := range sorted {
	testPkg(t, conf.Fset, pkg, fset2, pkgs2[i])
	}
	}

	func testPkgData(t testing.T, fset token.FileSet, pkg *types.Package, exportdata []byte) {
	imports := make(map[string]*types.Package)
	fset2 := token.NewFileSet()
	_, pkg2, err := gcimporter.IImportData(fset2, imports, exportdata, pkg.Path())
	if err != nil {
	t.Errorf("IImportData(%s): %v", pkg.Path(), err)
	}

	testPkg(t, fset, pkg, fset2, pkg2)
	}

	func testPkg(t testing.T, fset token.FileSet, pkg types.Package, fset2 token.FileSet, pkg2 *types.Package) {
	if _, err := iexport(fset2, pkg2); err != nil {
	t.Errorf("reexport %q: %v", pkg.Path(), err)
	}

	// Compare the packages' corresponding members.
	for _, name := range pkg.Scope().Names() {
	if !ast.IsExported(name) {
	continue
	}
	obj1 := pkg.Scope().Lookup(name)
	obj2 := pkg2.Scope().Lookup(name)
	if obj2 == nil {
	t.Errorf("%s.%s not found, want %s", pkg.Path(), name, obj1)
	continue
	}

	fl1 := fileLine(fset, obj1)
	fl2 := fileLine(fset2, obj2)
	if fl1 != fl2 {
	t.Errorf("%s.%s: got posn %s, want %s",
	pkg.Path(), name, fl2, fl1)
	}

	if err := cmpObj(obj1, obj2); err != nil {
	t.Errorf("%s.%s: %s\ngot: %s\nwant: %s",
	pkg.Path(), name, err, obj2, obj1)
	}
	}
	}

	// TestVeryLongFile tests the position of an import object declared in
	// a very long input file. Line numbers greater than maxlines are
	// reported as line 1, not garbage or token.NoPos.
	func TestIExportData_long(t *testing.T) {
	// parse and typecheck
	longFile := "package foo" + strings.Repeat("\n", 123456) + "var X int"
	fset1 := token.NewFileSet()
	f, err := parser.ParseFile(fset1, "foo.go", longFile, 0)
	if err != nil {
	t.Fatal(err)
	}
	var conf types.Config
	pkg, err := conf.Check("foo", fset1, []*ast.File{f}, nil)
	if err != nil {
	t.Fatal(err)
	}

	// export
	exportdata, err := iexport(fset1, pkg)
	if err != nil {
	t.Fatal(err)
	}

	// import
	imports := make(map[string]*types.Package)
	fset2 := token.NewFileSet()
	_, pkg2, err := gcimporter.IImportData(fset2, imports, exportdata, pkg.Path())
	if err != nil {
	t.Fatalf("IImportData(%s): %v", pkg.Path(), err)
	}

	// compare
	posn1 := fset1.Position(pkg.Scope().Lookup("X").Pos())
	posn2 := fset2.Position(pkg2.Scope().Lookup("X").Pos())
	if want := "foo.go:1:1"; posn2.String() != want {
	t.Errorf("X position = %s, want %s (orig was %s)",
	posn2, want, posn1)
	}
	}

	func TestIExportData_typealiases(t *testing.T) {
	// parse and typecheck
	fset1 := token.NewFileSet()
	f, err := parser.ParseFile(fset1, "p.go", src, 0)
	if err != nil {
	t.Fatal(err)
	}
	var conf types.Config
	pkg1, err := conf.Check("p", fset1, []*ast.File{f}, nil)
	if err == nil {
	// foo in undeclared in src; we should see an error
	t.Fatal("invalid source type-checked without error")
	}
	if pkg1 == nil {
	// despite incorrect src we should see a (partially) type-checked package
	t.Fatal("nil package returned")
	}
	checkPkg(t, pkg1, "export")

	// export
	// use a nil fileset here to confirm that it doesn't panic
	exportdata, err := iexport(nil, pkg1)
	if err != nil {
	t.Fatal(err)
	}

	// import
	imports := make(map[string]*types.Package)
	fset2 := token.NewFileSet()
	_, pkg2, err := gcimporter.IImportData(fset2, imports, exportdata, pkg1.Path())
	if err != nil {
	t.Fatalf("IImportData(%s): %v", pkg1.Path(), err)
	}
	checkPkg(t, pkg2, "import")
	}

	// cmpObj reports how x and y differ. They are assumed to belong to different
	// universes so cannot be compared directly. It is an adapted version of
	// equalObj in bexport_test.go.
	func cmpObj(x, y types.Object) error {
	if reflect.TypeOf(x) != reflect.TypeOf(y) {
	return fmt.Errorf("%T vs %T", x, y)
	}
	xt := x.Type()
	yt := y.Type()
	switch x.(type) {
	case types.Var, types.Func:
	// ok
	case *types.Const:
	xval := x.(*types.Const).Val()
	yval := y.(*types.Const).Val()
	equal := constant.Compare(xval, token.EQL, yval)
	if !equal {
	// try approx. comparison
	xkind := xval.Kind()
	ykind := yval.Kind()
	if xkind == constant.Complex \|\| ykind == constant.Complex {
	equal = same(constant.Real(xval), constant.Real(yval)) &&
	same(constant.Imag(xval), constant.Imag(yval))
	} else if xkind == constant.Float \|\| ykind == constant.Float {
	equal = same(xval, yval)
	} else if xkind == constant.Unknown && ykind == constant.Unknown {
	equal = true
	}
	}
	if !equal {
	return fmt.Errorf("unequal constants %s vs %s", xval, yval)
	}
	case *types.TypeName:
	xt = xt.Underlying()
	yt = yt.Underlying()
	default:
	return fmt.Errorf("unexpected %T", x)
	}
	return equalType(xt, yt)
	}

	// Use the same floating-point precision (512) as cmd/compile
	// (see Mpprec in cmd/compile/internal/gc/mpfloat.go).
	const mpprec = 512

	// same compares non-complex numeric values and reports if they are approximately equal.
	func same(x, y constant.Value) bool {
	xf := constantToFloat(x)
	yf := constantToFloat(y)
	d := new(big.Float).Sub(xf, yf)
	d.Abs(d)
	eps := big.NewFloat(1.0 / (1 << (mpprec - 1))) // allow for 1 bit of error
	return d.Cmp(eps) < 0
	}

	// copy of the function with the same name in iexport.go.
	func constantToFloat(x constant.Value) *big.Float {
	var f big.Float
	f.SetPrec(mpprec)
	if v, exact := constant.Float64Val(x); exact {
	// float64
	f.SetFloat64(v)
	} else if num, denom := constant.Num(x), constant.Denom(x); num.Kind() == constant.Int {
	// TODO(gri): add big.Rat accessor to constant.Value.
	n := valueToRat(num)
	d := valueToRat(denom)
	f.SetRat(n.Quo(n, d))
	} else {
	// Value too large to represent as a fraction => inaccessible.
	// TODO(gri): add big.Float accessor to constant.Value.
	_, ok := f.SetString(x.ExactString())
	if !ok {
	panic("should not reach here")
	}
	}
	return &f
	}

	// copy of the function with the same name in iexport.go.
	func valueToRat(x constant.Value) *big.Rat {
	// Convert little-endian to big-endian.
	// I can't believe this is necessary.
	bytes := constant.Bytes(x)
	for i := 0; i < len(bytes)/2; i++ {
	bytes[i], bytes[len(bytes)-1-i] = bytes[len(bytes)-1-i], bytes[i]
	}
	return new(big.Rat).SetInt(new(big.Int).SetBytes(bytes))
	}