make the formatter faster by not counting chars of comments

Author: giacomocavalieri

format/src/lib.rs

@@ -271,7 +271,7 @@ impl<'a, 'doc> Formatter<'a> {
             self.doc_comments.iter().map(|comment| {
                 DOC_COMMENT_DOCUMENT
                     .to_doc(arena)
-                    .append(arena, EcoString::from(comment.content))
+                    .append(arena, arena.zero_width_str(comment.content))
             }),
             LINE_DOCUMENT,
         );
@@ -286,7 +286,7 @@ impl<'a, 'doc> Formatter<'a> {
             let comments = self.module_comments.iter().map(|s| {
                 MODULE_COMMENT_DOCUMENT
                     .to_doc(arena)
-                    .append(arena, EcoString::from(s.content))
+                    .append(arena, arena.zero_width_str(s.content))
             });
             arena
                 .join(comments, LINE_DOCUMENT)
@@ -845,7 +845,7 @@ impl<'a, 'doc> Formatter<'a> {
                 .join(
                     comments.map(|comment| match comment {
                         Some(comment) => {
-                            DOC_COMMENT_DOCUMENT.append(arena, EcoString::from(comment))
+                            DOC_COMMENT_DOCUMENT.append(arena, arena.zero_width_str(comment))
                         }
                         None => unreachable!("empty lines dropped by pop_doc_comments"),
                     }),
@@ -3660,7 +3660,9 @@ impl<'a, 'doc> Formatter<'a> {
                 }
                 (_, None) => continue,
             };
-            doc.push(COMMENT_DOCUMENT.append(arena, EcoString::from(comment)));
+            doc.push(
+                COMMENT_DOCUMENT.append(arena, arena.zero_width_string(EcoString::from(comment))),
+            );
             match comments.peek() {
                 // Next line is a comment
                 Some((_, Some(_))) => doc.push(LINE_DOCUMENT),
@@ -4246,12 +4248,17 @@ fn printed_comments<'a, 'doc>(
     let _ = comments.peek()?;
 
     let mut doc = Vec::new();
-    while let Some(c) = comments.next() {
-        let c = match c {
-            Some(c) => c,
-            None => continue,
-        };
-        doc.push("//".to_doc(arena).append(arena, EcoString::from(c)));
+    while let Some(comment) = comments.next() {
+        let Some(comment) = comment else { continue };
+
+        // The comment is turned into a zero width string rather than a regular
+        // string document: comment lines are never touched by the formatter and
+        // we don't need to know how long each one is.
+        // So we can do this to avoid counting the graphemes of each one, which
+        // is a lot of wasted work.
+        let comment = arena.zero_width_str(comment);
+
+        doc.push(COMMENT_DOCUMENT.append(arena, comment));
         match comments.peek() {
             // Next line is a comment
             Some(Some(_)) => doc.push(LINE_DOCUMENT),

pretty-arena/src/lib.rs

@@ -1087,6 +1087,17 @@ impl<'string, 'doc> DocumentArena<'string, 'doc> {
         )
     }
 
+    /// Same as the `zero_width_string` but this works with string references.
+    /// This is useful when you already have a string reference and you want to
+    /// avoid allocating a further string.
+    ///
+    pub fn zero_width_str(&'doc self, string: &'string str) -> Document<'string, 'doc> {
+        Document(
+            self.documents
+                .alloc(PrintableDocument::ZeroWidthStr { string }),
+        )
+    }
+
     /// Joins together an iterator of documents into a single document.
     /// All the documents are gonna be rendered next to each other with no
     /// spaces in between.