diff --git a/src/cmd/cgo/gcc.go b/src/cmd/cgo/gcc.go
index c78197896c3..997a830994f 100644
--- a/src/cmd/cgo/gcc.go
+++ b/src/cmd/cgo/gcc.go
@@ -29,7 +29,7 @@ import (
 	"unicode"
 	"unicode/utf8"
 
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 )
 
 var debugDefine = flag.Bool("debug-define", false, "print relevant #defines")
@@ -1568,7 +1568,7 @@ func checkGCCBaseCmd() ([]string, error) {
 	if value == "" {
 		value = defaultCC(goos, goarch)
 	}
-	args, err := str.SplitQuotedFields(value)
+	args, err := quoted.Split(value)
 	if err != nil {
 		return nil, err
 	}
diff --git a/src/cmd/compile/internal/ssa/stmtlines_test.go b/src/cmd/compile/internal/ssa/stmtlines_test.go
index 843db8c07ef..90dd261c557 100644
--- a/src/cmd/compile/internal/ssa/stmtlines_test.go
+++ b/src/cmd/compile/internal/ssa/stmtlines_test.go
@@ -2,7 +2,7 @@ package ssa_test
 
 import (
 	cmddwarf "cmd/internal/dwarf"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 	"debug/dwarf"
 	"debug/elf"
 	"debug/macho"
@@ -58,7 +58,7 @@ func TestStmtLines(t *testing.T) {
 		if extld == "" {
 			extld = "gcc"
 		}
-		extldArgs, err := str.SplitQuotedFields(extld)
+		extldArgs, err := quoted.Split(extld)
 		if err != nil {
 			t.Fatal(err)
 		}
diff --git a/src/cmd/dist/buildtool.go b/src/cmd/dist/buildtool.go
index 320c62f8505..75f04a975c9 100644
--- a/src/cmd/dist/buildtool.go
+++ b/src/cmd/dist/buildtool.go
@@ -46,8 +46,8 @@ var bootstrapDirs = []string{
 	"cmd/internal/obj/...",
 	"cmd/internal/objabi",
 	"cmd/internal/pkgpath",
+	"cmd/internal/quoted",
 	"cmd/internal/src",
-	"cmd/internal/str",
 	"cmd/internal/sys",
 	"cmd/link",
 	"cmd/link/internal/...",
diff --git a/src/cmd/go/internal/base/base.go b/src/cmd/go/internal/base/base.go
index 0144525e307..954ce47a989 100644
--- a/src/cmd/go/internal/base/base.go
+++ b/src/cmd/go/internal/base/base.go
@@ -17,7 +17,7 @@ import (
 	"sync"
 
 	"cmd/go/internal/cfg"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 // A Command is an implementation of a go command
diff --git a/src/cmd/go/internal/base/flag.go b/src/cmd/go/internal/base/flag.go
index 7e5121bffbe..2c72c7e562b 100644
--- a/src/cmd/go/internal/base/flag.go
+++ b/src/cmd/go/internal/base/flag.go
@@ -9,7 +9,7 @@ import (
 
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 )
 
 // A StringsFlag is a command-line flag that interprets its argument
@@ -18,7 +18,7 @@ type StringsFlag []string
 
 func (v *StringsFlag) Set(s string) error {
 	var err error
-	*v, err = str.SplitQuotedFields(s)
+	*v, err = quoted.Split(s)
 	if *v == nil {
 		*v = []string{}
 	}
diff --git a/src/cmd/go/internal/envcmd/env.go b/src/cmd/go/internal/envcmd/env.go
index 181d2a2ca1f..e56dd8223f0 100644
--- a/src/cmd/go/internal/envcmd/env.go
+++ b/src/cmd/go/internal/envcmd/env.go
@@ -26,7 +26,7 @@ import (
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/work"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 )
 
