go/build: add help info for unset $GOPATH

We relay this info in a few places, in a few different ways, but not
consistently everywhere.  This led one of our users to start googling
and not find https://golang.org/doc/code.html#Workspaces, of which `go
help gopath` is the most equivalent.

Change-Id: I28a94375739f3aa4f200e145293ca2a5f65101e1
Reviewed-on: https://go-review.googlesource.com/28690
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/code.html

@@ -149,7 +149,7 @@ $ <b>export PATH=$PATH:$GOPATH/bin</b>
 <p>
 To learn more about setting up the <code>GOPATH</code> environment variable,
 please see
-<a href="/cmd/go/#hdr-GOPATH_environment_variable"><code>go help gopath</code></a>
+<a href="/cmd/go/#hdr-GOPATH_environment_variable"><code>'go help gopath'</code></a>
 </p>
 
 <h3 id="ImportPaths">Import paths</h3>

src/cmd/go/alldocs.go

@@ -514,7 +514,7 @@
 //
 // When checking out a new package, get creates the target directory
 // GOPATH/src/<import-path>. If the GOPATH contains multiple entries,
-// get uses the first one. See 'go help gopath'.
+// get uses the first one. For more details see: 'go help gopath'.
 //
 // When checking out or updating a package, get looks for a branch or tag
 // that matches the locally installed version of Go. The most important
@@ -1074,7 +1074,7 @@
 // 		The operating system for which to compile code.
 // 		Examples are linux, darwin, windows, netbsd.
 // 	GOPATH
-// 		See 'go help gopath'.
+// 		For more details see: 'go help gopath'.
 // 	GORACE
 // 		Options for the race detector.
 // 		See https://golang.org/doc/articles/race_detector.html.
@@ -1125,10 +1125,10 @@
 //
 // Import path syntax
 //
-// An import path (see 'go help packages') denotes a package
-// stored in the local file system.  In general, an import path denotes
-// either a standard package (such as "unicode/utf8") or a package
-// found in one of the work spaces (see 'go help gopath').
+// An import path (see 'go help packages') denotes a package stored in the local
+// file system.  In general, an import path denotes either a standard package (such
+// as "unicode/utf8") or a package found in one of the work spaces (For more
+// details see: 'go help gopath').
 //
 // Relative import paths
 //
@@ -1260,8 +1260,8 @@
 // same meta tag and then git clone https://code.org/r/p/exproj into
 // GOPATH/src/example.org.
 //
-// New downloaded packages are written to the first directory
-// listed in the GOPATH environment variable (see 'go help gopath').
+// New downloaded packages are written to the first directory listed in the GOPATH
+// environment variable (For more details see: 'go help gopath').
 //
 // The go command attempts to download the version of the
 // package appropriate for the Go release being used.
@@ -1305,7 +1305,7 @@
 //
 // Otherwise, the import path P denotes the package found in
 // the directory DIR/src/P for some DIR listed in the GOPATH
-// environment variable (see 'go help gopath').
+// environment variable (For more details see: 'go help gopath').
 //
 // If no import paths are given, the action applies to the
 // package in the current directory.

src/cmd/go/build.go

@@ -605,7 +605,7 @@ func installPackages(args []string, forGet bool) {
 				errorf("go install: no install location for %s: hidden by %s", p.Dir, p.ConflictDir)
 			default:
 				errorf("go install: no install location for directory %s outside GOPATH\n"+
-					"\tFor more details see: go help gopath", p.Dir)
+					"\tFor more details see: 'go help gopath'", p.Dir)
 			}
 		}
 	}

src/cmd/go/get.go

@@ -49,7 +49,7 @@ Get also accepts build flags to control the installation. See 'go help build'.
 
 When checking out a new package, get creates the target directory
 GOPATH/src/<import-path>. If the GOPATH contains multiple entries,
-get uses the first one. See 'go help gopath'.
+get uses the first one. For more details see: 'go help gopath'.
 
