Preserve format as much as possible.

- Add a new line between the last table element and newelements when there were no previous elements.
- Preserve the decor prefix of previous elements.

Signed-off-by: David Calavera <david.calavera@gmail.com>

Author: calavera

src/cargo/ops/cargo_new.rs

@@ -997,16 +997,59 @@ fn update_manifest_with_new_member(
                 return Ok(());
             }
         }
+        let decor = if members.is_empty() {
+            None
+        } else {
+            members.get(members.len() - 1).map(|m| m.decor()).cloned()
+        };
 
         let Some(toml_members) = workspace_document["workspace"]["members"].as_array_mut() else {
             anyhow::bail!("[workspace.members] is not an array");
         };
 
         toml_members.push(relpath);
+
+        if let Some(decor) = decor {
+            if let Some(last) = toml_members
+                .get_mut(toml_members.len() - 1)
+                .map(|m| m.decor_mut())
+            {
+                if let Some(prefix) = decor.prefix() {
+                    last.set_prefix(prefix.to_owned());
+                }
+                if let Some(suffix) = decor.suffix() {
+                    last.set_suffix(suffix.to_owned());
+                }
+            }
+        }
     } else {
+        let ws = workspace_document["workspace"]
+            .as_table_mut()
+            .with_context(|| format!("[workspace] is not a table"))?;
+
+        let mut last = None;
+        for (_, value) in ws.get_values() {
+            if !value.is_inline_table() {
+                last = Some(value.clone());
+            }
+        }
+
+        let assume_new_line = last.is_some();
+        let mut prefix = None;
+        if let Some(value) = last {
+            prefix = value.decor().prefix().cloned();
+        }
+
         let mut array = Array::new();
         array.push(relpath);
-        workspace_document["workspace"]["members"] = toml_edit::value(array);
+        if !assume_new_line {
+            array.decor_mut().set_suffix("\n");
+        }
+        if let Some(prefix) = prefix {
+            array.decor_mut().set_prefix(prefix);
+        }
+
+        ws.insert("members", toml_edit::value(array));
     }
 
     paths::write(&root_manifest_path, workspace_document.to_string())