diff --git a/src/cmd/go/build.go b/src/cmd/go/build.go
index e76c6fc2f57..caffa1f05d3 100644
--- a/src/cmd/go/build.go
+++ b/src/cmd/go/build.go
@@ -23,7 +23,7 @@ import (
 )
 
 var cmdBuild = &Command{
-	UsageLine: "build [-a] [-n] [-o output] [-p n] [-v] [-x] [importpath... | gofiles...]",
+	UsageLine: "build [-a] [-n] [-o output] [-p n] [-v] [-x] [-work] [importpath... | gofiles...]",
 	Short:     "compile packages and dependencies",
 	Long: `
 Build compiles the packages named by the import paths,
@@ -48,6 +48,9 @@ It is an error to use -o when the command line specifies multiple packages.
 The -p flag specifies the number of builds that can be run in parallel.
 The default is the number of CPUs available.
 
+The -work flag causes build to print the name of the temporary work
+directory and not delete it when exiting.
+
 For more about import paths, see 'go help importpath'.
 
 See also: go install, go get, go clean.
@@ -70,6 +73,7 @@ var buildP = runtime.NumCPU() // -p flag
 var buildV bool               // -v flag
 var buildX bool               // -x flag
 var buildO = cmdBuild.Flag.String("o", "", "output file")
+var buildWork bool // -work flag
 
 var buildContext = build.DefaultContext
 
@@ -80,6 +84,7 @@ func addBuildFlags(cmd *Command) {
 	cmd.Flag.IntVar(&buildP, "p", buildP, "")
 	cmd.Flag.BoolVar(&buildV, "v", false, "")
 	cmd.Flag.BoolVar(&buildX, "x", false, "")
+	cmd.Flag.BoolVar(&buildWork, "work", false, "")
 
 	// TODO(rsc): This -t flag is used by buildscript.sh but
 	// not documented.  Should be documented but the
@@ -140,7 +145,7 @@ func runBuild(cmd *Command, args []string) {
 }
 
 var cmdInstall = &Command{
-	UsageLine: "install [-a] [-n] [-p n] [-v] [-x] [importpath...]",
+	UsageLine: "install [-a] [-n] [-p n] [-v] [-x] [-work] [importpath...]",
 	Short:     "compile and install packages and dependencies",
 	Long: `
 Install compiles and installs the packages named by the import paths,
@@ -154,6 +159,9 @@ The -x flag prints the commands.
 The -p flag specifies the number of builds that can be run in parallel.
 The default is the number of CPUs available.
 
+The -work flag causes build to print the name of the temporary work
+directory and not delete it when exiting.
+
 For more about import paths, see 'go help importpath'.
 
 See also: go build, go get, go clean.
@@ -263,10 +271,12 @@ func (b *builder) init() {
 		if err != nil {
 			fatalf("%s", err)
 		}
-		if buildX {
+		if buildX || buildWork {
 			fmt.Printf("WORK=%s\n", b.work)
 		}
-		atexit(func() { os.RemoveAll(b.work) })
+		if !buildWork {
+			atexit(func() { os.RemoveAll(b.work) })
+		}
 	}
 }
 
@@ -300,7 +310,7 @@ func goFilesPackage(gofiles []string, target string) *Package {
 	ctxt.ReadDir = func(string) ([]os.FileInfo, error) { return dir, nil }
 	pwd, _ := os.Getwd()
 	var stk importStack
-	pkg := scanPackage(&ctxt, &build.Tree{Path: "."}, "<command line>", "<command line>", pwd+"/.", &stk)
+	pkg := scanPackage(&ctxt, &build.Tree{Path: "."}, "<command line>", "<command line>", pwd+"/.", &stk, true)
 	if pkg.Error != nil {
 		fatalf("%s", pkg.Error)
 	}
@@ -686,8 +696,10 @@ func (b *builder) install(a *action) error {
 	// garbage down in a large build.  On an operating system
 	// with aggressive buffering, cleaning incrementally like
 	// this keeps the intermediate objects from hitting the disk.
-	defer os.RemoveAll(a1.objdir)
-	defer os.Remove(a1.target)
+	if !buildWork {
+		defer os.RemoveAll(a1.objdir)
+		defer os.Remove(a1.target)
+	}
 
 	return b.copyFile(a.target, a1.target, perm)
 }
@@ -745,6 +757,18 @@ func (b *builder) copyFile(dst, src string, perm os.FileMode) error {
 	}
 	defer sf.Close()
 
