new: ignore vendored rust artifacts in the generated gitignore

src/new/debian.rs

@@ -323,15 +323,25 @@ pub const ROOT_GITIGNORE_ENTRIES: [&str; 6] = [
     "target/",
 ];
 
-/// Merge the root `.gitignore` entries into `existing` (the current file
-/// contents, when there is one): missing entries are appended, an existing
-/// file is never overwritten just to duplicate entries. Returns the new
-/// contents, or `None` when nothing has to be written.
-pub fn merge_root_gitignore(existing: Option<&str>) -> Option<String> {
+/// Comment heading a root `.gitignore` freshly created by pkh (the skeleton
+/// build-artifact section).
+pub const ROOT_GITIGNORE_HEADER: &str = "# pkh build artifacts";
+
+/// Merge `entries` into the root `.gitignore` contents `existing` (the
+/// current file contents, when there is one): missing entries are appended,
+/// an existing file is never overwritten just to duplicate entries. A fresh
+/// file is headed by the `header` comment when one is given; appending to a
+/// user file adds bare entries. Returns the new contents, or `None` when
+/// nothing has to be written.
+pub fn merge_gitignore_entries(
+    existing: Option<&str>,
+    entries: &[&str],
+    header: Option<&str>,
+) -> Option<String> {
     let have: HashSet<&str> = existing
         .map(|content| content.lines().map(str::trim).collect())
         .unwrap_or_default();
-    let missing: Vec<&str> = ROOT_GITIGNORE_ENTRIES
+    let missing: Vec<&str> = entries
         .iter()
         .copied()
         .filter(|entry| !have.contains(entry))
@@ -346,8 +356,11 @@ pub fn merge_root_gitignore(existing: Option<&str>) -> Option<String> {
     }
     // Section comment only for a fresh file; appending to a user file adds
     // bare entries.
-    if existing.is_none() {
-        out.push_str("# pkh build artifacts\n");
+    if existing.is_none()
+        && let Some(header) = header
+    {
+        out.push_str(header);
+        out.push('\n');
     }
     for entry in missing {
         out.push_str(entry);
@@ -746,17 +759,30 @@ mod tests {
     }
 
     #[test]
-    fn root_gitignore_merge() {
+    fn gitignore_merge() {
         // Fresh file: header + all entries.
-        let fresh = merge_root_gitignore(None).unwrap();
+        let fresh =
+            merge_gitignore_entries(None, &ROOT_GITIGNORE_ENTRIES, Some(ROOT_GITIGNORE_HEADER))
+                .unwrap();
         assert!(fresh.starts_with("# pkh build artifacts\n"));
         for entry in ROOT_GITIGNORE_ENTRIES {
             assert!(fresh.contains(entry), "{entry} missing");
         }
 
+        // A fresh file without a header carries the bare entries.
+        assert_eq!(
+            merge_gitignore_entries(None, &["a/", "b"], None).unwrap(),
+            "a/\nb\n"
+        );
+
         // Existing file: only the missing entries are appended, nothing lost.
         let existing = "*.deb\nnode_modules/\n";
-        let merged = merge_root_gitignore(Some(existing)).unwrap();
+        let merged = merge_gitignore_entries(
+            Some(existing),
+            &ROOT_GITIGNORE_ENTRIES,
+            Some(ROOT_GITIGNORE_HEADER),
+        )
+        .unwrap();
         assert!(merged.starts_with(existing));
         assert!(merged.contains("*.dsc\n"));
         assert!(!merged.contains("*.deb\n*.deb"));
@@ -766,7 +792,31 @@ mod tests {
             .iter()
             .map(|e| format!("{e}\n"))
             .collect();
-        assert!(merge_root_gitignore(Some(&full)).is_none());
+        assert!(
+            merge_gitignore_entries(
+                Some(&full),
+                &ROOT_GITIGNORE_ENTRIES,
+                Some(ROOT_GITIGNORE_HEADER),
+            )
+            .is_none()
+        );
+    }
+
+    /// The vendoring entries of the rust template merge into an existing
+    /// user `.gitignore` like any other entry set: appended after the
+    /// user's lines, no header comment, idempotent.
+    #[test]
+    fn gitignore_merge_appends_template_entries() {
+        let entries = ["vendor/", ".cargo/config.toml"];
+        let merged =
+            merge_gitignore_entries(Some("# my project\n*.log\n"), &entries, None).unwrap();
+        assert_eq!(merged, "# my project\n*.log\nvendor/\n.cargo/config.toml\n");
+
+        // Already ignored: nothing to write.
+        assert!(
+            merge_gitignore_entries(Some("vendor/\n.cargo/config.toml\n"), &entries, None)
+                .is_none()
+        );
     }
 
     #[test]