refactor: split apply

flake.nix

@@ -105,6 +105,7 @@
           cargo-tarpaulin
           clippy
           jq
+          linuxPackages_latest.perf
           nodejs
           nodePackages.prettier
           nodePackages.prettier-plugin-toml

src/alejandra_engine/src/children.rs

@@ -96,10 +96,6 @@ impl Children {
         self.get(self.current_index)
     }
 
-    pub fn peek_prev(&mut self) -> Option<rnix::SyntaxElement> {
-        self.get(self.current_index - 1)
-    }
-
     pub fn move_next(&mut self) {
         self.current_index += 1
     }

src/alejandra_engine/src/parsers/apply.rs

@@ -0,0 +1,38 @@
+use std::collections::LinkedList;
+
+#[derive(Debug, Default)]
+pub(crate) struct Apply {
+    pub left:     Option<rnix::SyntaxElement>,
+    pub comments: LinkedList<String>,
+    pub newline:  bool,
+    pub right:    Option<rnix::SyntaxElement>,
+}
+
+pub(crate) fn parse(
+    build_ctx: &crate::builder::BuildCtx,
+    node: &rnix::SyntaxNode,
+) -> Apply {
+    let mut apply = Apply::default();
+
+    let mut children = crate::children::Children::new(build_ctx, node);
+
+    // left
+    apply.left = Some(children.get_next().unwrap());
+
+    // /**/
+    children.drain_trivia(|element| match element {
+        crate::children::Trivia::Comment(text) => {
+            apply.comments.push_back(text);
+        }
+        crate::children::Trivia::Whitespace(text) => {
+            if !apply.newline {
+                apply.newline = crate::utils::count_newlines(&text) > 0;
+            }
+        }
+    });
+
+    // right
+    apply.right = Some(children.get_next().unwrap());
+
+    apply
+}

src/alejandra_engine/src/parsers/mod.rs

@@ -1,2 +1,3 @@
+pub(crate) mod apply;
 pub(crate) mod if_else;
 pub(crate) mod pattern;

src/alejandra_engine/src/rules/apply.rs

@@ -4,55 +4,51 @@ pub(crate) fn rule(
 ) -> std::collections::LinkedList<crate::builder::Step> {
     let mut steps = std::collections::LinkedList::new();
 
-    let mut children = crate::children::Children::new(build_ctx, node);
+    let apply = crate::parsers::apply::parse(build_ctx, node);
 
-    let vertical = children.has_comments()
-        || children.has_newlines()
-        || build_ctx.vertical;
+    let vertical =
+        build_ctx.vertical || apply.newline || !apply.comments.is_empty();
 
     // left
-    let child = children.get_next().unwrap();
-
+    let element = apply.left.unwrap();
     if vertical {
-        steps.push_back(crate::builder::Step::FormatWider(child));
+        steps.push_back(crate::builder::Step::FormatWider(element));
     } else {
-        steps.push_back(crate::builder::Step::Format(child));
+        steps.push_back(crate::builder::Step::Format(element));
     }
 
     // /**/
-    children.drain_trivia(|element| match element {
-        crate::children::Trivia::Comment(text) => {
+    let comments = !apply.comments.is_empty();
+    if comments {
+        for text in apply.comments {
             steps.push_back(crate::builder::Step::NewLine);
             steps.push_back(crate::builder::Step::Pad);
             steps.push_back(crate::builder::Step::Comment(text));
         }
-        crate::children::Trivia::Whitespace(_) => {}
-    });
-
-    let child_prev = children.peek_prev().unwrap();
+    }
 
     // right
-    let child = children.get_next().unwrap();
+    let element = apply.right.unwrap();
     if vertical {
-        if let rnix::SyntaxKind::TOKEN_COMMENT
-        | rnix::SyntaxKind::TOKEN_WHITESPACE = child_prev.kind()
-        {
-            steps.push_back(crate::builder::Step::NewLine);
-            steps.push_back(crate::builder::Step::Pad);
-        } else if let rnix::SyntaxKind::NODE_ATTR_SET
-        | rnix::SyntaxKind::NODE_LIST
-        | rnix::SyntaxKind::NODE_PAREN
-        | rnix::SyntaxKind::NODE_STRING = child.kind()
+        if !apply.newline
+            && !comments
+            && matches!(
+                element.kind(),
+                rnix::SyntaxKind::NODE_ATTR_SET
+                    | rnix::SyntaxKind::NODE_LIST
+                    | rnix::SyntaxKind::NODE_PAREN
+                    | rnix::SyntaxKind::NODE_STRING
+            )
         {
             steps.push_back(crate::builder::Step::Whitespace);
         } else {
             steps.push_back(crate::builder::Step::NewLine);
             steps.push_back(crate::builder::Step::Pad);
         };
-        steps.push_back(crate::builder::Step::FormatWider(child));
+        steps.push_back(crate::builder::Step::FormatWider(element));
     } else {
         steps.push_back(crate::builder::Step::Whitespace);
-        steps.push_back(crate::builder::Step::Format(child));
+        steps.push_back(crate::builder::Step::Format(element));
     }
 
     steps