cmd/cue: support cue fix - to fix CUE via stdin and stdout

We were missing the extra few lines of code to handle it,
just like other commands such as `cue fmt` or `cue import`.

Fixes #3417.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Id2d88b32c02e4218db4ad119489b4f68f1f39783
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200554
Reviewed-by: Matthew Sackman <matthew@cue.works>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/fix.go

@@ -89,7 +89,7 @@ func runFixAll(cmd *Command, args []string) error {
 
 	for _, i := range instances {
 		for _, f := range i.Files {
-			if done[f] || !strings.HasSuffix(f.Filename, ".cue") {
+			if done[f] || (f.Filename != "-" && !strings.HasSuffix(f.Filename, ".cue")) {
 				continue
 			}
 			done[f] = true
@@ -99,9 +99,14 @@ func runFixAll(cmd *Command, args []string) error {
 				errs = errors.Append(errs, errors.Promote(err, "format"))
 			}
 
-			err = os.WriteFile(f.Filename, b, 0644)
-			if err != nil {
-				errs = errors.Append(errs, errors.Promote(err, "write"))
+			if f.Filename == "-" {
+				if _, err := cmd.OutOrStdout().Write(b); err != nil {
+					return err
+				}
+			} else {
+				if err := os.WriteFile(f.Filename, b, 0644); err != nil {
+					errs = errors.Append(errs, errors.Promote(err, "write"))
+				}
 			}
 		}
 	}

cmd/cue/cmd/testdata/script/fix.txtar

@@ -1,8 +1,14 @@
-exec cue fix ./...
+# Just like other commands, we can fix with stdin/stdout.
+stdin p/three.cue
+exec cue fix -
+cmp stdout p/three.cue.fixed
+
 # Make sure we fix all files in a directory, even if they're a mix of packages (or no packages).
+exec cue fix ./...
 cmp p/one.cue p/one.cue.fixed
 cmp p/two.cue p/two.cue.fixed
 cmp p/three.cue p/three.cue.fixed
+
 -- p/one.cue --
 package one
 
@@ -28,4 +34,4 @@ out: list.Repeat(["baz"], 3)
 -- p/three.cue.fixed --
 import "list"
 
-out: list.Concat([["a"], (list.Concat([(list.Repeat(["a"], 7)), ["gh"]]))])
+out: list.Concat([["a"], (list.Concat([(list.Repeat(["a"], 7)), ["gh"]]))])