new: keep generated builds away from local build outputs and vendored autotools files

src/new/debian.rs

@@ -107,9 +107,19 @@ fn source_format(opts: &NewOptions) -> OutputFile {
 
 /// `debian/source/local-options` with `single-debian-patch`, so later
 /// upstream-tree edits stay representable as one `debian/patches/debian-changes-*`
-/// patch instead of failing the build (quilt only).
+/// patch instead of failing the build (quilt only). Common build-output
+/// directories are excluded from the delta as well: compiling locally before
+/// a source build must not turn `target/`, `node_modules/` or `.venv/`
+/// binaries into unrepresentable changes (dpkg ignores `__pycache__` and
+/// friends by default, but not those).
 fn local_options() -> OutputFile {
-    OutputFile::new("debian/source/local-options", "single-debian-patch\n")
+    OutputFile::new(
+        "debian/source/local-options",
+        "single-debian-patch\n\
+         extend-diff-ignore = ^target/\n\
+         extend-diff-ignore = ^node_modules/\n\
+         extend-diff-ignore = ^\\.venv/\n",
+    )
 }
 
 /// `debian/changelog`: the single initial entry, distribution UNRELEASED by
@@ -558,7 +568,10 @@ mod tests {
         assert_eq!(find("debian/source/format").contents, "3.0 (quilt)\n");
         assert_eq!(
             find("debian/source/local-options").contents,
-            "single-debian-patch\n"
+            "single-debian-patch\n\
+             extend-diff-ignore = ^target/\n\
+             extend-diff-ignore = ^node_modules/\n\
+             extend-diff-ignore = ^\\.venv/\n"
         );
 
         let native = NewOptions {

src/new/mod.rs

@@ -428,7 +428,10 @@ mod tests {
         );
         assert_eq!(
             std::fs::read_to_string(tree.join("debian/source/local-options")).unwrap(),
-            "single-debian-patch\n"
+            "single-debian-patch\n\
+             extend-diff-ignore = ^target/\n\
+             extend-diff-ignore = ^node_modules/\n\
+             extend-diff-ignore = ^\\.venv/\n"
         );
 
         // Orig tarball: contains the skeleton file, excludes debian/.

src/new/templates/rust.rs

@@ -96,6 +96,11 @@ impl Template for Rust {
     /// created it, see [`patch_rules_locked`]); omitting it is always safe.
     /// The built artifact of a skeleton is named after its crate (a
     /// sanitized package name) and installed under the command name.
+    ///
+    /// `dh_update_autotools_config` is overridden away: crates embedding C
+    /// 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`.
     fn rules_extra(&self, opts: &NewOptions) -> String {
         let locked = if lockfile_present(opts) {
             " --locked"
@@ -118,6 +123,8 @@ impl Template for Rust {
              override_dh_auto_test:\n\
              \tcargo test --release --offline{locked}\n\
              \n\
+             override_dh_update_autotools_config:\n\
+             \n\
              override_dh_auto_clean:\n\
              \tcargo clean\n",
             locked = locked,
@@ -564,6 +571,10 @@ mod tests {
         let extra = template.rules_extra(&with_lock);
         assert!(extra.contains("\tcargo build --release --offline --locked\n"));
         assert!(extra.contains("\tcargo test --release --offline --locked\n"));
+        // debhelper refreshing embedded autotools files (e.g. `-sys` crates
+        // shipping config.sub/config.guess) would break cargo's per-file
+        // vendored checksums — the override skips the refresh entirely.
+        assert!(extra.contains("override_dh_update_autotools_config:\n"));
     }
 
     /// Package names may carry `+`/`.` (legal dpkg, rejected by cargo): the