fix(add): Preserve comments when updating simple deps

A case the tests showed but isn't covered here is when a `[features]`
table is created, the dependencies-end comment gets attached to that,
e.g. see cargo_add/overwrite_optional

Fixes #13645

src/cargo/util/toml_mut/dependency.rs

@@ -474,9 +474,18 @@ impl Dependency {
         item: &mut toml_edit::Item,
     ) {
         if str_or_1_len_table(item) {
-            // Nothing to preserve
-            *item = self.to_toml(crate_root);
-            key.fmt();
+            // Little to preserve
+            let mut new_item = self.to_toml(crate_root);
+            match (&item, &mut new_item) {
+                (toml_edit::Item::Value(old), toml_edit::Item::Value(new)) => {
+                    *new.decor_mut() = old.decor().clone();
+                }
+                (toml_edit::Item::Table(old), toml_edit::Item::Table(new)) => {
+                    *new.decor_mut() = old.decor().clone();
+                }
+                (_, _) => {}
+            }
+            *item = new_item;
         } else if let Some(table) = item.as_table_like_mut() {
             match &self.source {
                 Some(Source::Registry(src)) => {

tests/testsuite/cargo_add/overwrite_default_features/out/Cargo.toml

@@ -6,6 +6,8 @@ version = "0.0.0"
 edition = "2015"
 
 [dependencies]
-my-package1 = "99999.0.0"
-my-package2 = "0.4.1"
+# Before my-package1
+my-package1 = "99999.0.0"  # After my-package1
+# Before my-package2
+my-package2 = "0.4.1"  # After my-package2
 # End

tests/testsuite/cargo_add/overwrite_no_default_features/out/Cargo.toml

@@ -6,6 +6,8 @@ version = "0.0.0"
 edition = "2015"
 
 [dependencies]
-my-package1 = { version = "99999.0.0", default-features = false }
-my-package2 = { version = "0.4.1", default-features = false }
+# Before my-package1
+my-package1 = { version = "99999.0.0", default-features = false }  # After my-package1
+# Before my-package2
+my-package2 = { version = "0.4.1", default-features = false }  # After my-package2
 # End

tests/testsuite/cargo_add/overwrite_no_optional/out/Cargo.toml

@@ -6,5 +6,6 @@ version = "0.0.0"
 edition = "2015"
 
 [dependencies]
-my-package = "0.1.0"
+# Before my-package
+my-package = "0.1.0"  # After my-package
 # End

tests/testsuite/cargo_add/overwrite_no_public/out/Cargo.toml

@@ -7,5 +7,6 @@ version = "0.0.0"
 edition = "2015"
 
 [dependencies]
-my-package = "0.1.0"
+# Before my-package
+my-package = "0.1.0"  # After my-package
 # End

tests/testsuite/cargo_add/overwrite_optional/out/Cargo.toml

@@ -6,7 +6,8 @@ version = "0.0.0"
 edition = "2015"
 
 [dependencies]
-my-package = { version = "0.1.0", optional = true }
+# Before my-package
+my-package = { version = "0.1.0", optional = true }  # After my-package
 
 [features]
 my-package = ["dep:my-package"]

tests/testsuite/cargo_add/overwrite_public/out/Cargo.toml

@@ -7,5 +7,6 @@ version = "0.0.0"
 edition = "2015"
 
 [dependencies]
-my-package = { version = "0.1.0", public = true }
+# Before my-package
+my-package = { version = "0.1.0", public = true }  # After my-package
 # End