+	// Be careful about removing/overwriting dst.
+	// Do not remove/overwrite if dst exists and is a directory
+	// or a non-object file.
+	if fi, err := os.Stat(dst); err == nil {
+		if fi.IsDir() {
+			return fmt.Errorf("build output %q already exists and is a directory", dst)
+		}
+		if !isObject(dst) {
+			return fmt.Errorf("build output %q already exists and is not an object file", dst)
+		}
+	}
+
 	// On Windows, remove lingering ~ file from last attempt.
 	if toolIsWindows {
 		if _, err := os.Stat(dst + "~"); err == nil {
@@ -777,6 +801,32 @@ func (b *builder) copyFile(dst, src string, perm os.FileMode) error {
 	return nil
 }
 
+var objectMagic = [][]byte{
+	{'!', '<', 'a', 'r', 'c', 'h', '>', '\n'},        // Package archive
+	{'\x7F', 'E', 'L', 'F'},                          // ELF
+	{0xFE, 0xED, 0xFA, 0xCE},                         // Mach-O big-endian 32-bit
+	{0xFE, 0xED, 0xFA, 0xCF},                         // Mach-O big-endian 64-bit
+	{0xCE, 0xFA, 0xED, 0xFE},                         // Mach-O little-endian 32-bit
+	{0xCF, 0xFA, 0xED, 0xFE},                         // Mach-O little-endian 64-bit
+	{0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00, 0x04, 0x00}, // PE (Windows) as generated by 6l/8l
+}
+
+func isObject(s string) bool {
+	f, err := os.Open(s)
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+	buf := make([]byte, 64)
+	io.ReadFull(f, buf)
+	for _, magic := range objectMagic {
+		if bytes.HasPrefix(buf, magic) {
+			return true
+		}
+	}
+	return false
+}
+
 // fmtcmd formats a command in the manner of fmt.Sprintf but also:
 //
 //	If dir is non-empty and the script is not in dir right now,
diff --git a/src/cmd/go/main.go b/src/cmd/go/main.go
index 68ab582df1d..44f33d4f00c 100644
--- a/src/cmd/go/main.go
+++ b/src/cmd/go/main.go
@@ -398,7 +398,7 @@ func allPackages(pattern string) []string {
 			have[name] = true
 
 			_, err = build.ScanDir(path)
-			if err != nil {
+			if err != nil && strings.Contains(err.Error(), "no Go source files") {
 				return nil
 			}
 
diff --git a/src/cmd/go/pkg.go b/src/cmd/go/pkg.go
index 00c5d30b50d..c855fa6c4c8 100644
--- a/src/cmd/go/pkg.go
+++ b/src/cmd/go/pkg.go
@@ -6,6 +6,7 @@ package main
 
 import (
 	"bytes"
+	"fmt"
 	"go/build"
 	"go/scanner"
 	"os"
@@ -135,6 +136,7 @@ func loadPackage(arg string, stk *importStack) *Package {
 	}
 
 	// Find basic information about package path.
+	isCmd := false
 	t, importPath, err := build.FindTree(arg)
 	dir := ""
 	// Maybe it is a standard command.
@@ -146,6 +148,7 @@ func loadPackage(arg string, stk *importStack) *Package {
 			importPath = arg
 			dir = p
 			err = nil
+			isCmd = true
 		}
 	}
 	// Maybe it is a path to a standard command.
@@ -158,6 +161,7 @@ func loadPackage(arg string, stk *importStack) *Package {
 			importPath = filepath.FromSlash(arg[len(cmd):])
 			dir = arg
 			err = nil
+			isCmd = true
 		}
 	}
 	if err != nil {
@@ -178,12 +182,23 @@ func loadPackage(arg string, stk *importStack) *Package {
 	}
 
