Make target dir self-ignoring

src/cargo/core/workspace.rs

@@ -402,9 +402,16 @@ impl<'gctx> Workspace<'gctx> {
     }
 
     pub fn target_dir(&self) -> Filesystem {
-        self.target_dir
+        let target_dir = self
+            .target_dir
             .clone()
-            .unwrap_or_else(|| self.default_target_dir())
+            .unwrap_or_else(|| self.default_target_dir());
+
+        let path = target_dir.as_path_unlocked();
+        paths::create_dir_all(path).unwrap();
+        std::fs::write(path.join(".gitignore"), "*\n").unwrap();
+
+        target_dir
     }
 
     fn default_target_dir(&self) -> Filesystem {

src/cargo/ops/cargo_new.rs

@@ -680,6 +680,10 @@ fn write_ignore_file(base_path: &Path, list: &IgnoreList, vcs: VersionControl) -
     }
 
     for fp_ignore in match vcs {
+        // TODO: Stop writing the .gitignore file by default once enough
+        // time has passed. The target directory generated by cargo is now
+        // self-ignoring, but the .gitignore file should still be generated
+        // while people are still using older toolchains.
         VersionControl::Git => vec![base_path.join(".gitignore")],
         VersionControl::Hg => vec![base_path.join(".hgignore")],
         VersionControl::Pijul => vec![base_path.join(".ignore")],