 When checking out or updating a package, get looks for a branch or tag
 that matches the locally installed version of Go. The most important
@@ -393,11 +393,11 @@ func downloadPackage(p *Package) error {
 		// Package not found. Put in first directory of $GOPATH.
 		list := filepath.SplitList(buildContext.GOPATH)
 		if len(list) == 0 {
-			return fmt.Errorf("cannot download, $GOPATH not set. For more details see: go help gopath")
+			return fmt.Errorf("cannot download, $GOPATH not set. For more details see: 'go help gopath'")
 		}
 		// Guard against people setting GOPATH=$GOROOT.
 		if list[0] == goroot {
-			return fmt.Errorf("cannot download, $GOPATH must not be set to $GOROOT. For more details see: go help gopath")
+			return fmt.Errorf("cannot download, $GOPATH must not be set to $GOROOT. For more details see: 'go help gopath'")
 		}
 		p.build.SrcRoot = filepath.Join(list[0], "src")
 		p.build.PkgRoot = filepath.Join(list[0], "pkg")

src/cmd/go/go_test.go

@@ -1622,8 +1622,8 @@ func TestMissingGOPATHIsReported(t *testing.T) {
 	defer tg.cleanup()
 	tg.setenv("GOPATH", "")
 	tg.runFail("install", "foo/quxx")
-	if tg.grepCountBoth(`\(\$GOPATH not set\)$`) != 1 {
-		t.Error(`go install foo/quxx expected error: ($GOPATH not set)`)
+	if tg.grepCountBoth(`\(\$GOPATH not set\. For more details see: 'go help gopath'\)$`) != 1 {
+		t.Error(`go install foo/quxx expected error: ($GOPATH not set. For more details see: 'go help gopath')`)
 	}
 }
 

src/cmd/go/help.go

@@ -42,7 +42,7 @@ denotes the package in that directory.
 
 Otherwise, the import path P denotes the package found in
 the directory DIR/src/P for some DIR listed in the GOPATH
-environment variable (see 'go help gopath').
+environment variable (For more details see: 'go help gopath').
 
 If no import paths are given, the action applies to the
 package in the current directory.
@@ -102,10 +102,10 @@ var helpImportPath = &Command{
 	Short:     "import path syntax",
 	Long: `
 
-An import path (see 'go help packages') denotes a package
-stored in the local file system.  In general, an import path denotes
-either a standard package (such as "unicode/utf8") or a package
-found in one of the work spaces (see 'go help gopath').
+An import path (see 'go help packages') denotes a package stored in the local
+file system.  In general, an import path denotes either a standard package (such
+as "unicode/utf8") or a package found in one of the work spaces (For more
+details see: 'go help gopath').
 
 Relative import paths
 
@@ -237,8 +237,8 @@ the go tool will verify that https://example.org/?go-get=1 contains the
 same meta tag and then git clone https://code.org/r/p/exproj into
 GOPATH/src/example.org.
 
-New downloaded packages are written to the first directory
-listed in the GOPATH environment variable (see 'go help gopath').
+New downloaded packages are written to the first directory listed in the GOPATH
+environment variable (For more details see: 'go help gopath').
 
 The go command attempts to download the version of the
 package appropriate for the Go release being used.
@@ -439,7 +439,7 @@ General-purpose environment variables:
 		The operating system for which to compile code.
 		Examples are linux, darwin, windows, netbsd.
 	GOPATH
-		See 'go help gopath'.
+		For more details see: 'go help gopath'.
 	GORACE
 		Options for the race detector.
 		See https://golang.org/doc/articles/race_detector.html.

src/cmd/go/main.go

@@ -147,7 +147,7 @@ func main() {
 				os.Exit(2)
 			}
 			if !filepath.IsAbs(p) {
-				fmt.Fprintf(os.Stderr, "go: GOPATH entry is relative; must be absolute path: %q.\nRun 'go help gopath' for usage.\n", p)
+				fmt.Fprintf(os.Stderr, "go: GOPATH entry is relative; must be absolute path: %q.\nFor more details see: 'go help gopath'\n", p)
 				os.Exit(2)
 			}
 		}

src/go/build/build.go

@@ -636,7 +636,7 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 			format = "\t%s"
 		}
 		if len(tried.gopath) == 0 {
-			paths = append(paths, "\t($GOPATH not set)")
+			paths = append(paths, "\t($GOPATH not set. For more details see: 'go help gopath')")
 		}
 		return p, fmt.Errorf("cannot find package %q in any of:\n%s", path, strings.Join(paths, "\n"))
 	}