cmd/go: fix error for 'go install x.go' when GOBIN is not set

Fixes #6191.
Fixes #5426.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13234052

src/cmd/go/build.go

@@ -311,7 +311,9 @@ func runInstall(cmd *Command, args []string) {
 
 	for _, p := range pkgs {
 		if p.Target == "" && (!p.Standard || p.ImportPath != "unsafe") {
-			if p.ConflictDir != "" {
+			if p.cmdline {
+				errorf("go install: no install location for .go files listed on command line (GOBIN not set)")
+			} else if p.ConflictDir != "" {
 				errorf("go install: no install location for %s: hidden by %s", p.Dir, p.ConflictDir)
 			} else {
 				errorf("go install: no install location for directory %s outside GOPATH", p.Dir)
@@ -486,6 +488,7 @@ func goFilesPackage(gofiles []string) *Package {
 	bp, err := ctxt.ImportDir(dir, 0)
 	pkg := new(Package)
 	pkg.local = true
+	pkg.cmdline = true
 	pkg.load(&stk, bp, err)
 	pkg.localPrefix = dirToImportPath(dir)
 	pkg.ImportPath = "command-line-arguments"

src/cmd/go/pkg.go

@@ -82,6 +82,7 @@ type Package struct {
 	fake         bool                 // synthesized package
 	forceBuild   bool                 // this package must be rebuilt
 	forceLibrary bool                 // this package is a library (even if named "main")
+	cmdline      bool                 // defined by files listed on command line
 	local        bool                 // imported via local path (./ or ../)
 	localPrefix  string               // interpret ./ and ../ imports relative to this prefix
 	exeName      string               // desired name for temporary executable

src/cmd/go/test.bash

@@ -150,11 +150,16 @@ fi
 
 # Without $GOBIN set, installing a program outside $GOPATH should fail
 # (there is nowhere to install it).
-TEST install without destination
-if ./testgo install testdata/src/go-cmd-test/helloworld.go; then
+TEST install without destination fails
+if ./testgo install testdata/src/go-cmd-test/helloworld.go 2>testdata/err; then
 	echo "go install testdata/src/go-cmd-test/helloworld.go should have failed, did not"
 	ok=false
+elif ! grep 'no install location for .go files listed on command line' testdata/err; then
+	echo "wrong error:"
+	cat testdata/err
+	ok=false
 fi
+rm -f testdata/err
 
 # With $GOBIN set, should install there.
 TEST install to GOBIN '(command-line package)'