Avoid duplicating first-entry comments in `uv add` (#9109)

## Summary

Closes https://github.com/astral-sh/uv/issues/9105.

crates/uv-workspace/src/pyproject_mut.rs

@@ -957,6 +957,23 @@ pub fn add_dependency(
 
             let decor = value.decor_mut();
 
+            // If we're adding to the end of the list, treat trailing comments as leading comments
+            // on the added dependency.
+            //
+            // For example, given:
+            // ```toml
+            // dependencies = [
+            //     "anyio", # trailing comment
+            // ]
+            // ```
+            //
+            // If we add `flask` to the end, we want to retain the comment on `anyio`:
+            // ```toml
+            // dependencies = [
+            //     "anyio", # trailing comment
+            //     "flask",
+            // ]
+            // ```
             if index == deps.len() {
                 decor.set_prefix(deps.trailing().clone());
                 deps.set_trailing("");
@@ -979,7 +996,76 @@ pub fn add_dependency(
                     .unwrap()
                     .clone();
 
-                deps.get_mut(index).unwrap().decor_mut().set_prefix(prefix);
+                // However, if the prefix includes a comment, we don't want to duplicate it.
+                // Depending on the location of the comment, we either want to leave it as-is, or
+                // attach it to the entry that's being moved to the next line.
+                //
+                // For example, given:
+                // ```toml
+                // dependencies = [ # comment
+                //     "flask",
+                // ]
+                // ```
+                //
+                // If we add `anyio` to the beginning, we want to retain the comment on the open
+                // bracket:
+                // ```toml
+                // dependencies = [ # comment
+                //     "anyio",
+                //     "flask",
+                // ]
+                // ```
+                //
+                // However, given:
+                // ```toml
+                // dependencies = [
+                //     # comment
+                //     "flask",
+                // ]
+                // ```
+                //
+                // If we add `anyio` to the beginning, we want the comment to move down with the
+                // existing entry:
+                // entry:
+                // ```toml
+                // dependencies = [
+                //     "anyio",
+                //     # comment
+                //     "flask",
+                // ]
+                if let Some(prefix) = prefix.as_str() {
+                    // Treat anything before the first own-line comment as a prefix on the new
+                    // entry; anything after the first own-line comment is a prefix on the existing
+                    // entry.
+                    //
+                    // This is equivalent to using the first and last line content as the prefix for
+                    // the new entry, and the rest as the prefix for the existing entry.
+                    if let Some((first_line, rest)) = prefix.split_once(['\r', '\n']) {
+                        // Determine the appropriate newline character.
+                        let newline = {
+                            let mut chars = prefix[first_line.len()..].chars();
+                            match (chars.next(), chars.next()) {
+                                (Some('\r'), Some('\n')) => "\r\n",
+                                (Some('\r'), _) => "\r",
+                                (Some('\n'), _) => "\n",
+                                _ => "\n",
+                            }
+                        };
+                        let last_line = rest.lines().last().unwrap_or_default();
+                        let prefix = format!("{first_line}{newline}{last_line}");
+                        deps.get_mut(index).unwrap().decor_mut().set_prefix(prefix);
+
+                        let prefix = format!("{newline}{rest}");
+                        deps.get_mut(index + 1)
+                            .unwrap()
+                            .decor_mut()
+                            .set_prefix(prefix);
+                    } else {
+                        deps.get_mut(index).unwrap().decor_mut().set_prefix(prefix);
+                    }
+                } else {
+                    deps.get_mut(index).unwrap().decor_mut().set_prefix(prefix);
+                }
             }
 
             reformat_array_multiline(deps);

crates/uv/tests/it/edit.rs

@@ -7681,3 +7681,152 @@ fn add_preserves_trailing_depth() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn add_preserves_first_own_line_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 = [
+            # comment
+            "sniffio==1.3.1",
+        ]
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("charset-normalizer"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    Prepared 2 packages in [TIME]
+    Installed 2 packages in [TIME]
+     + charset-normalizer==3.3.2
+     + 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 = [
+            "charset-normalizer>=3.3.2",
+            # comment
+            "sniffio==1.3.1",
+        ]
+        "###
+        );
+    });
+    Ok(())
+}
+
+#[test]
+fn add_preserves_first_line_bracket_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 = [ # comment
+            "sniffio==1.3.1",
+        ]
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("charset-normalizer"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    Prepared 2 packages in [TIME]
+    Installed 2 packages in [TIME]
+     + charset-normalizer==3.3.2
+     + 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 = [ # comment
+            "charset-normalizer>=3.3.2",
+            "sniffio==1.3.1",
+        ]
+        "###
+        );
+    });
+    Ok(())
+}
+
+#[test]
+fn add_no_indent() -> 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 = [
+"sniffio==1.3.1"
+        ]
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("charset-normalizer"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    Prepared 2 packages in [TIME]
+    Installed 2 packages in [TIME]
+     + charset-normalizer==3.3.2
+     + 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 = [
+            "charset-normalizer>=3.3.2",
+            "sniffio==1.3.1",
+        ]
+        "###
+        );
+    });
+    Ok(())
+}