cmd/cue: avoid confusing "unknown flag" errors in cue mod

If the user runs

    cue mod nosuchcommand --nosuchflag

we'd fail with "unknown flag --nosuchflag" which is technically correct,
but "no such subcommand" is a much more useful error message.

For example, if a user on CUE v0.12.0 tries to run a newer command
which doesn't exist yet, such as `cue mod mirror --to=...`,
the error would not clearly point to `mod mirror` not existing yet.

Fixes #3810.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Iabd3780335f3d30488bb667d9115ec2af843d0d9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211267
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: mvdan

cmd/cue/cmd/root.go

@@ -90,6 +90,13 @@ type Stats struct {
 // do anything useful, but we need RunE for good error messages, as Cobra itself is lacking:
 // https://github.com/spf13/cobra/issues/706
 func commandGroup(c *Command, cmd *cobra.Command) *cobra.Command {
+	// If the user ran `cue mod nosuchcommand --nosuchflag`, we run the `cue mod` command
+	// because `nosuchcommand` was not a known subcommand.
+	// In such a case, do not fail with "unknown flag"; what matters to the user is that
+	// `nosuchcommand`, which might have such a flag if it existed, was not found at all.
+	// The logic below does not need to parse flags in any case.
+	cmd.FParseErrWhitelist.UnknownFlags = true
+
 	name := cmd.Name()
 	cmd.RunE = mkRunE(c, func(cmd *Command, args []string) error {
 		stderr := cmd.Stderr()

cmd/cue/cmd/testdata/script/unknown_args.txtar

@@ -16,26 +16,32 @@ cmp stderr unknown_mod_cmd.out
 
 ! exec cue mod --unknown
 ! stdout .
-cmp stderr unknown_flag.out
+cmp stderr missing_mod_cmd.out
 
-# TODO: this should result in an "unknown subcommand" error.
 ! exec cue mod unknown --unknown
 ! stdout .
-cmp stderr unknown_flag.out
+cmp stderr unknown_mod_cmd.out
+
+! exec cue -v mod -E unknown --unknown
+! stdout .
+cmp stderr unknown_mod_cmd.out
 
 # A rather convoluted edge case, but Cobra allows mixing flags with args,
 # even when the args are subcommand names. -v and -E are global flags.
 # TODO: this should result in an "unknown subcommand" error.
 ! exec cue -v mod -E unknown --unknown
 ! stdout .
-cmp stderr unknown_flag.out
+cmp stderr unknown_mod_cmd.out
 
 ! exec cue evla
 ! stdout .
 cmp stderr typo_cmd.stdout
 
 -- unknown_cmd.out --
 unknown command "unknown" for "cue"
+-- missing_mod_cmd.out --
+mod must be run as one of its subcommands
+Run 'cue help mod' for known subcommands.
 -- unknown_mod_cmd.out --
 mod must be run as one of its subcommands: unknown subcommand "unknown"
 Run 'cue help mod' for known subcommands.