 	// Maybe we know the package by its directory.
-	if p := packageCache[dir]; p != nil {
+	p := packageCache[dir]
+	if p != nil {
 		packageCache[importPath] = p
-		return reusePackage(p, stk)
+		p = reusePackage(p, stk)
+	} else {
+		p = scanPackage(&buildContext, t, arg, importPath, dir, stk, false)
 	}
 
-	return scanPackage(&buildContext, t, arg, importPath, dir, stk)
+	// If we loaded the files from the Go root's cmd/ tree,
+	// it must be a command (package main).
+	if isCmd && p.Error == nil && p.Name != "main" {
+		p.Error = &PackageError{
+			ImportStack: stk.copy(),
+			Err:         fmt.Sprintf("expected package main in %q; found package %s", dir, p.Name),
+		}
+	}
+	return p
 }
 
 func reusePackage(p *Package, stk *importStack) *Package {
@@ -243,7 +258,7 @@ var isGoTool = map[string]bool{
 	"exp/ebnflint": true,
 }
 
-func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string, stk *importStack) *Package {
+func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string, stk *importStack, useAllFiles bool) *Package {
 	// Read the files in the directory to learn the structure
 	// of the package.
 	p := &Package{
@@ -255,7 +270,9 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 	packageCache[dir] = p
 	packageCache[importPath] = p
 
+	ctxt.UseAllFiles = useAllFiles
 	info, err := ctxt.ScanDir(dir)
+	useAllFiles = false // flag does not apply to dependencies
 	if err != nil {
 		p.Error = &PackageError{
 			ImportStack: stk.copy(),
diff --git a/src/cmd/go/test.go b/src/cmd/go/test.go
index e2bf44ed9d4..a291262b3f8 100644
--- a/src/cmd/go/test.go
+++ b/src/cmd/go/test.go
@@ -273,6 +273,10 @@ func runTest(cmd *Command, args []string) {
 			}
 		}
 
+		// Ignore pseudo-packages.
+		delete(deps, "C")
+		delete(deps, "unsafe")
+
 		all := []string{}
 		for path := range deps {
 			all = append(all, path)
diff --git a/src/pkg/crypto/tls/generate_cert.go b/src/pkg/crypto/tls/generate_cert.go
index 7c0718b82ab..84be5bfd856 100644
--- a/src/pkg/crypto/tls/generate_cert.go
+++ b/src/pkg/crypto/tls/generate_cert.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // Generate a self-signed X.509 certificate for a TLS server. Outputs to
 // 'cert.pem' and 'key.pem' and will overwrite existing files.
 
diff --git a/src/pkg/encoding/gob/dump.go b/src/pkg/encoding/gob/dump.go
index f7d822c11e4..e23a11e48bf 100644
--- a/src/pkg/encoding/gob/dump.go
+++ b/src/pkg/encoding/gob/dump.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 package main
 
 // Need to compile package gob with debug.go to build this program.
diff --git a/src/pkg/exp/norm/maketables.go b/src/pkg/exp/norm/maketables.go
index 6e6317198f0..bb21bb58109 100644
--- a/src/pkg/exp/norm/maketables.go
+++ b/src/pkg/exp/norm/maketables.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // Normalization table generator.
 // Data read from the web.
 // See forminfo.go for a description of the trie values associated with each rune.
diff --git a/src/pkg/exp/norm/maketesttables.go b/src/pkg/exp/norm/maketesttables.go
index 20eb889ddef..d3112b4041c 100644
--- a/src/pkg/exp/norm/maketesttables.go
+++ b/src/pkg/exp/norm/maketesttables.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // Generate test data for trie code.
 
