From 5a7662ca79840f31cb4682ab23c0176ba2b711fb Mon Sep 17 00:00:00 2001
From: Joe Neeman <joeneeman@gmail.com>
Date: Fri, 5 Apr 2024 19:29:10 -0500
Subject: [PATCH] Avoid backslashes before a new block.

---
 fuzz/Cargo.lock         |  2 +-
 src/cm.rs               | 15 ++++++++++++---
 src/tests/commonmark.rs | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/fuzz/Cargo.lock b/fuzz/Cargo.lock
index 74bb410a..5e73097d 100644
--- a/fuzz/Cargo.lock
+++ b/fuzz/Cargo.lock
@@ -128,7 +128,7 @@ checksum = "4f0807fb6f644c83f3e4ec014fec9858c1c8b26a7db8eb5f0bde5817df9c1df7"
 
 [[package]]
 name = "comrak"
-version = "0.20.0"
+version = "0.22.0"
 dependencies = [
  "arbitrary",
  "clap",
diff --git a/src/cm.rs b/src/cm.rs
index 3c2cfbc4..17b30888 100644
--- a/src/cm.rs
+++ b/src/cm.rs
@@ -326,6 +326,9 @@ impl<'a, 'o> CommonMarkFormatter<'a, 'o> {
                 )
                 && self.get_in_tight_list_item(node);
         }
+        let next_is_block = node
+            .next_sibling()
+            .map_or(true, |next| next.data.borrow().value.block());
 
         match node.data.borrow().value {
             NodeValue::Document => (),
@@ -345,7 +348,7 @@ impl<'a, 'o> CommonMarkFormatter<'a, 'o> {
             NodeValue::Text(ref literal) => {
                 self.format_text(literal.as_bytes(), allow_wrap, entering)
             }
-            NodeValue::LineBreak => self.format_line_break(entering),
+            NodeValue::LineBreak => self.format_line_break(entering, next_is_block),
             NodeValue::SoftBreak => self.format_soft_break(allow_wrap, entering),
             NodeValue::Code(ref code) => {
                 self.format_code(code.literal.as_bytes(), allow_wrap, entering)
@@ -571,10 +574,16 @@ impl<'a, 'o> CommonMarkFormatter<'a, 'o> {
         }
     }
 
-    fn format_line_break(&mut self, entering: bool) {
+    fn format_line_break(&mut self, entering: bool, next_is_block: bool) {
         if entering {
             if !self.options.render.hardbreaks {
-                write!(self, "\\").unwrap();
+                if !next_is_block {
+                    // If the next element is a block, a backslash means a
+                    // literal backslash instead of a line break. In this case
+                    // we can just skip the line break since it's meaningless
+                    // before a block.
+                    write!(self, "\\").unwrap();
+                }
             }
             self.cr();
         }
diff --git a/src/tests/commonmark.rs b/src/tests/commonmark.rs
index 622b4d81..3f297e02 100644
--- a/src/tests/commonmark.rs
+++ b/src/tests/commonmark.rs
@@ -1,3 +1,7 @@
+use std::cell::RefCell;
+
+use self::nodes::{Ast, LineColumn};
+
 use super::*;
 use ntest::test_case;
 
@@ -11,6 +15,36 @@ fn commonmark_removes_redundant_strong() {
     commonmark(input, output, Some(&options));
 }
 
+#[test]
+fn commonmark_avoids_spurious_backslash() {
+    let arena = Arena::new();
+    let options = Options::default();
+    let empty = LineColumn { line: 0, column: 0 };
+
+    let ast = |val: NodeValue| arena.alloc(AstNode::new(RefCell::new(Ast::new(val, empty))));
+    let root = ast(NodeValue::Document);
+
+    let p1 = ast(NodeValue::Paragraph);
+    p1.append(ast(NodeValue::Text("Line 1".to_owned())));
+    root.append(p1);
+
+    root.append(ast(NodeValue::LineBreak));
+
+    let p2 = ast(NodeValue::Paragraph);
+    p2.append(ast(NodeValue::Text("Line 2".to_owned())));
+    root.append(p2);
+
+    let mut output = vec![];
+    cm::format_document(root, &options, &mut output).unwrap();
+
+    compare_strs(
+        &String::from_utf8(output).unwrap(),
+        "Line 1\n\nLine 2\n",
+        "rendered",
+        "<synthetic>",
+    );
+}
+
 #[test_case("$$x^2$$ and $1 + 2$ and $`y^2`$", "$$x^2$$ and $1 + 2$ and $`y^2`$\n")]
 #[test_case("$$\nx^2\n$$", "$$\nx^2\n$$\n")]
 #[test_case("```math\nx^2\n```", "``` math\nx^2\n```\n")]