diff --git a/src/cmd/go.mod b/src/cmd/go.mod
index 21670b9..5c5c99e 100644
--- a/src/cmd/go.mod
+++ b/src/cmd/go.mod
@@ -7,7 +7,7 @@
 	github.com/ianlancetaylor/demangle v0.0.0-20200414190113-039b1ae3a340 // indirect
 	golang.org/x/arch v0.0.0-20200511175325-f7c78586839d
 	golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9
-	golang.org/x/mod v0.3.1-0.20200625141748-0b26df4a2231
+	golang.org/x/mod v0.3.1-0.20200824162228-c0d644d00ab8
 	golang.org/x/sys v0.0.0-20200501145240-bc7a7d42d5c3 // indirect
 	golang.org/x/tools v0.0.0-20200616133436-c1934b75d054
 	golang.org/x/xerrors v0.0.0-20200806184451-1a77d5e9f316 // indirect
diff --git a/src/cmd/go.sum b/src/cmd/go.sum
index 1b5ef51..69cebe1 100644
--- a/src/cmd/go.sum
+++ b/src/cmd/go.sum
@@ -14,8 +14,8 @@
 golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9 h1:psW17arqaxU48Z5kZ0CQnkZWQJsqcURM6tKiBApRjXI=
 golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
 golang.org/x/mod v0.2.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
-golang.org/x/mod v0.3.1-0.20200625141748-0b26df4a2231 h1:R11LxkoUvECaAHdM5/ZOevSR7n+016EgTw8nbE1l+XM=
-golang.org/x/mod v0.3.1-0.20200625141748-0b26df4a2231/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
+golang.org/x/mod v0.3.1-0.20200824162228-c0d644d00ab8 h1:Qbq3laTJZip3mEOreFwHF81RGkkhIvmraRMINHNyWHE=
+golang.org/x/mod v0.3.1-0.20200824162228-c0d644d00ab8/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
 golang.org/x/net v0.0.0-20200226121028-0de0cce0169b/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
@@ -32,7 +32,6 @@
 golang.org/x/tools v0.0.0-20200616133436-c1934b75d054/go.mod h1:EkVYQZoAsY45+roYkvgYkIh4xh/qjgUK9TdY2XT94GE=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
-golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20200806184451-1a77d5e9f316 h1:Jhw4VC65LaKnpq9FvcK+a8ZzrFm3D+UygvMMrhkOw70=
 golang.org/x/xerrors v0.0.0-20200806184451-1a77d5e9f316/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
diff --git a/src/cmd/vendor/golang.org/x/mod/modfile/read.go b/src/cmd/vendor/golang.org/x/mod/modfile/read.go
index c1f2008..2a961ca 100644
--- a/src/cmd/vendor/golang.org/x/mod/modfile/read.go
+++ b/src/cmd/vendor/golang.org/x/mod/modfile/read.go
@@ -477,9 +477,17 @@
 
 // endToken marks the end of an input token.
 // It records the actual token string in tok.text.
+// A single trailing newline (LF or CRLF) will be removed from comment tokens.
 func (in *input) endToken(kind tokenKind) {
 	in.token.kind = kind
 	text := string(in.tokenStart[:len(in.tokenStart)-len(in.remaining)])
+	if kind.isComment() {
+		if strings.HasSuffix(text, "\r\n") {
+			text = text[:len(text)-2]
+		} else {
+			text = strings.TrimSuffix(text, "\n")
+		}
+	}
 	in.token.text = text
 	in.token.endPos = in.pos
 }
diff --git a/src/cmd/vendor/golang.org/x/mod/modfile/rule.go b/src/cmd/vendor/golang.org/x/mod/modfile/rule.go
index 91ca682..83398dd 100644
--- a/src/cmd/vendor/golang.org/x/mod/modfile/rule.go
+++ b/src/cmd/vendor/golang.org/x/mod/modfile/rule.go
@@ -30,6 +30,7 @@
 
 	"golang.org/x/mod/internal/lazyregexp"
 	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
 )
 
 // A File is the parsed, interpreted form of a go.mod file.
@@ -39,6 +40,7 @@
 	Require []*Require
 	Exclude []*Exclude
 	Replace []*Replace
+	Retract []*Retract
 
