main: Special error message for invalid top-level command

Previously Terraform would react to an invalid top-level command the same
way as for typing no command at all: just printing out the long top-level
help directory.

If someone's tried to type a command, it's more helpful to respond to that
request by explaining directly that the command is invalid, rather than
leaving the user to puzzle that out themselves by referring to the help
text.

As a bonus, this also allows us to use our "didyoumean" package to suggest
possible alternatives if it seems like the user made a typo.

main.go

@@ -17,6 +17,7 @@ import (
 	"github.com/hashicorp/terraform/command/cliconfig"
 	"github.com/hashicorp/terraform/command/format"
 	"github.com/hashicorp/terraform/httpclient"
+	"github.com/hashicorp/terraform/internal/didyoumean"
 	"github.com/hashicorp/terraform/internal/logging"
 	"github.com/hashicorp/terraform/version"
 	"github.com/mattn/go-shellwords"
@@ -282,6 +283,32 @@ func wrappedMain() int {
 		AutocompleteUninstall: "uninstall-autocomplete",
 	}
 
+	// Before we continue we'll check whether the requested command is
+	// actually known. If not, we might be able to suggest an alternative
+	// if it seems like the user made a typo.
+	// (This bypasses the built-in help handling in cli.CLI for the situation
+	// where a command isn't found, because it's likely more helpful to
+	// mention what specifically went wrong, rather than just printing out
+	// a big block of usage information.)
+	if cmd := cliRunner.Subcommand(); cmd != "" {
+		// Due to the design of cli.CLI, this special error message only works
+		// for typos of top-level commands. For a subcommand typo, like
+		// "terraform state posh", cmd would be "state" here and thus would
+		// be considered to exist, and it would print out its own usage message.
+		if _, exists := Commands[cmd]; !exists {
+			suggestions := make([]string, 0, len(Commands))
+			for name := range Commands {
+				suggestions = append(suggestions, name)
+			}
+			suggestion := didyoumean.NameSuggestion(cmd, suggestions)
+			if suggestion != "" {
+				suggestion = fmt.Sprintf(" Did you mean %q?", suggestion)
+			}
+			fmt.Fprintf(os.Stderr, "Terraform has no command named %q.%s\n\nTo see all of Terraform's top-level commands, run:\n  terraform -help\n\n", cmd, suggestion)
+			return 1
+		}
+	}
+
 	exitCode, err := cliRunner.Run()
 	if err != nil {
 		Ui.Error(fmt.Sprintf("Error executing CLI: %s", err.Error()))