new: keep vendored *.orig files through dh_clean in the rust rules

dh_clean unlinks *.orig patch backups, and vendored crates carry
Cargo.toml.orig (and the occasional *.xml.orig) that cargo's per-file
checksums require on cold builds. Override dh_clean with -X .orig.

src/new/templates/rust.rs

@@ -101,6 +101,9 @@ impl Template for Rust {
     /// sources (e.g. `-sys` crates shipping `config.sub`/`config.guess`)
     /// carry per-file cargo checksums, and debhelper refreshing those files
     /// with the system's newer copies would break `cargo build --offline`.
+    /// `dh_clean` gets `-X Cargo.toml.orig` for the same reason: it treats
+    /// every vendored `Cargo.toml.orig` as a patch backup and deletes it,
+    /// which breaks the checksums on any build without a warm cache.
     fn rules_extra(&self, opts: &NewOptions) -> String {
         let locked = if lockfile_present(opts) {
             " --locked"
@@ -125,6 +128,12 @@ impl Template for Rust {
              \n\
              override_dh_update_autotools_config:\n\
              \n\
+             override_dh_clean:\n\
+             \t# dh_clean unlinks `*.orig` patch backups, but vendored crates\n\
+             \t# ship files like `Cargo.toml.orig` that cargo's per-file\n\
+             \t# checksums require on cold builds (chroots, Launchpad).\n\
+             \tdh_clean -X .orig\n\
+             \n\
              override_dh_auto_clean:\n\
              \tcargo clean\n",
             locked = locked,
@@ -552,6 +561,8 @@ mod tests {
         );
         assert!(extra.contains("override_dh_auto_test:\n\tcargo test --release --offline\n"));
         assert!(extra.contains("override_dh_auto_clean:\n\tcargo clean"));
+        assert!(extra.contains("override_dh_clean:"));
+        assert!(extra.contains("\tdh_clean -X .orig"));
         assert!(!extra.contains("--locked"));
     }