 	Syntax *FileSyntax
 }
@@ -75,6 +77,21 @@
 	Syntax *Line
 }
 
+// A Retract is a single retract statement.
+type Retract struct {
+	VersionInterval
+	Rationale string
+	Syntax    *Line
+}
+
+// A VersionInterval represents a range of versions with upper and lower bounds.
+// Intervals are closed: both bounds are included. When Low is equal to High,
+// the interval may refer to a single version ('v1.2.3') or an interval
+// ('[v1.2.3, v1.2.3]'); both have the same representation.
+type VersionInterval struct {
+	Low, High string
+}
+
 func (f *File) AddModuleStmt(path string) error {
 	if f.Syntax == nil {
 		f.Syntax = new(FileSyntax)
@@ -138,7 +155,7 @@
 	for _, x := range fs.Stmt {
 		switch x := x.(type) {
 		case *Line:
-			f.add(&errs, x, x.Token[0], x.Token[1:], fix, strict)
+			f.add(&errs, nil, x, x.Token[0], x.Token[1:], fix, strict)
 
 		case *LineBlock:
 			if len(x.Token) > 1 {
@@ -161,9 +178,9 @@
 					})
 				}
 				continue
-			case "module", "require", "exclude", "replace":
+			case "module", "require", "exclude", "replace", "retract":
 				for _, l := range x.Line {
-					f.add(&errs, l, x.Token[0], l.Token, fix, strict)
+					f.add(&errs, x, l, x.Token[0], l.Token, fix, strict)
 				}
 			}
 		}
@@ -177,7 +194,7 @@
 
 var GoVersionRE = lazyregexp.New(`^([1-9][0-9]*)\.(0|[1-9][0-9]*)$`)
 
-func (f *File) add(errs *ErrorList, line *Line, verb string, args []string, fix VersionFixer, strict bool) {
+func (f *File) add(errs *ErrorList, block *LineBlock, line *Line, verb string, args []string, fix VersionFixer, strict bool) {
 	// If strict is false, this module is a dependency.
 	// We ignore all unknown directives as well as main-module-only
 	// directives like replace and exclude. It will work better for
@@ -186,7 +203,7 @@
 	// and simply ignore those statements.
 	if !strict {
 		switch verb {
-		case "module", "require", "go":
+		case "go", "module", "retract", "require":
 			// want these even for dependency go.mods
 		default:
 			return
@@ -232,6 +249,7 @@
 
 		f.Go = &Go{Syntax: line}
 		f.Go.Version = args[0]
+
 	case "module":
 		if f.Module != nil {
 			errorf("repeated module statement")
@@ -248,6 +266,7 @@
 			return
 		}
 		f.Module.Mod = module.Version{Path: s}
+
 	case "require", "exclude":
 		if len(args) != 2 {
 			errorf("usage: %s module/path v1.2.3", verb)
@@ -284,6 +303,7 @@
 				Syntax: line,
 			})
 		}
+
 	case "replace":
 		arrow := 2
 		if len(args) >= 2 && args[1] == "=>" {
@@ -347,6 +367,33 @@
 			New:    module.Version{Path: ns, Version: nv},
 			Syntax: line,
 		})
+
+	case "retract":
+		rationale := parseRetractRationale(block, line)
+		vi, err := parseVersionInterval(verb, &args, fix)
+		if err != nil {
+			if strict {
+				wrapError(err)
+				return
+			} else {
+				// Only report errors parsing intervals in the main module. We may
+				// support additional syntax in the future, such as open and half-open
+				// intervals. Those can't be supported now, because they break the
+				// go.mod parser, even in lax mode.
+				return
+			}
+		}
+		if len(args) > 0 && strict {
+			// In the future, there may be additional information after the version.
+			errorf("unexpected token after version: %q", args[0])
+			return
+		}
+		retract := &Retract{
+			VersionInterval: vi,
+			Rationale:       rationale,
+			Syntax:          line,
+		}
+		f.Retract = append(f.Retract, retract)
 	}
 }
 
