fix(fmt): don't corrupt a for-loop's header with a trailing comment on the init clause

.changelog/fmt-forloop-comment-corruption.md

@@ -0,0 +1,5 @@
+---
+forge: patch
+---
+
+Fixed `forge fmt` silently corrupting `for` loops that have a trailing `//` comment on or after the header's closing brace, which could destroy source code with no error or warning.

crates/fmt/src/state/sol.rs

@@ -2006,12 +2006,22 @@ impl<'ast> State<'_, 'ast> {
     /// Prints the given statement in the source code, handling formatting, inline documentation,
     /// trailing comments and layout logic for various statement kinds.
     fn print_stmt(&mut self, stmt: &'ast ast::Stmt<'ast>) {
+        self.print_stmt_bound(stmt, None);
+    }
+
+    /// Like [`Self::print_stmt`], but bounds the statement's own trailing-comment scan by
+    /// `next_pos` instead of scanning unbounded. Every caller other than a `for`-loop's init
+    /// clause prints one statement per line, where an unbounded scan is harmless because
+    /// nothing else follows on that line. A `for`-loop's init clause is different: it shares
+    /// its source line with the condition/increment clauses that get printed afterwards, so an
+    /// unbounded scan can swallow those clauses into the comment (see `print_for_stmt`).
+    fn print_stmt_bound(&mut self, stmt: &'ast ast::Stmt<'ast>, next_pos: Option<BytePos>) {
         let ast::Stmt { ref docs, span, ref kind } = *stmt;
         self.print_docs(docs);
 
         // Handle disabled statements.
         if self.handle_span(span, false) {
-            self.print_trailing_comment_no_break(stmt.span.hi(), None);
+            self.print_trailing_comment_no_break(stmt.span.hi(), next_pos);
             return;
         }
 
@@ -2080,7 +2090,7 @@ impl<'ast> State<'_, 'ast> {
             stmt.span.hi(),
             CommentConfig::default().trailing_no_break().mixed_no_break().mixed_prev_space(),
         );
-        self.print_trailing_comment_no_break(stmt.span.hi(), None);
+        self.print_trailing_comment_no_break(stmt.span.hi(), next_pos);
     }
 
     /// Prints an `assembly` statement, including optional dialect and flags,
@@ -2180,7 +2190,9 @@ impl<'ast> State<'_, 'ast> {
         // Print init.
         self.s.cbox(0);
         match init {
-            Some(init_stmt) => self.print_stmt(init_stmt),
+            Some(init_stmt) => {
+                self.print_stmt_bound(init_stmt, Some(init_stmt.span.hi()));
+            }
             None => self.print_word(";"),
         }
 

crates/fmt/testdata/ForStatementComments/fmt.sol

@@ -0,0 +1,49 @@
+pragma solidity ^0.8.8;
+
+contract ForStatementComments {
+    function bracedTrailingComment() external {
+        uint256 x;
+        for (uint256 i = 0; i < 10; ++i) {
+            // step
+            x++;
+        }
+    }
+
+    function bracelessTrailingComment() external {
+        uint256 x;
+        for (uint256 i = 0; i < 10; ++i) {
+            x++; // step
+        }
+    }
+
+    function emptyBodyTrailingComment() external {
+        for (uint256 i = 0; i < 10; ++i) { // step
+        }
+    }
+
+    function leadingCommentUnaffected() external {
+        // leading comment, must not move
+        for (uint256 i = 0; i < 10; ++i) {
+            i;
+        }
+    }
+
+    function commentAfterHeaderNoCondNoNext() external {
+        for (uint256 i = 0;;)  // after header
+        {}
+    }
+
+    function missingConditionTrailingComment() external {
+        for (uint256 i = 0;; ++i) {
+            // c
+            i;
+        }
+    }
+
+    function missingIncrementTrailingComment() external {
+        for (uint256 i = 0; i < 10;) {
+            // c
+            i;
+        }
+    }
+}

crates/fmt/testdata/ForStatementComments/original.sol

@@ -0,0 +1,44 @@
+pragma solidity ^0.8.8;
+
+contract ForStatementComments {
+    function bracedTrailingComment() external {
+        uint256 x;
+        for (uint256 i = 0; i < 10; ++i) { // step
+            x++;
+        }
+    }
+
+    function bracelessTrailingComment() external {
+        uint256 x;
+        for (uint256 i = 0; i < 10; ++i) x++; // step
+    }
+
+    function emptyBodyTrailingComment() external {
+        for (uint256 i = 0; i < 10; ++i) { // step
+        }
+    }
+
+    function leadingCommentUnaffected() external {
+        // leading comment, must not move
+        for (uint256 i = 0; i < 10; ++i) {
+            i;
+        }
+    }
+
+    function commentAfterHeaderNoCondNoNext() external {
+        for (uint256 i = 0;;) // after header
+        {}
+    }
+
+    function missingConditionTrailingComment() external {
+        for (uint256 i = 0; ; ++i) { // c
+            i;
+        }
+    }
+
+    function missingIncrementTrailingComment() external {
+        for (uint256 i = 0; i < 10;) { // c
+            i;
+        }
+    }
+}

crates/fmt/tests/formatter.rs

@@ -247,6 +247,7 @@ fmt_tests! {
     ErrorDefinition,
     EventDefinition,
     ForStatement,
+    ForStatementComments,
     FunctionCall,
     FunctionCallArgsStatement,
     FunctionDefinition,