 var CmdEnv = &base.Command{
@@ -470,7 +470,7 @@ func checkEnvWrite(key, val string) error {
 		if val == "" {
 			break
 		}
-		args, err := str.SplitQuotedFields(val)
+		args, err := quoted.Split(val)
 		if err != nil {
 			return fmt.Errorf("invalid %s: %v", key, err)
 		}
diff --git a/src/cmd/go/internal/fix/fix.go b/src/cmd/go/internal/fix/fix.go
index cc5940fccd8..988d45e71cc 100644
--- a/src/cmd/go/internal/fix/fix.go
+++ b/src/cmd/go/internal/fix/fix.go
@@ -10,7 +10,7 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 	"context"
 	"fmt"
 	"os"
diff --git a/src/cmd/go/internal/generate/generate.go b/src/cmd/go/internal/generate/generate.go
index 5981e5ecdbe..a3873d11387 100644
--- a/src/cmd/go/internal/generate/generate.go
+++ b/src/cmd/go/internal/generate/generate.go
@@ -26,7 +26,7 @@ import (
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/work"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 var CmdGenerate = &base.Command{
diff --git a/src/cmd/go/internal/get/get.go b/src/cmd/go/internal/get/get.go
index 0412506b9e8..f46313dcff6 100644
--- a/src/cmd/go/internal/get/get.go
+++ b/src/cmd/go/internal/get/get.go
@@ -20,7 +20,7 @@ import (
 	"cmd/go/internal/vcs"
 	"cmd/go/internal/web"
 	"cmd/go/internal/work"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 )
diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
index 821e622abbe..8c85ddcf211 100644
--- a/src/cmd/go/internal/list/list.go
+++ b/src/cmd/go/internal/list/list.go
@@ -24,7 +24,7 @@ import (
 	"cmd/go/internal/modinfo"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/work"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 var CmdList = &base.Command{
diff --git a/src/cmd/go/internal/load/flag.go b/src/cmd/go/internal/load/flag.go
index d0d5716c3fc..de079decdf2 100644
--- a/src/cmd/go/internal/load/flag.go
+++ b/src/cmd/go/internal/load/flag.go
@@ -6,7 +6,7 @@ package load
 
 import (
 	"cmd/go/internal/base"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 	"fmt"
 	"strings"
 )
@@ -63,7 +63,7 @@ func (f *PerPackageFlag) set(v, cwd string) error {
 		match = MatchPackage(pattern, cwd)
 		v = v[i+1:]
 	}
-	flags, err := str.SplitQuotedFields(v)
+	flags, err := quoted.Split(v)
 	if err != nil {
 		return err
 	}
diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
index dfe7849516a..c6c5fb00a8d 100644
--- a/src/cmd/go/internal/load/pkg.go
+++ b/src/cmd/go/internal/load/pkg.go
@@ -38,9 +38,9 @@ import (
 	"cmd/go/internal/modload"
 	"cmd/go/internal/par"
 	"cmd/go/internal/search"
+	"cmd/go/internal/str"
 	"cmd/go/internal/trace"
 	"cmd/go/internal/vcs"
-	"cmd/internal/str"
 	"cmd/internal/sys"
 
 	"golang.org/x/mod/modfile"
diff --git a/src/cmd/go/internal/load/test.go b/src/cmd/go/internal/load/test.go
index 4cefb62d51a..8a18dfbe931 100644
--- a/src/cmd/go/internal/load/test.go
+++ b/src/cmd/go/internal/load/test.go
@@ -23,7 +23,7 @@ import (
 
 	"cmd/go/internal/fsys"
 	"cmd/go/internal/trace"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 var TestMainDeps = []string{
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index 57189b4607f..484e095cc76 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -24,7 +24,7 @@ import (
 	"cmd/go/internal/imports"
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
diff --git a/src/cmd/go/internal/modfetch/codehost/codehost.go b/src/cmd/go/internal/modfetch/codehost/codehost.go
index efb4b1516a2..378fbae34f9 100644
--- a/src/cmd/go/internal/modfetch/codehost/codehost.go
+++ b/src/cmd/go/internal/modfetch/codehost/codehost.go
@@ -21,7 +21,7 @@ import (
 
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/lockedfile"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 // Downloaded size limits.
diff --git a/src/cmd/go/internal/modfetch/codehost/vcs.go b/src/cmd/go/internal/modfetch/codehost/vcs.go
index 5d810d2621c..c2cca084e30 100644
--- a/src/cmd/go/internal/modfetch/codehost/vcs.go
+++ b/src/cmd/go/internal/modfetch/codehost/vcs.go
@@ -20,7 +20,7 @@ import (
 
