cmd/cue: improve stdin input handling in cue fmt --check

I was playing around with the `gofmt` command, and noticed that
it adds an indication in standard input when the -l flag is used:

    $ cat some_file.go | gofmt -l
      <standard input>

The current `cue fmt --check` implementation intentionally avoids
displaying such output, but I think this is less intuitive as if
a user runs `cue fmt --check -`, the empty output can be confusing.

This change modifies `cue fmt --check` to write `-` in case `-`
was specified as an argument.

Signed-off-by: Noam Dolovich <[email protected]>
Change-Id: I4cb9e1e75a5f2bfc12c723cc909268315151e552
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193777
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: NoamTD

cmd/cue/cmd/fmt.go

@@ -131,15 +131,13 @@ func newFmtCmd(c *Command) *cobra.Command {
 					if check && !bytes.Equal(formatted.Bytes(), original) {
 						foundBadlyFormatted = true
 
-						if file.Filename != "-" {
-							f := file.Filename
-							var path string
-							path, err = filepath.Rel(cwd, f)
-							if err != nil {
-								path = f
-							}
-							fmt.Fprintln(stdout, path)
+						var path string
+						f := file.Filename
+						path, err = filepath.Rel(cwd, f)
+						if err != nil {
+							path = f
 						}
+						fmt.Fprintln(stdout, path)
 					}
 				}
 			}

cmd/cue/cmd/testdata/script/fmt_check.txtar

@@ -15,10 +15,15 @@ cmpenv stdout expected-output
 ! exec cue fmt --check not_formatted.cue another/not_formatted.cue
 cmpenv stdout expected-output
 
-# stdin fails with no output
+# stdin fails with indicative output
 stdin not_formatted.cue
 ! exec cue fmt --check -
-! stdout .
+cmp stdout stdin-expected-output
+
+# fails with indicative output when both stdin and files provided
+stdin not_formatted.cue
+! exec cue fmt --check - another/not_formatted.cue
+cmpenv stdout stdin-mixed-expected-output
 
 cd ../module
 
@@ -39,6 +44,11 @@ x:   1
 -- standalone/expected-output --
 not_formatted.cue
 another${/}not_formatted.cue
+-- standalone/stdin-expected-output --
+-
+-- standalone/stdin-mixed-expected-output --
+-
+another${/}not_formatted.cue
 -- module/cue.mod/module.cue --
 module: "example.com"
 -- module/not_formatted.cue --