new: ignore vendored rust artifacts in the generated gitignore

src/new/templates/mod.rs

@@ -137,6 +137,15 @@ pub trait Template: Sync {
         Vec::new()
     }
 
+    /// Root `.gitignore` entries the template contributes in every mode
+    /// (the skeleton-mode build-artifact entries of
+    /// [`crate::new::debian::ROOT_GITIGNORE_ENTRIES`] are separate), merged
+    /// into the root `.gitignore` after the files are written — missing
+    /// ones appended, an existing file never overwritten. Default: none.
+    fn gitignore_entries(&self, _opts: &NewOptions) -> Vec<String> {
+        Vec::new()
+    }
+
     /// Extra defaults derived from project metadata in `dir` (detect.rs);
     /// `None` when the project carries nothing this template can read.
     fn probe(&self, _dir: &Path) -> Option<ProbeResult> {
@@ -248,6 +257,48 @@ mod tests {
         );
     }
 
+    /// Only the rust template contributes root `.gitignore` entries of its
+    /// own — exactly the vendoring pair, and unconditionally (its vendoring
+    /// hook runs in every mode); every other template contributes nothing.
+    #[test]
+    fn gitignore_entries_are_rust_only() {
+        let o = NewOptions {
+            name: "mytool".into(),
+            template: TemplateId::Rust,
+            source_dir: SourceDir::Here,
+            upstream_version: "0.1.0".into(),
+            revision: 1,
+            summary: "A tool".into(),
+            long_description: "A tool".into(),
+            homepage: None,
+            license: crate::new::options::License::Mit,
+            command: "mytool".into(),
+            maintainer: ("Jane".into(), "jane@example.com".into()),
+            dist: "ubuntu".into(),
+            series: "resolute".into(),
+            release: false,
+            depends: Vec::new(),
+            source_format: crate::new::options::SourceFormat::Quilt,
+            orig: None,
+            git: false,
+            autopkgtest: false,
+            pkg_config: false,
+            watch: None,
+        };
+
+        for id in TemplateId::all() {
+            let entries = get(id).unwrap().gitignore_entries(&o);
+            if id == TemplateId::Rust {
+                assert_eq!(
+                    entries,
+                    vec!["vendor/".to_string(), ".cargo/config.toml".to_string()]
+                );
+            } else {
+                assert!(entries.is_empty(), "{id} contributes {entries:?}");
+            }
+        }
+    }
+
     /// The final `debian/rules` of every template must be valid-looking
     /// make: `#!/usr/bin/make -f` shebang, exactly one `%:` target whose
     /// recipe is the template's dh line, tab-indented recipes only, and no