Fix uv add comment handling for empty arrays (#8504)

## Summary

Closes https://github.com/astral-sh/uv/issues/8496.

crates/uv-workspace/src/pyproject_mut.rs

@@ -366,7 +366,6 @@ impl PyProjectTomlMut {
 
         // Set the position to the minimum, if it's not already the first element.
         if let Some(min) = existing.iter().filter_map(toml_edit::Table::position).min() {
-            // if !table.position().is_some_and(|position| position < min) {
             table.set_position(min);
 
             // Increment the position of all existing elements.
@@ -375,7 +374,6 @@ impl PyProjectTomlMut {
                     table.set_position(position + 1);
                 }
             }
-            // }
         }
 
         // Push the item to the table.
@@ -805,11 +803,16 @@ pub fn add_dependency(
             let index = index.unwrap_or(deps.len());
 
             let mut value = Value::from(req_string.as_str());
+
             let decor = value.decor_mut();
-            decor.set_prefix(deps.trailing().clone());
+
-            deps.set_trailing("");
+            if index == deps.len() {
+                decor.set_prefix(deps.trailing().clone());
+                deps.set_trailing("");
+            }
 
             deps.insert_formatted(index, value);
+
             // `reformat_array_multiline` uses the indentation of the first dependency entry.
             // Therefore, we retrieve the indentation of the first dependency entry and apply it to
             // the new entry. Note that it is only necessary if the newly added dependency is going
@@ -992,6 +995,11 @@ fn reformat_array_multiline(deps: &mut Array) {
                 .and_then(|s| s.lines().last())
                 .unwrap_or_default();
 
+            let decor_prefix = decor_prefix
+                .split_once('#')
+                .map(|(s, _)| s)
+                .unwrap_or(decor_prefix);
+
             // If there is no indentation, use four-space.
             indentation_prefix = Some(if decor_prefix.is_empty() {
                 "    ".to_string()
@@ -1023,7 +1031,16 @@ fn reformat_array_multiline(deps: &mut Array) {
         let mut rv = String::new();
         if comments.peek().is_some() {
             for comment in comments {
-                rv.push_str("\n    ");
+                match comment.comment_type {
+                    CommentType::OwnLine => {
+                        let indentation_prefix_str =
+                            format!("\n{}", indentation_prefix.as_ref().unwrap());
+                        rv.push_str(&indentation_prefix_str);
+                    }
+                    CommentType::EndOfLine => {
+                        rv.push(' ');
+                    }
+                }
                 rv.push_str(&comment.text);
             }
         }

crates/uv/tests/it/edit.rs

@@ -6591,3 +6591,240 @@ fn add_preserves_open_bracket_comment() -> Result<()> {
     });
     Ok(())
 }
+
+#[test]
+fn add_preserves_empty_comment() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+            # First line.
+            # Second line.
+        ]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("anyio==3.7.0"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 4 packages in [TIME]
+    Prepared 4 packages in [TIME]
+    Installed 4 packages in [TIME]
+     + anyio==3.7.0
+     + idna==3.6
+     + project==0.1.0 (from file://[TEMP_DIR]/)
+     + sniffio==1.3.1
+    "###);
+
+    let pyproject_toml = context.read("pyproject.toml");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+            # First line.
+            # Second line.
+            "anyio==3.7.0",
+        ]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+        "###
+        );
+    });
+
+    Ok(())
+}
+
+#[test]
+fn add_preserves_trailing_comment() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+            "idna",
+            "iniconfig",  # Use iniconfig.
+            # First line.
+            # Second line.
+        ]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("anyio==3.7.0"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 5 packages in [TIME]
+    Prepared 5 packages in [TIME]
+    Installed 5 packages in [TIME]
+     + anyio==3.7.0
+     + idna==3.6
+     + iniconfig==2.0.0
+     + project==0.1.0 (from file://[TEMP_DIR]/)
+     + sniffio==1.3.1
+    "###);
+
+    let pyproject_toml = context.read("pyproject.toml");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+            "anyio==3.7.0",
+            "idna",
+            "iniconfig", # Use iniconfig.
+            # First line.
+            # Second line.
+        ]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+        "###
+        );
+    });
+
+    uv_snapshot!(context.filters(), context.add().arg("typing-extensions"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 6 packages in [TIME]
+    Prepared 2 packages in [TIME]
+    Uninstalled 1 package in [TIME]
+    Installed 2 packages in [TIME]
+     ~ project==0.1.0 (from file://[TEMP_DIR]/)
+     + typing-extensions==4.10.0
+    "###);
+
+    let pyproject_toml = context.read("pyproject.toml");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+            "anyio==3.7.0",
+            "idna",
+            "iniconfig", # Use iniconfig.
+            # First line.
+            # Second line.
+            "typing-extensions>=4.10.0",
+        ]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+        "###
+        );
+    });
+
+    Ok(())
+}
+
+#[test]
+fn add_preserves_trailing_depth() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+          "idna",
+          "iniconfig",# Use iniconfig.
+            # First line.
+            # Second line.
+        ]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("anyio==3.7.0"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 5 packages in [TIME]
+    Prepared 5 packages in [TIME]
+    Installed 5 packages in [TIME]
+     + anyio==3.7.0
+     + idna==3.6
+     + iniconfig==2.0.0
+     + project==0.1.0 (from file://[TEMP_DIR]/)
+     + sniffio==1.3.1
+    "###);
+
+    let pyproject_toml = context.read("pyproject.toml");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            pyproject_toml, @r###"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+          "anyio==3.7.0",
+          "idna",
+          "iniconfig", # Use iniconfig.
+          # First line.
+          # Second line.
+        ]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+        "###
+        );
+    });
+
+    Ok(())
+}