feat(help): Include environment-only variables

The environment only variables don't accept command
line options, but should still be listed in the
help message.

Instead of listing the command line flags the
placeholder text '(environment only)' is displayed.

README.md

@@ -634,6 +634,7 @@ Change the default registry to push to.
 
   --host HOST            URL of the repository [default: docker.io]
   --user USER            username to connect as [env: REPO_USERNAME]
+  (environment only)     password to connect with [env: REPO_PASSWORD]
 
 Global options:
   --quiet, -q            Quiet

example_test.go

@@ -296,6 +296,7 @@ func Example_helpTextWithGroups() {
 	//
 	//   --host HOST            URL of the repository [default: docker.io]
 	//   --user USER            username to connect as [env: REPO_USERNAME]
+	//   (environment only)     password to connect with [env: REPO_PASSWORD]
 	//
 	// Global options:
 	//   --quiet, -q            Quiet

usage.go

@@ -285,7 +285,7 @@ func (p *Parser) writeHelpForSubcommand(w io.Writer, cmd *command) {
 // writeHelpForArguments writes the list of short, long, and environment-only
 // options in order.
 func (p *Parser) writeHelpForArguments(w io.Writer, cmd *command, header, help string) {
-	var positionals, longOptions, shortOptions []*spec
+	var positionals, longOptions, shortOptions, envOnly []*spec
 	for _, spec := range cmd.options {
 		switch {
 		case spec.positional:
@@ -294,10 +294,12 @@ func (p *Parser) writeHelpForArguments(w io.Writer, cmd *command, header, help s
 			longOptions = append(longOptions, spec)
 		case spec.short != "":
 			shortOptions = append(shortOptions, spec)
+		case spec.env != "":
+			envOnly = append(envOnly, spec)
 		}
 	}
 
-	if cmd.parent != nil && len(shortOptions)+len(longOptions) == 0 {
+	if cmd.parent != nil && len(shortOptions)+len(longOptions)+len(envOnly) == 0 {
 		return
 	}
 
@@ -312,6 +314,9 @@ func (p *Parser) writeHelpForArguments(w io.Writer, cmd *command, header, help s
 	for _, spec := range longOptions {
 		p.printOption(w, spec)
 	}
+	for _, spec := range envOnly {
+		p.printOption(w, spec)
+	}
 
 	// write the list of argument groups
 	if len(cmd.groups) > 0 {

usage_test.go

@@ -510,6 +510,7 @@ This block represents related arguments.
   --host HOST            hostname to connect to [default: localhost, env: DB_HOST]
   --port PORT            port to connect to [default: 3306, env: DB_PORT]
   --user USER            username to connect as [env: DB_USERNAME]
+  (environment only)     password to connect with [env: DB_PASSWORD]
 
 Global options:
   --help, -h             display this help and exit
@@ -746,6 +747,8 @@ Usage: example [-s SHORT]
 
 Options:
   -s SHORT [env: SHORT]
+  (environment only) [env: ENVONLY]
+  (environment only) [env: CUSTOM]
   --help, -h             display this help and exit
 `
 	var args struct {