Add pending deprecation warning for --accept-unseen (#668)

Open to this being shot down. If others tend to agree, this is a
low-cost way of figuring out whether anyone is using it

CHANGELOG.md

@@ -5,6 +5,11 @@ All notable changes to insta and cargo-insta are documented here.
 ## 1.42.1
 
 - Improved handling of control characters in inline snapshots.  #713
+- Add pending deprecation warning for `--accept-unseen`. We've left an issue
+  open at <https://github.com/mitsuhiko/insta/issues/659> eliciting feedback on
+  whether anyone uses this for a few months.  A warning will now be printed when
+  `--accept-unseen` is used, and we'll eventually remove the feature unless we
+  get some feedback that it's useful.  #668
 
 ## 1.42.0
 

cargo-insta/src/cli.rs

@@ -637,6 +637,13 @@ fn process_snapshots(
 fn test_run(mut cmd: TestCommand, color: ColorWhen) -> Result<(), Box<dyn Error>> {
     let loc = handle_target_args(&cmd.target_args, &cmd.test_runner_options.package)?;
 
+    if cmd.accept_unseen {
+        eprintln!(
+            "{} If this option is materially helpful to you, please add a comment at https://github.com/mitsuhiko/insta/issues/659.", 
+            style("`--accept-unseen` is pending deprecation.").bold().yellow()
+        )
+    }
+
     // Based on any configs in the config file, update the test command. Default
     // is `SnapshotUpdate::Auto`.
     match loc.tool_config.snapshot_update() {