 	"cmd/go/internal/lockedfile"
 	"cmd/go/internal/par"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 // A VCSError indicates an error using a version control system.
diff --git a/src/cmd/go/internal/modget/query.go b/src/cmd/go/internal/modget/query.go
index d7341e7813d..887cb51b317 100644
--- a/src/cmd/go/internal/modget/query.go
+++ b/src/cmd/go/internal/modget/query.go
@@ -14,7 +14,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/search"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 )
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 0f5b0150003..845bf2f8a23 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -119,7 +119,7 @@ import (
 	"cmd/go/internal/mvs"
 	"cmd/go/internal/par"
 	"cmd/go/internal/search"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
index c9ed129dbfa..1eb484de9d0 100644
--- a/src/cmd/go/internal/modload/query.go
+++ b/src/cmd/go/internal/modload/query.go
@@ -22,7 +22,7 @@ import (
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/search"
 	"cmd/go/internal/trace"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
diff --git a/src/cmd/go/internal/run/run.go b/src/cmd/go/internal/run/run.go
index 11e2c81b9aa..03895d27ebf 100644
--- a/src/cmd/go/internal/run/run.go
+++ b/src/cmd/go/internal/run/run.go
@@ -19,7 +19,7 @@ import (
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/work"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 var CmdRun = &base.Command{
diff --git a/src/cmd/internal/str/path.go b/src/cmd/go/internal/str/path.go
similarity index 100%
rename from src/cmd/internal/str/path.go
rename to src/cmd/go/internal/str/path.go
diff --git a/src/cmd/go/internal/str/str.go b/src/cmd/go/internal/str/str.go
new file mode 100644
index 00000000000..5bc521b9df4
--- /dev/null
+++ b/src/cmd/go/internal/str/str.go
@@ -0,0 +1,111 @@
+// Copyright 2017 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 str provides string manipulation utilities.
+package str
+
+import (
+	"bytes"
+	"fmt"
+	"unicode"
+	"unicode/utf8"
+)
+
+// StringList flattens its arguments into a single []string.
+// Each argument in args must have type string or []string.
+func StringList(args ...interface{}) []string {
+	var x []string
+	for _, arg := range args {
+		switch arg := arg.(type) {
+		case []string:
+			x = append(x, arg...)
+		case string:
+			x = append(x, arg)
+		default:
+			panic("stringList: invalid argument of type " + fmt.Sprintf("%T", arg))
+		}
+	}
+	return x
+}
+
+// ToFold returns a string with the property that
+//	strings.EqualFold(s, t) iff ToFold(s) == ToFold(t)
+// This lets us test a large set of strings for fold-equivalent
+// duplicates without making a quadratic number of calls
+// to EqualFold. Note that strings.ToUpper and strings.ToLower
+// do not have the desired property in some corner cases.
+func ToFold(s string) string {
+	// Fast path: all ASCII, no upper case.
+	// Most paths look like this already.
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf || 'A' <= c && c <= 'Z' {
+			goto Slow
+		}
+	}
+	return s
+
+Slow:
+	var buf bytes.Buffer
+	for _, r := range s {
+		// SimpleFold(x) cycles to the next equivalent rune > x
+		// or wraps around to smaller values. Iterate until it wraps,
+		// and we've found the minimum value.
+		for {
+			r0 := r
+			r = unicode.SimpleFold(r0)
+			if r <= r0 {
+				break
+			}
+		}
+		// Exception to allow fast path above: A-Z => a-z
+		if 'A' <= r && r <= 'Z' {
+			r += 'a' - 'A'
+		}
+		buf.WriteRune(r)
+	}
+	return buf.String()
+}
+
+// FoldDup reports a pair of strings from the list that are
+// equal according to strings.EqualFold.
+// It returns "", "" if there are no such strings.
+func FoldDup(list []string) (string, string) {
+	clash := map[string]string{}
+	for _, s := range list {
+		fold := ToFold(s)
+		if t := clash[fold]; t != "" {
+			if s > t {
+				s, t = t, s
+			}
+			return s, t
+		}
+		clash[fold] = s
+	}
+	return "", ""
+}
+
+// Contains reports whether x contains s.
+func Contains(x []string, s string) bool {
+	for _, t := range x {
+		if t == s {
+			return true
+		}
+	}
+	return false
+}
+
+// Uniq removes consecutive duplicate strings from ss.
+func Uniq(ss *[]string) {
+	if len(*ss) <= 1 {
+		return
+	}
+	uniq := (*ss)[:1]
+	for _, s := range *ss {
+		if s != uniq[len(uniq)-1] {
+			uniq = append(uniq, s)
+		}
+	}
+	*ss = uniq
+}
diff --git a/src/cmd/go/internal/str/str_test.go b/src/cmd/go/internal/str/str_test.go
new file mode 100644
index 00000000000..8ea758e0a8b
--- /dev/null
+++ b/src/cmd/go/internal/str/str_test.go
@@ -0,0 +1,29 @@
+// Copyright 2020 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 str
+
+import (
+	"testing"
+)
+
+var foldDupTests = []struct {
+	list   []string
+	f1, f2 string
+}{
+	{StringList("math/rand", "math/big"), "", ""},
+	{StringList("math", "strings"), "", ""},
+	{StringList("strings"), "", ""},
+	{StringList("strings", "strings"), "strings", "strings"},
+	{StringList("Rand", "rand", "math", "math/rand", "math/Rand"), "Rand", "rand"},
+}
+
+func TestFoldDup(t *testing.T) {
+	for _, tt := range foldDupTests {
+		f1, f2 := FoldDup(tt.list)
+		if f1 != tt.f1 || f2 != tt.f2 {
+			t.Errorf("foldDup(%q) = %q, %q, want %q, %q", tt.list, f1, f2, tt.f1, tt.f2)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/test/test.go b/src/cmd/go/internal/test/test.go
index dc1bea505bd..ea1d4ff20e9 100644
--- a/src/cmd/go/internal/test/test.go
+++ b/src/cmd/go/internal/test/test.go
@@ -33,7 +33,7 @@ import (
 	"cmd/go/internal/search"
 	"cmd/go/internal/trace"
 	"cmd/go/internal/work"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 	"cmd/internal/test2json"
 )
 