@@ -444,6 +491,53 @@
 	return s
 }
 
+func parseVersionInterval(verb string, args *[]string, fix VersionFixer) (VersionInterval, error) {
+	toks := *args
+	if len(toks) == 0 || toks[0] == "(" {
+		return VersionInterval{}, fmt.Errorf("expected '[' or version")
+	}
+	if toks[0] != "[" {
+		v, err := parseVersion(verb, "", &toks[0], fix)
+		if err != nil {
+			return VersionInterval{}, err
+		}
+		*args = toks[1:]
+		return VersionInterval{Low: v, High: v}, nil
+	}
+	toks = toks[1:]
+
+	if len(toks) == 0 {
+		return VersionInterval{}, fmt.Errorf("expected version after '['")
+	}
+	low, err := parseVersion(verb, "", &toks[0], fix)
+	if err != nil {
+		return VersionInterval{}, err
+	}
+	toks = toks[1:]
+
+	if len(toks) == 0 || toks[0] != "," {
+		return VersionInterval{}, fmt.Errorf("expected ',' after version")
+	}
+	toks = toks[1:]
+
+	if len(toks) == 0 {
+		return VersionInterval{}, fmt.Errorf("expected version after ','")
+	}
+	high, err := parseVersion(verb, "", &toks[0], fix)
+	if err != nil {
+		return VersionInterval{}, err
+	}
+	toks = toks[1:]
+
+	if len(toks) == 0 || toks[0] != "]" {
+		return VersionInterval{}, fmt.Errorf("expected ']' after version")
+	}
+	toks = toks[1:]
+
+	*args = toks
+	return VersionInterval{Low: low, High: high}, nil
+}
+
 func parseString(s *string) (string, error) {
 	t := *s
 	if strings.HasPrefix(t, `"`) {
@@ -461,6 +555,27 @@
 	return t, nil
 }
 
+// parseRetractRationale extracts the rationale for a retract directive from the
+// surrounding comments. If the line does not have comments and is part of a
+// block that does have comments, the block's comments are used.
+func parseRetractRationale(block *LineBlock, line *Line) string {
+	comments := line.Comment()
+	if block != nil && len(comments.Before) == 0 && len(comments.Suffix) == 0 {
+		comments = block.Comment()
+	}
+	groups := [][]Comment{comments.Before, comments.Suffix}
+	var lines []string
+	for _, g := range groups {
+		for _, c := range g {
+			if !strings.HasPrefix(c.Token, "//") {
+				continue // blank line
+			}
+			lines = append(lines, strings.TrimSpace(strings.TrimPrefix(c.Token, "//")))
+		}
+	}
+	return strings.Join(lines, "\n")
+}
+
 type ErrorList []Error
 
 func (e ErrorList) Error() string {
@@ -494,6 +609,8 @@
 	var directive string
 	if e.ModPath != "" {
 		directive = fmt.Sprintf("%s %s: ", e.Verb, e.ModPath)
+	} else if e.Verb != "" {
+		directive = fmt.Sprintf("%s: ", e.Verb)
 	}
 
 	return pos + directive + e.Err.Error()
@@ -585,6 +702,15 @@
 	}
 	f.Replace = f.Replace[:w]
 
+	w = 0
+	for _, r := range f.Retract {
+		if r.Low != "" || r.High != "" {
+			f.Retract[w] = r
+			w++
+		}
+	}
+	f.Retract = f.Retract[:w]
+
 	f.Syntax.Cleanup()
 }
 
@@ -778,6 +904,34 @@
 	return nil
 }
 
+func (f *File) AddRetract(vi VersionInterval, rationale string) error {
+	r := &Retract{
+		VersionInterval: vi,
+	}
+	if vi.Low == vi.High {
+		r.Syntax = f.Syntax.addLine(nil, "retract", AutoQuote(vi.Low))
+	} else {
+		r.Syntax = f.Syntax.addLine(nil, "retract", "[", AutoQuote(vi.Low), ",", AutoQuote(vi.High), "]")
+	}
+	if rationale != "" {
+		for _, line := range strings.Split(rationale, "\n") {
+			com := Comment{Token: "// " + line}
+			r.Syntax.Comment().Before = append(r.Syntax.Comment().Before, com)
+		}
+	}
+	return nil
+}
+
+func (f *File) DropRetract(vi VersionInterval) error {
+	for _, r := range f.Retract {
+		if r.VersionInterval == vi {
+			f.Syntax.removeLine(r.Syntax)
+			*r = Retract{}
+		}
+	}
+	return nil
+}
+
 func (f *File) SortBlocks() {
 	f.removeDups() // otherwise sorting is unsafe
 
@@ -786,28 +940,38 @@
 		if !ok {
 			continue
 		}
-		sort.Slice(block.Line, func(i, j int) bool {
-			li := block.Line[i]
-			lj := block.Line[j]
-			for k := 0; k < len(li.Token) && k < len(lj.Token); k++ {
-				if li.Token[k] != lj.Token[k] {
-					return li.Token[k] < lj.Token[k]
-				}
-			}
-			return len(li.Token) < len(lj.Token)
+		less := lineLess
+		if block.Token[0] == "retract" {
+			less = lineRetractLess
+		}
+		sort.SliceStable(block.Line, func(i, j int) bool {
+			return less(block.Line[i], block.Line[j])
 		})
 	}
 }
 
