cmd/cue/cmd: disallow commands in non-tool files

This is not strictly necessary, but it is mostly to be consistent
with v0.2 and not relax restrictions we may want to preserve
before the v0.3 release.

Fixes #654

Change-Id: I7701bd3ce80ca44475cced8cb4e639160b2afd78
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/9063
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/custom.go

@@ -30,6 +30,7 @@ import (
 	"cuelang.org/go/cue"
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/internal"
+	"cuelang.org/go/internal/core/adt"
 	itask "cuelang.org/go/internal/task"
 	_ "cuelang.org/go/pkg/tool/cli" // Register tasks
 	_ "cuelang.org/go/pkg/tool/exec"
@@ -70,6 +71,25 @@ func addCustom(c *Command, parent *cobra.Command, typ, name string, tools *cue.I
 	if !o.Exists() {
 		return nil, o.Err()
 	}
+
+	// Ensure there is at least one tool file.
+	// TODO: remove this block to allow commands to be defined in any file.
+outer:
+	for _, v := range []cue.Value{tools.Lookup(typ), o} {
+		_, vv := internal.CoreValue(v)
+		w := vv.(*adt.Vertex)
+		for _, c := range w.Conjuncts {
+			src := c.Source()
+			if src == nil {
+				continue
+			}
+			if strings.HasSuffix(src.Pos().Filename(), "_tool.cue") {
+				break outer
+			}
+		}
+		return nil, errors.New("no command %q defined in a *_tool.cue file")
+	}
+
 	docs := o.Doc()
 	var usage, short, long string
 	if len(docs) > 0 {

cmd/cue/cmd/testdata/script/cmd_issue650.txt

@@ -11,7 +11,7 @@ package blah
 }
 -- cue.mod/module.cue --
 module: "mod.com"
--- x.cue --
+-- x_tool.cue --
 package kube
 
 import (

cmd/cue/cmd/testdata/script/cmd_notool.txt

@@ -1,5 +1,3 @@
-skip 'error messages'
-
 ! cue cmd notool
 ! stdout .
 cmp stderr cmd_baddisplay.out

cmd/cue/cmd/testdata/script/cmd_notool2.txt

@@ -1,13 +1,11 @@
-skip 'wrong error message'
-
 ! cue notool
 ! stdout .
 cmp stderr cmd_baddisplay.out
 
 -- cmd_baddisplay.out --
-cmd must be run as one of its subcommands: unknown subcommand "notool"
+command "notool" is not defined
 Ensure commands are defined in a "_tool.cue" file.
-Run 'cue help cmd' for known subcommands.
+Run 'cue help' to show available commands.
 -- task.cue --
 package home
 message: "Hello world!"