diff --git a/src/cmd/go/internal/vcs/vcs.go b/src/cmd/go/internal/vcs/vcs.go
index 941bd57147e..c4853d7ae31 100644
--- a/src/cmd/go/internal/vcs/vcs.go
+++ b/src/cmd/go/internal/vcs/vcs.go
@@ -27,7 +27,7 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/search"
 	"cmd/go/internal/web"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 )
diff --git a/src/cmd/go/internal/work/buildid.go b/src/cmd/go/internal/work/buildid.go
index 15f944d2af2..d4f2a716d7b 100644
--- a/src/cmd/go/internal/work/buildid.go
+++ b/src/cmd/go/internal/work/buildid.go
@@ -16,7 +16,7 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
 	"cmd/internal/buildid"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 // Build IDs
diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
index 62d81438288..03f8866cf2c 100644
--- a/src/cmd/go/internal/work/exec.go
+++ b/src/cmd/go/internal/work/exec.go
@@ -34,8 +34,9 @@ import (
 	"cmd/go/internal/fsys"
 	"cmd/go/internal/load"
 	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
 	"cmd/go/internal/trace"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 	"cmd/internal/sys"
 )
 
@@ -2666,7 +2667,7 @@ func envList(key, def string) []string {
 	if v == "" {
 		v = def
 	}
-	args, err := str.SplitQuotedFields(v)
+	args, err := quoted.Split(v)
 	if err != nil {
 		panic(fmt.Sprintf("could not parse environment variable %s with value %q: %v", key, v, err))
 	}
diff --git a/src/cmd/go/internal/work/gc.go b/src/cmd/go/internal/work/gc.go
index 3eb9b35f408..e3b4a817e78 100644
--- a/src/cmd/go/internal/work/gc.go
+++ b/src/cmd/go/internal/work/gc.go
@@ -20,8 +20,9 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
 	"cmd/go/internal/load"
+	"cmd/go/internal/str"
 	"cmd/internal/objabi"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 	"cmd/internal/sys"
 	"crypto/sha1"
 )
@@ -565,7 +566,7 @@ func setextld(ldflags []string, compiler []string) ([]string, error) {
 			return ldflags, nil
 		}
 	}
-	joined, err := str.JoinAndQuoteFields(compiler)
+	joined, err := quoted.Join(compiler)
 	if err != nil {
 		return nil, err
 	}
diff --git a/src/cmd/go/internal/work/gccgo.go b/src/cmd/go/internal/work/gccgo.go
index 3cb7b641833..60181b99e47 100644
--- a/src/cmd/go/internal/work/gccgo.go
+++ b/src/cmd/go/internal/work/gccgo.go
@@ -17,7 +17,7 @@ import (
 	"cmd/go/internal/fsys"
 	"cmd/go/internal/load"
 	"cmd/internal/pkgpath"
-	"cmd/internal/str"
+	"cmd/go/internal/str"
 )
 