+// removeDups removes duplicate exclude and replace directives.
+//
+// Earlier exclude directives take priority.
+//
+// Later replace directives take priority.
+//
+// require directives are not de-duplicated. That's left up to higher-level
+// logic (MVS).
+//
+// retract directives are not de-duplicated since comments are
+// meaningful, and versions may be retracted multiple times.
 func (f *File) removeDups() {
-	have := make(map[module.Version]bool)
 	kill := make(map[*Line]bool)
+
+	// Remove duplicate excludes.
+	haveExclude := make(map[module.Version]bool)
 	for _, x := range f.Exclude {
-		if have[x.Mod] {
+		if haveExclude[x.Mod] {
 			kill[x.Syntax] = true
 			continue
 		}
-		have[x.Mod] = true
+		haveExclude[x.Mod] = true
 	}
 	var excl []*Exclude
 	for _, x := range f.Exclude {
@@ -817,15 +981,16 @@
 	}
 	f.Exclude = excl
 
-	have = make(map[module.Version]bool)
+	// Remove duplicate replacements.
 	// Later replacements take priority over earlier ones.
+	haveReplace := make(map[module.Version]bool)
 	for i := len(f.Replace) - 1; i >= 0; i-- {
 		x := f.Replace[i]
-		if have[x.Old] {
+		if haveReplace[x.Old] {
 			kill[x.Syntax] = true
 			continue
 		}
-		have[x.Old] = true
+		haveReplace[x.Old] = true
 	}
 	var repl []*Replace
 	for _, x := range f.Replace {
@@ -835,6 +1000,9 @@
 	}
 	f.Replace = repl
 
+	// Duplicate require and retract directives are not removed.
+
+	// Drop killed statements from the syntax tree.
 	var stmts []Expr
 	for _, stmt := range f.Syntax.Stmt {
 		switch stmt := stmt.(type) {
@@ -858,3 +1026,38 @@
 	}
 	f.Syntax.Stmt = stmts
 }
+
+// lineLess returns whether li should be sorted before lj. It sorts
+// lexicographically without assigning any special meaning to tokens.
+func lineLess(li, lj *Line) bool {
+	for k := 0; k < len(li.Token) && k < len(lj.Token); k++ {
+		if li.Token[k] != lj.Token[k] {
+			return li.Token[k] < lj.Token[k]
+		}
+	}
+	return len(li.Token) < len(lj.Token)
+}
+
+// lineRetractLess returns whether li should be sorted before lj for lines in
+// a "retract" block. It treats each line as a version interval. Single versions
+// are compared as if they were intervals with the same low and high version.
+// Intervals are sorted in descending order, first by low version, then by
+// high version, using semver.Compare.
+func lineRetractLess(li, lj *Line) bool {
+	interval := func(l *Line) VersionInterval {
+		if len(l.Token) == 1 {
+			return VersionInterval{Low: l.Token[0], High: l.Token[0]}
+		} else if len(l.Token) == 5 && l.Token[0] == "[" && l.Token[2] == "," && l.Token[4] == "]" {
+			return VersionInterval{Low: l.Token[1], High: l.Token[3]}
+		} else {
+			// Line in unknown format. Treat as an invalid version.
+			return VersionInterval{}
+		}
+	}
+	vii := interval(li)
+	vij := interval(lj)
+	if cmp := semver.Compare(vii.Low, vij.Low); cmp != 0 {
+		return cmp > 0
+	}
+	return semver.Compare(vii.High, vij.High) > 0
+}
diff --git a/src/cmd/vendor/modules.txt b/src/cmd/vendor/modules.txt
index 7272f04..ab2f81a 100644
--- a/src/cmd/vendor/modules.txt
+++ b/src/cmd/vendor/modules.txt
@@ -29,7 +29,7 @@
 golang.org/x/crypto/ed25519
 golang.org/x/crypto/ed25519/internal/edwards25519
 golang.org/x/crypto/ssh/terminal
-# golang.org/x/mod v0.3.1-0.20200625141748-0b26df4a2231
+# golang.org/x/mod v0.3.1-0.20200824162228-c0d644d00ab8
 ## explicit
 golang.org/x/mod/internal/lazyregexp
 golang.org/x/mod/modfile