 package main
diff --git a/src/pkg/exp/norm/normregtest.go b/src/pkg/exp/norm/normregtest.go
index 57ba7032981..c2ab25bc99d 100644
--- a/src/pkg/exp/norm/normregtest.go
+++ b/src/pkg/exp/norm/normregtest.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 package main
 
 import (
diff --git a/src/pkg/exp/norm/triegen.go b/src/pkg/exp/norm/triegen.go
index 4ad9e0e057c..2e275a06254 100644
--- a/src/pkg/exp/norm/triegen.go
+++ b/src/pkg/exp/norm/triegen.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // Trie table generator.
 // Used by make*tables tools to generate a go file with trie data structures
 // for mapping UTF-8 to a 16-bit value. All but the last byte in a UTF-8 byte
diff --git a/src/pkg/go/build/dir.go b/src/pkg/go/build/dir.go
index 0917e736aa4..6b30f76265b 100644
--- a/src/pkg/go/build/dir.go
+++ b/src/pkg/go/build/dir.go
@@ -25,10 +25,11 @@ import (
 
 // A Context specifies the supporting context for a build.
 type Context struct {
-	GOARCH     string   // target architecture
-	GOOS       string   // target operating system
-	CgoEnabled bool     // whether cgo can be used
-	BuildTags  []string // additional tags to recognize in +build lines
+	GOARCH      string   // target architecture
+	GOOS        string   // target operating system
+	CgoEnabled  bool     // whether cgo can be used
+	BuildTags   []string // additional tags to recognize in +build lines
+	UseAllFiles bool     // use files regardless of +build lines, file names
 
 	// By default, ScanDir uses the operating system's
 	// file system calls to read directories and files.
@@ -225,6 +226,7 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 
 	var Sfiles []string // files with ".S" (capital S)
 	var di DirInfo
+	var firstFile string
 	imported := make(map[string][]token.Position)
 	testImported := make(map[string][]token.Position)
 	fset := token.NewFileSet()
@@ -237,7 +239,7 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 			strings.HasPrefix(name, ".") {
 			continue
 		}
-		if !ctxt.goodOSArchFile(name) {
+		if !ctxt.UseAllFiles && !ctxt.goodOSArchFile(name) {
 			continue
 		}
 
@@ -250,12 +252,13 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 			continue
 		}
 
-		// Look for +build comments to accept or reject the file.
 		filename, data, err := ctxt.readFile(dir, name)
 		if err != nil {
 			return nil, err
 		}
-		if !ctxt.shouldBuild(data) {
+
+		// Look for +build comments to accept or reject the file.
+		if !ctxt.UseAllFiles && !ctxt.shouldBuild(data) {
 			continue
 		}
 
@@ -281,9 +284,6 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 		}
 
 		pkg := string(pf.Name.Name)
-		if pkg == "main" && di.Package != "" && di.Package != "main" {
-			continue
-		}
 		if pkg == "documentation" {
 			continue
 		}
@@ -293,15 +293,11 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 			pkg = pkg[:len(pkg)-len("_test")]
 		}
 
-		if pkg != di.Package && di.Package == "main" {
-			// Found non-main package but was recording
-			// information about package main.  Reset.
-			di = DirInfo{}
-		}
 		if di.Package == "" {
 			di.Package = pkg
+			firstFile = name
 		} else if pkg != di.Package {
-			return nil, fmt.Errorf("%s: found packages %s and %s", dir, pkg, di.Package)
+			return nil, fmt.Errorf("%s: found packages %s (%s) and %s (%s)", dir, di.Package, firstFile, pkg, name)
 		}
 		if pf.Doc != nil {
 			if di.PackageComment != nil {
diff --git a/src/pkg/go/doc/headscan.go b/src/pkg/go/doc/headscan.go
index 37486b126fd..f5593476382 100644
--- a/src/pkg/go/doc/headscan.go
+++ b/src/pkg/go/doc/headscan.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 /*
 	The headscan command extracts comment headings from package files;
 	it is used to detect false positives which may require an adjustment
diff --git a/src/pkg/net/http/triv.go b/src/pkg/net/http/triv.go
index 994fc0e32f6..c88a0fbce73 100644
--- a/src/pkg/net/http/triv.go
+++ b/src/pkg/net/http/triv.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 package main
 
 import (
diff --git a/src/pkg/unicode/maketables.go b/src/pkg/unicode/maketables.go
index 15e3f20774d..16bc83cea91 100644
--- a/src/pkg/unicode/maketables.go
+++ b/src/pkg/unicode/maketables.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 // Unicode table generator.
 // Data read from the web.