 // The Gccgo toolchain.
diff --git a/src/cmd/go/internal/work/init.go b/src/cmd/go/internal/work/init.go
index 56e39f8c52a..4dbbd2a13f2 100644
--- a/src/cmd/go/internal/work/init.go
+++ b/src/cmd/go/internal/work/init.go
@@ -11,7 +11,7 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/fsys"
 	"cmd/go/internal/modload"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 	"cmd/internal/sys"
 	"fmt"
 	"os"
@@ -46,7 +46,7 @@ func BuildInit() {
 	// Make sure CC, CXX, and FC are absolute paths.
 	for _, key := range []string{"CC", "CXX", "FC"} {
 		value := cfg.Getenv(key)
-		args, err := str.SplitQuotedFields(value)
+		args, err := quoted.Split(value)
 		if err != nil {
 			base.Fatalf("go: %s environment variable could not be parsed: %v", key, err)
 		}
diff --git a/src/cmd/internal/quoted/quoted.go b/src/cmd/internal/quoted/quoted.go
new file mode 100644
index 00000000000..e7575dfc669
--- /dev/null
+++ b/src/cmd/internal/quoted/quoted.go
@@ -0,0 +1,127 @@
+// Copyright 2017 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 quoted provides string manipulation utilities.
+package quoted
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+	"unicode"
+)
+
+func isSpaceByte(c byte) bool {
+	return c == ' ' || c == '\t' || c == '\n' || c == '\r'
+}
+
+// Split splits s into a list of fields,
+// allowing single or double quotes around elements.
+// There is no unescaping or other processing within
+// quoted fields.
+func Split(s string) ([]string, error) {
+	// Split fields allowing '' or "" around elements.
+	// Quotes further inside the string do not count.
+	var f []string
+	for len(s) > 0 {
+		for len(s) > 0 && isSpaceByte(s[0]) {
+			s = s[1:]
+		}
+		if len(s) == 0 {
+			break
+		}
+		// Accepted quoted string. No unescaping inside.
+		if s[0] == '"' || s[0] == '\'' {
+			quote := s[0]
+			s = s[1:]
+			i := 0
+			for i < len(s) && s[i] != quote {
+				i++
+			}
+			if i >= len(s) {
+				return nil, fmt.Errorf("unterminated %c string", quote)
+			}
+			f = append(f, s[:i])
+			s = s[i+1:]
+			continue
+		}
+		i := 0
+		for i < len(s) && !isSpaceByte(s[i]) {
+			i++
+		}
+		f = append(f, s[:i])
+		s = s[i:]
+	}
+	return f, nil
+}
+
+// Join joins a list of arguments into a string that can be parsed
+// with Split. Arguments are quoted only if necessary; arguments
+// without spaces or quotes are kept as-is. No argument may contain both
+// single and double quotes.
+func Join(args []string) (string, error) {
+	var buf []byte
+	for i, arg := range args {
+		if i > 0 {
+			buf = append(buf, ' ')
+		}
+		var sawSpace, sawSingleQuote, sawDoubleQuote bool
+		for _, c := range arg {
+			switch {
+			case c > unicode.MaxASCII:
+				continue
+			case isSpaceByte(byte(c)):
+				sawSpace = true
+			case c == '\'':
+				sawSingleQuote = true
+			case c == '"':
+				sawDoubleQuote = true
+			}
+		}
+		switch {
+		case !sawSpace && !sawSingleQuote && !sawDoubleQuote:
+			buf = append(buf, []byte(arg)...)
+
+		case !sawSingleQuote:
+			buf = append(buf, '\'')
+			buf = append(buf, []byte(arg)...)
+			buf = append(buf, '\'')
+
+		case !sawDoubleQuote:
+			buf = append(buf, '"')
+			buf = append(buf, []byte(arg)...)
+			buf = append(buf, '"')
+
+		default:
+			return "", fmt.Errorf("argument %q contains both single and double quotes and cannot be quoted", arg)
+		}
+	}
+	return string(buf), nil
+}
+
+// A Flag parses a list of string arguments encoded with Join.
+// It is useful for flags like cmd/link's -extldflags.
+type Flag []string
+
+var _ flag.Value = (*Flag)(nil)
+
+func (f *Flag) Set(v string) error {
+	fs, err := Split(v)
+	if err != nil {
+		return err
+	}
+	*f = fs[:len(fs):len(fs)]
+	return nil
+}
+
+func (f *Flag) String() string {
+	if f == nil {
+		return ""
+	}
+	s, err := Join(*f)
+	if err != nil {
+		return strings.Join(*f, " ")
+	}
+	return s
+}
diff --git a/src/cmd/internal/str/str_test.go b/src/cmd/internal/quoted/quoted_test.go
similarity index 79%
rename from src/cmd/internal/str/str_test.go
rename to src/cmd/internal/quoted/quoted_test.go
index 3609af6a06d..d76270c87b4 100644
--- a/src/cmd/internal/str/str_test.go
+++ b/src/cmd/internal/quoted/quoted_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package str
+package quoted
 
