cmd/cue: implement cue mod tidy --check

This wires up `modload.CheckTidy` to the cue command.

Specifically, `cue mod tidy --check` succeeds only if the
module file is contains exactly the required dependencies
and also contains a CUE language version.

Fixes #2910.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I49379d28d7ab156016cfeb21bd26834e42dd2df1
Dispatch-Trailer: {"type":"trybot","CL":1178021,"patchset":2,"ref":"refs/changes/21/1178021/2","targetBranch":"master"}

cmd/cue/cmd/flags.go

@@ -21,6 +21,7 @@ import (
 // Common flags
 const (
 	flagAll           flagName = "all"
+	flagCheck         flagName = "check"
 	flagDryrun        flagName = "dryrun"
 	flagVerbose       flagName = "verbose"
 	flagAllErrors     flagName = "all-errors"

cmd/cue/cmd/modtidy.go

@@ -50,6 +50,7 @@ for this command to work.
 		RunE: mkRunE(c, runModTidy),
 		Args: cobra.ExactArgs(0),
 	}
+	cmd.Flags().Bool(string(flagCheck), false, "check for tidiness only; do not update module.cue file")
 
 	return cmd
 }
@@ -67,11 +68,13 @@ func runModTidy(cmd *Command, args []string) error {
 	if err != nil {
 		return err
 	}
+	if flagCheck.Bool(cmd) {
+		return modload.CheckTidy(ctx, os.DirFS(modRoot), ".", reg)
+	}
 	mf, err := modload.Tidy(ctx, os.DirFS(modRoot), ".", reg, versionForModFile())
 	if err != nil {
 		return err
 	}
-	// TODO check whether it's changed or not.
 	data, err := mf.Format()
 	if err != nil {
 		return fmt.Errorf("internal error: invalid module.cue file generated: %v", err)

cmd/cue/cmd/testdata/script/modtidy_check.txtar

@@ -0,0 +1,26 @@
+# Check that cue mod tidy --check succeeds OK
+# even when the module file is formatted differently
+# e.g. language.version in a single line.
+
+exec cue mod tidy --check
+
+-- cue.mod/module.cue --
+module: "main.org@v0"
+language: version: "v0.99.99"
+
+// A comment.
+deps: "example.com@v0": v: "v0.0.1"
+
+-- main.cue --
+package main
+import "example.com@v0:main"
+
+main
+
+-- _registry/example.com_v0.0.1/cue.mod/module.cue --
+module: "example.com@v0"
+
+-- _registry/example.com_v0.0.1/top.cue --
+package main
+
+"example.com@v0": "v0.0.1"

cmd/cue/cmd/testdata/script/modtidy_check_fail.txtar

@@ -0,0 +1,25 @@
+# Check that cue mod tidy --check fails when some dependencies aren't present.
+
+! exec cue mod tidy --check
+cmp stderr want-stderr
+
+-- want-stderr --
+module is not tidy: cannot find module providing package example.com@v0:main
+-- cue.mod/module.cue --
+module: "main.org@v0"
+language: version: "v0.99.99"
+
+-- main.cue --
+package main
+import "example.com@v0:main"
+
+main
+
+-- _registry/example.com_v0.0.1/cue.mod/module.cue --
+module: "example.com@v0"
+
+-- _registry/example.com_v0.0.1/top.cue --
+package main
+
+"example.com@v0": "v0.0.1"
+

cmd/cue/cmd/testdata/script/modtidy_initial.txtar

@@ -9,6 +9,9 @@ cmp cue.mod/module.cue want-module
 # Check that the resulting module evaluates as expected.
 exec cue export .
 cmp stdout want-stdout
+
+# Check that the tidy check succeeds
+exec cue mod tidy --check
 -- want-module --
 module: "main.org@v0"
 language: {