Not moving the first line comment of blocks

src/formatting/syntux/session.rs

@@ -1,4 +1,5 @@
 use std::cell::RefCell;
+use std::ops::Range;
 use std::path::Path;
 use std::rc::Rc;
 
@@ -189,6 +190,15 @@ impl ParseSess {
         }
     }
 
+    pub(crate) fn line_bounds(&self, pos: BytePos) -> Option<Range<BytePos>> {
+        let line = self.parse_sess.source_map().lookup_line(pos).ok();
+
+        match line {
+            Some(line_info) => Some(line_info.sf.line_bounds(line_info.line)),
+            None => None,
+        }
+    }
+
     pub(crate) fn line_of_byte_pos(&self, pos: BytePos) -> usize {
         self.parse_sess.source_map().lookup_char_pos(pos).line
     }

src/formatting/visitor.rs

@@ -7,7 +7,7 @@ use rustc_span::{symbol, BytePos, Pos, Span, DUMMY_SP};
 use crate::config::{BraceStyle, Config};
 use crate::formatting::{
     attr::*,
-    comment::{contains_comment, rewrite_comment, CodeCharKind, CommentCodeSlices},
+    comment::{comment_style, contains_comment, rewrite_comment, CodeCharKind, CommentCodeSlices},
     items::{
         format_impl, format_trait, format_trait_alias, is_mod_decl, is_use_item,
         rewrite_associated_impl_type, rewrite_extern_crate, rewrite_opaque_impl_type,
@@ -263,6 +263,36 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
         self.block_indent = self.block_indent.block_indent(self.config);
         self.push_str("{");
 
+        if has_braces {
+            let block_line_range = self.parse_sess.lookup_line_range(b.span);
+            if block_line_range.lo != block_line_range.hi { // Skipping if a single line block
+                let first_line_contains_stmt = if let Some(first_stmt) = b.stmts.first() {
+                    self.parse_sess.lookup_line_range(first_stmt.span).lo == block_line_range.lo
+                } else {
+                    false
+                };
+
+                let first_line_bounds = self.parse_sess.line_bounds(self.last_pos).unwrap();
+                let first_line_snip = self
+                    .snippet(mk_sp(self.last_pos, first_line_bounds.end))
+                    .trim();
+
+                if !first_line_contains_stmt
+                    && contains_comment(first_line_snip)
+                    && comment_style(first_line_snip, self.config.normalize_comments())
+                        .is_line_comment()
+                {
+                    if let Some(comment) =
+                        rewrite_comment(first_line_snip, false, self.shape(), self.config)
+                    {
+                        self.push_str(" ");
+                        self.push_str(&comment);
+                        self.last_pos = self.last_pos + BytePos(first_line_snip.len() as u32);
+                    }
+                }
+            }
+        }
+
         let first_non_ws = inner_attrs
             .and_then(|attrs| attrs.first().map(|attr| attr.span.lo()))
             .or_else(|| b.stmts.first().map(|s| s.span().lo()));

tests/source/issue-3255.rs

@@ -0,0 +1,26 @@
+fn foo(){  
+    if true {      // Sample comment
+        1
+    }
+}
+
+
+fn foo(){  
+    if true {
+        // Sample comment
+        1
+    }
+}
+
+fn foo(){  
+    if true {    /* Sample comment */
+        1
+    }
+}
+
+fn foo(){  
+    if true {     /* Sample
+                 comment */
+        1
+    }
+}

tests/target/async_fn.rs

@@ -13,8 +13,7 @@ async unsafe fn foo() {
 }
 
 async unsafe fn rust() {
-    async move {
-        // comment
+    async move { // comment
         Ok(())
     }
 }

tests/target/control-brace-style-always-next-line.rs

@@ -23,8 +23,7 @@ fn main() {
 
 
     'while_label: while cond
-    {
-        // while comment
+    { // while comment
         let foo = ();
     }
 

tests/target/control-brace-style-always-same-line.rs

@@ -18,8 +18,7 @@ fn main() {
     }
 
 
-    'while_label: while cond {
-        // while comment
+    'while_label: while cond { // while comment
         let foo = ();
     }
 

tests/target/issue-3255.rs

@@ -0,0 +1,27 @@
+fn foo() {
+    if true { // Sample comment
+        1
+    }
+}
+
+fn foo() {
+    if true {
+        // Sample comment
+        1
+    }
+}
+
+fn foo() {
+    if true {
+        /* Sample comment */
+        1
+    }
+}
+
+fn foo() {
+    if true {
+        /* Sample
+        comment */
+        1
+    }
+}

tests/target/match.rs

@@ -7,8 +7,7 @@ fn foo() {
         // Some comment.
         a => foo(),
         b if 0 < 42 => foo(),
-        c => {
-            // Another comment.
+        c => { // Another comment.
             // Comment.
             an_expression;
             foo()