 import (
 	"reflect"
@@ -10,27 +10,7 @@ import (
 	"testing"
 )
 
-var foldDupTests = []struct {
-	list   []string
-	f1, f2 string
-}{
-	{StringList("math/rand", "math/big"), "", ""},
-	{StringList("math", "strings"), "", ""},
-	{StringList("strings"), "", ""},
-	{StringList("strings", "strings"), "strings", "strings"},
-	{StringList("Rand", "rand", "math", "math/rand", "math/Rand"), "Rand", "rand"},
-}
-
-func TestFoldDup(t *testing.T) {
-	for _, tt := range foldDupTests {
-		f1, f2 := FoldDup(tt.list)
-		if f1 != tt.f1 || f2 != tt.f2 {
-			t.Errorf("foldDup(%q) = %q, %q, want %q, %q", tt.list, f1, f2, tt.f1, tt.f2)
-		}
-	}
-}
-
-func TestSplitQuotedFields(t *testing.T) {
+func TestSplit(t *testing.T) {
 	for _, test := range []struct {
 		name    string
 		value   string
@@ -54,7 +34,7 @@ func TestSplitQuotedFields(t *testing.T) {
 		{name: "quote_unclosed", value: `'a`, wantErr: "unterminated ' string"},
 	} {
 		t.Run(test.name, func(t *testing.T) {
-			got, err := SplitQuotedFields(test.value)
+			got, err := Split(test.value)
 			if err != nil {
 				if test.wantErr == "" {
 					t.Fatalf("unexpected error: %v", err)
@@ -73,7 +53,7 @@ func TestSplitQuotedFields(t *testing.T) {
 	}
 }
 
