Preserve comments when auto-correcting unused dependencies

CHANGELOG.md

@@ -142,6 +142,21 @@ So `pks check` may report a violation it didn't, or no longer report one, and a
 `package_todo.yml` entry for one it no longer reports is stale until you run
 `pks update`.
 
+#### `check-unused-dependencies --auto-correct` keeps comments in `package.yml`
+
+Auto-correct used to rewrite every `package.yml` it touched from scratch, which
+deleted all of its comments and reordered its keys. It now removes the unused
+entries from the `dependencies:` list and leaves the rest of the file as it
+was, line endings included. A comment covers the entries below it, up to the
+next blank line or comment, and is removed only when all of those entries are.
+A commented-out entry such as `# - packs/a` is always kept, along with any
+comment directly above it. If no dependencies remain, the whole `dependencies:`
+block is removed, including the comments and blank lines inside it.
+
+If a list is written in a form that cannot be edited this way, such as
+`dependencies: [packs/a, packs/b]`, the file is rewritten as before and a
+warning names the file and the dependencies removed.
+
 ## 0.7.0
 
 ### Features

src/packs/checker.rs

@@ -3,6 +3,7 @@ mod dependency;
 pub(crate) mod layer;
 
 mod common_test;
+mod dependency_removal;
 mod folder_privacy;
 pub(crate) mod pack_checker;
 mod privacy;
@@ -606,15 +607,40 @@ fn remove_reference_to_dependency(
     pack: &Pack,
     dependency_names: &[String],
 ) -> anyhow::Result<()> {
-    let without_dependency = pack
-        .dependencies
-        .iter()
-        .filter(|dependency| !dependency_names.contains(dependency));
-    let updated_pack = Pack {
-        dependencies: without_dependency.cloned().collect(),
-        ..pack.clone()
-    };
-    write_pack_to_disk(&updated_pack)?;
+    let contents = std::fs::read_to_string(&pack.yml).map_err(|e| {
+        anyhow::Error::new(e)
+            .context(format!("Failed to read pack {:?}", pack.yml))
+    })?;
+
+    match dependency_removal::remove_dependencies(&contents, dependency_names) {
+        Some(updated) => std::fs::write(&pack.yml, updated).map_err(|e| {
+            anyhow::Error::new(e)
+                .context(format!("Failed to write pack to disk {:?}", pack.yml))
+        })?,
+        None => {
+            eprintln!(
+                "Warning: could not edit the dependencies list in `{}` in \
+                 place to remove {}, so the file was rewritten: its comments \
+                 were not preserved and its keys may be reordered.",
+                pack.relative_yml().display(),
+                dependency_names
+                    .iter()
+                    .map(|name| format!("`{name}`"))
+                    .collect::<Vec<_>>()
+                    .join(", "),
+            );
+            let without_dependency = pack
+                .dependencies
+                .iter()
+                .filter(|dependency| !dependency_names.contains(dependency));
+            let updated_pack = Pack {
+                dependencies: without_dependency.cloned().collect(),
+                ..pack.clone()
+            };
+            write_pack_to_disk(&updated_pack)?;
+        }
+    }
+
     Ok(())
 }
 // Note: Display impl was removed from CheckAllResult. Use write_text() directly with Configuration.