Check for inline comments that will force hanging in function arguments (#1025)

* Add test case

* Check for inline comments that will force hanging in function arguments

* Update snapshots

* Update changelog

* Fix has_inline_comments to not recurse into nested function bodies

The previous implementation used contains_comments(lhs) which checked
all tokens in the entire LHS tree, incorrectly detecting comments
inside nested function bodies as "inline" comments of the outer binary
expression.

Now we only check for comments that are actually inline within the
binary expression: trailing comments on LHS, leading comments on RHS,
and recursively check nested compound expressions.

* Only match singleline comments

Author: JohnnyMorganz

CHANGELOG.md

@@ -16,6 +16,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Fixed syntax error in output when a single-line comment appears between an index suffix and a table call argument, e.g. `foo.bar -- comment { }` ([#873](https://github.com/JohnnyMorganz/StyLua/issues/873))
 
+### Fixed
+
+- Fixed malformed formatting when a binary expression inside of a function call with comments around the operators is incorrectly collapsed onto one line ([#996](https://github.com/JohnnyMorganz/StyLua/issues/996))
+
 ## [2.3.1] - 2025-11-01
 
 ### Fixed

src/formatters/functions.rs

@@ -147,11 +147,13 @@ fn function_args_contains_comments(
         true
     } else {
         arguments.pairs().any(|argument| {
-            // Leading / Trailing trivia of expression (ignore inline comments)
+            // Leading / Trailing trivia of expression
             argument.value().leading_trivia()
                 .iter()
                 .chain(argument.value().trailing_trivia().iter())
                 .any(function_trivia_contains_comments)
+            // Inline comments that will force hanging
+            || argument.value().has_inline_comments()
             // Punctuation contains comments
             || argument
                 .punctuation()

src/formatters/trivia_util.rs

@@ -1172,7 +1172,11 @@ impl HasInlineComments for Expression {
     fn has_inline_comments(&self) -> bool {
         match self {
             Expression::BinaryOperator { lhs, binop, rhs } => {
-                contains_comments(binop) || contains_comments(lhs) || rhs.has_inline_comments()
+                contains_comments(binop)
+                    || lhs.has_trailing_comments(CommentSearch::Single)
+                    || rhs.has_leading_comments(CommentSearch::Single)
+                    || lhs.has_inline_comments()
+                    || rhs.has_inline_comments()
             }
             Expression::UnaryOperator { unop, expression } => {
                 let op_contains_comments = match unop {

tests/inputs/hang-binop-comments-3.lua

@@ -0,0 +1,10 @@
+-- https://github.com/JohnnyMorganz/StyLua/issues/996
+
+local function f()
+  local a = "ab"
+  a = a:gsub("a".. -- "
+    'b', function() return "c" end)
+  print(a)
+end
+
+f()

tests/snapshots/[email protected]

@@ -0,0 +1,20 @@
+---
+source: tests/tests.rs
+expression: "format(&contents, LuaVersion::Lua51)"
+input_file: tests/inputs/hang-binop-comments-3.lua
+---
+-- https://github.com/JohnnyMorganz/StyLua/issues/996
+
+local function f()
+	local a = "ab"
+	a = a:gsub(
+		"a" -- "
+			.. "b",
+		function()
+			return "c"
+		end
+	)
+	print(a)
+end
+
+f()