-func TestJoinAndQuoteFields(t *testing.T) {
+func TestJoin(t *testing.T) {
 	for _, test := range []struct {
 		name          string
 		args          []string
@@ -88,7 +68,7 @@ func TestJoinAndQuoteFields(t *testing.T) {
 		{name: "unquoteable", args: []string{`'"`}, wantErr: "contains both single and double quotes and cannot be quoted"},
 	} {
 		t.Run(test.name, func(t *testing.T) {
-			got, err := JoinAndQuoteFields(test.args)
+			got, err := Join(test.args)
 			if err != nil {
 				if test.wantErr == "" {
 					t.Fatalf("unexpected error: %v", err)
diff --git a/src/cmd/internal/str/str.go b/src/cmd/internal/str/str.go
deleted file mode 100644
index 409cf8f7b4b..00000000000
--- a/src/cmd/internal/str/str.go
+++ /dev/null
@@ -1,227 +0,0 @@
-// Copyright 2017 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 str provides string manipulation utilities.
-package str
-
-import (
-	"bytes"
-	"flag"
-	"fmt"
-	"strings"
-	"unicode"
-	"unicode/utf8"
-)
-
-// StringList flattens its arguments into a single []string.
-// Each argument in args must have type string or []string.
-func StringList(args ...interface{}) []string {
-	var x []string
-	for _, arg := range args {
-		switch arg := arg.(type) {
-		case []string:
-			x = append(x, arg...)
-		case string:
-			x = append(x, arg)
-		default:
-			panic("stringList: invalid argument of type " + fmt.Sprintf("%T", arg))
-		}
-	}
-	return x
-}
-
-// ToFold returns a string with the property that
-//	strings.EqualFold(s, t) iff ToFold(s) == ToFold(t)
-// This lets us test a large set of strings for fold-equivalent
-// duplicates without making a quadratic number of calls
-// to EqualFold. Note that strings.ToUpper and strings.ToLower
-// do not have the desired property in some corner cases.
-func ToFold(s string) string {
-	// Fast path: all ASCII, no upper case.
-	// Most paths look like this already.
-	for i := 0; i < len(s); i++ {
-		c := s[i]
-		if c >= utf8.RuneSelf || 'A' <= c && c <= 'Z' {
-			goto Slow
-		}
-	}
-	return s
-
-Slow:
-	var buf bytes.Buffer
-	for _, r := range s {
-		// SimpleFold(x) cycles to the next equivalent rune > x
-		// or wraps around to smaller values. Iterate until it wraps,
-		// and we've found the minimum value.
-		for {
-			r0 := r
-			r = unicode.SimpleFold(r0)
-			if r <= r0 {
-				break
-			}
-		}
-		// Exception to allow fast path above: A-Z => a-z
-		if 'A' <= r && r <= 'Z' {
-			r += 'a' - 'A'
-		}
-		buf.WriteRune(r)
-	}
-	return buf.String()
-}
-
-// FoldDup reports a pair of strings from the list that are
-// equal according to strings.EqualFold.
-// It returns "", "" if there are no such strings.
-func FoldDup(list []string) (string, string) {
-	clash := map[string]string{}
-	for _, s := range list {
-		fold := ToFold(s)
-		if t := clash[fold]; t != "" {
-			if s > t {
-				s, t = t, s
-			}
-			return s, t
-		}
-		clash[fold] = s
-	}
-	return "", ""
-}
-
-// Contains reports whether x contains s.
-func Contains(x []string, s string) bool {
-	for _, t := range x {
-		if t == s {
-			return true
-		}
-	}
-	return false
-}
-
-// Uniq removes consecutive duplicate strings from ss.
-func Uniq(ss *[]string) {
-	if len(*ss) <= 1 {
-		return
-	}
-	uniq := (*ss)[:1]
-	for _, s := range *ss {
-		if s != uniq[len(uniq)-1] {
-			uniq = append(uniq, s)
-		}
-	}
-	*ss = uniq
-}
-
-func isSpaceByte(c byte) bool {
-	return c == ' ' || c == '\t' || c == '\n' || c == '\r'
-}
-
-// SplitQuotedFields splits s into a list of fields,
-// allowing single or double quotes around elements.
-// There is no unescaping or other processing within
-// quoted fields.
-func SplitQuotedFields(s string) ([]string, error) {
-	// Split fields allowing '' or "" around elements.
-	// Quotes further inside the string do not count.
-	var f []string
-	for len(s) > 0 {
-		for len(s) > 0 && isSpaceByte(s[0]) {
-			s = s[1:]
-		}
-		if len(s) == 0 {
-			break
-		}
-		// Accepted quoted string. No unescaping inside.
-		if s[0] == '"' || s[0] == '\'' {
-			quote := s[0]
-			s = s[1:]
-			i := 0
-			for i < len(s) && s[i] != quote {
-				i++
-			}
-			if i >= len(s) {
-				return nil, fmt.Errorf("unterminated %c string", quote)
-			}
-			f = append(f, s[:i])
-			s = s[i+1:]
-			continue
-		}
-		i := 0
-		for i < len(s) && !isSpaceByte(s[i]) {
-			i++
-		}
-		f = append(f, s[:i])
-		s = s[i:]
-	}
-	return f, nil
-}
-
-// JoinAndQuoteFields joins a list of arguments into a string that can be parsed
-// with SplitQuotedFields. Arguments are quoted only if necessary; arguments
-// without spaces or quotes are kept as-is. No argument may contain both
-// single and double quotes.
-func JoinAndQuoteFields(args []string) (string, error) {
-	var buf []byte
-	for i, arg := range args {
-		if i > 0 {
-			buf = append(buf, ' ')
-		}
-		var sawSpace, sawSingleQuote, sawDoubleQuote bool
-		for _, c := range arg {
-			switch {
-			case c > unicode.MaxASCII:
-				continue
-			case isSpaceByte(byte(c)):
-				sawSpace = true
-			case c == '\'':
-				sawSingleQuote = true
-			case c == '"':
-				sawDoubleQuote = true
-			}
-		}
-		switch {
-		case !sawSpace && !sawSingleQuote && !sawDoubleQuote:
-			buf = append(buf, []byte(arg)...)
-
-		case !sawSingleQuote:
-			buf = append(buf, '\'')
-			buf = append(buf, []byte(arg)...)
-			buf = append(buf, '\'')
-
-		case !sawDoubleQuote:
-			buf = append(buf, '"')
-			buf = append(buf, []byte(arg)...)
-			buf = append(buf, '"')
-
-		default:
-			return "", fmt.Errorf("argument %q contains both single and double quotes and cannot be quoted", arg)
-		}
-	}
-	return string(buf), nil
-}
-
-// A QuotedStringListFlag parses a list of string arguments encoded with
-// JoinAndQuoteFields. It is useful for flags like cmd/link's -extldflags.
-type QuotedStringListFlag []string
-
-var _ flag.Value = (*QuotedStringListFlag)(nil)
-
-func (f *QuotedStringListFlag) Set(v string) error {
-	fs, err := SplitQuotedFields(v)
-	if err != nil {
-		return err
-	}
-	*f = fs[:len(fs):len(fs)]
-	return nil
-}
-
-func (f *QuotedStringListFlag) String() string {
-	if f == nil {
-		return ""
-	}
-	s, err := JoinAndQuoteFields(*f)
-	if err != nil {
-		return strings.Join(*f, " ")
-	}
-	return s
-}
diff --git a/src/cmd/link/dwarf_test.go b/src/cmd/link/dwarf_test.go
index f7bbb014d9e..78ef3cfe971 100644
--- a/src/cmd/link/dwarf_test.go
+++ b/src/cmd/link/dwarf_test.go
@@ -8,7 +8,7 @@ import (
 	"bytes"
 	cmddwarf "cmd/internal/dwarf"
 	"cmd/internal/objfile"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 	"debug/dwarf"
 	"internal/testenv"
 	"os"
@@ -68,7 +68,7 @@ func testDWARF(t *testing.T, buildmode string, expectDWARF bool, env ...string)
 			if extld == "" {
 				extld = "gcc"
 			}
-			extldArgs, err := str.SplitQuotedFields(extld)
+			extldArgs, err := quoted.Split(extld)
 			if err != nil {
 				t.Fatal(err)
 			}
diff --git a/src/cmd/link/internal/ld/main.go b/src/cmd/link/internal/ld/main.go
index a5a5a71250f..a1d86965e43 100644
--- a/src/cmd/link/internal/ld/main.go
+++ b/src/cmd/link/internal/ld/main.go
@@ -34,7 +34,7 @@ import (
 	"bufio"
 	"cmd/internal/goobj"
 	"cmd/internal/objabi"
-	"cmd/internal/str"
+	"cmd/internal/quoted"
 	"cmd/internal/sys"
 	"cmd/link/internal/benchmark"
 	"flag"
@@ -76,8 +76,8 @@ var (
 	flagLibGCC     = flag.String("libgcc", "", "compiler support lib for internal linking; use \"none\" to disable")
 	flagTmpdir     = flag.String("tmpdir", "", "use `directory` for temporary files")
 
-	flagExtld      str.QuotedStringListFlag
-	flagExtldflags str.QuotedStringListFlag
+	flagExtld      quoted.Flag
+	flagExtldflags quoted.Flag
 	flagExtar      = flag.String("extar", "", "archive program for buildmode=c-archive")
 
 	flagA             = flag.Bool("a", false, "no-op (deprecated)")