Fix issue of comment indentation on opening line of a block

src/formatting/comment.rs

@@ -156,6 +156,11 @@ pub(crate) fn is_last_comment_block(s: &str) -> bool {
     s.trim_end().ends_with("*/")
 }
 
+/// Returns true if the first line of the passed string started with a block-comment.
+pub(crate) fn is_first_comment_block(s: &str) -> bool {
+    s.trim_start().starts_with("/*")
+}
+
 /// Combine `prev_str` and `next_str` into a single `String`. `span` may contain
 /// comments between two strings. If there are such comments, then that will be
 /// recovered. If `allow_extend` is true and there is no comment between the two

src/formatting/expr.rs

@@ -1205,7 +1205,7 @@ pub(crate) fn is_simple_block_stmt(
         && attrs.map_or(true, |a| a.is_empty())
 }
 
-fn block_has_statements(block: &ast::Block) -> bool {
+pub(crate) fn block_has_statements(block: &ast::Block) -> bool {
     !block
         .stmts
         .iter()

src/formatting/items.rs

@@ -18,7 +18,7 @@ use crate::formatting::{
         FindUncommented,
     },
     expr::{
-        is_empty_block, is_simple_block_stmt, rewrite_assign_rhs, rewrite_assign_rhs_with,
+        block_has_statements, is_simple_block_stmt, rewrite_assign_rhs, rewrite_assign_rhs_with,
         rewrite_assign_rhs_with_comments, RhsTactics,
     },
     lists::{definitive_tactic, itemize_list, write_list, ListFormatting, Separator},
@@ -457,8 +457,15 @@ impl<'a> FmtVisitor<'a> {
 
         let context = self.get_context();
 
+        let block_snip = self.snippet_provider.span_to_snippet(block.span)?;
+        let has_comments = if let Some(firstline_end) = block_snip.find('\n') {
+            contains_comment(&block_snip[firstline_end..])
+        } else {
+            false
+        };
         if self.config.empty_item_single_line()
-            && is_empty_block(&context, block, None)
+            && !block_has_statements(&block)
+            && !has_comments
             && self.block_indent.width() + fn_str.len() + 3 <= self.config.max_width()
             && !last_line_contains_single_line_comment(fn_str)
         {

src/formatting/visitor.rs

@@ -7,7 +7,10 @@ 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::{
+        contains_comment, is_first_comment_block, is_last_comment_block, 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,
@@ -443,6 +446,39 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
         if let Some((fn_str, fn_brace_style)) = rewrite {
             self.format_missing_with_indent(source!(self, s).lo());
 
+            let mut last_pos = source!(self, block.span).lo();
+            let block_snip = self.snippet(block.span);
+            if let Some(first_line) = block_snip.lines().next() {
+                let block_line_range = self.parse_sess.lookup_line_range(block.span);
+                let first_line_contains_stmt = if let Some(first_stmt) = block.stmts.first() {
+                    self.parse_sess.lookup_line_range(first_stmt.span).lo == block_line_range.lo
+                } else {
+                    false
+                };
+
+                let comment_snip = if block_line_range.lo == block_line_range.hi {
+                    // 1= {, 1 = }
+                    &first_line[1..first_line.len() - 1]
+                } else {
+                    // 1= {
+                    &first_line[1..]
+                };
+
+                if !first_line_contains_stmt
+                    && contains_comment(comment_snip)
+                    && (is_first_comment_block(comment_snip) && is_last_comment_block(comment_snip)
+                        || !is_first_comment_block(comment_snip))
+                {
+                    if let Some(comment) =
+                        rewrite_comment(comment_snip.trim(), false, self.shape(), self.config)
+                    {
+                        self.push_str(&comment);
+                        self.push_str(&self.block_indent.to_string_with_newline(self.config));
+                        last_pos = last_pos + BytePos(comment_snip.len() as u32);
+                    }
+                }
+            }
+
             if let Some(rw) = self.single_line_fn(&fn_str, block, inner_attrs) {
                 self.push_str(&rw);
                 self.last_pos = s.hi();
@@ -457,7 +493,7 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
                 }
                 _ => unreachable!(),
             }
-            self.last_pos = source!(self, block.span).lo();
+            self.last_pos = last_pos;
         } else {
             self.format_missing(source!(self, block.span).lo());
         }

tests/source/issue-3255.rs

@@ -0,0 +1,51 @@
+fn foo(){ // Sample comment
+    1
+}
+
+fn foo(){
+    // Sample comment
+    1
+}
+
+// Sample comment
+fn foo(){
+    1
+}
+
+fn bar(){
+fn foo(){ // Sample comment
+    1
+}
+}
+
+fn foo(
+    param: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA,
+    param2: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
+    ) { // Sample comment
+1
+}
+
+fn foo(){} // Sample comment
+
+fn foo(){ /* Sample comment */
+    1
+}
+
+fn foo(){ /* Sample comment */ }
+
+fn foo(){ /* Sample 
+             comment */
+    1
+}
+
+fn foo(){ // Some comment
+}
+
+// Test
+fn main() {}
+
+fn main() {/* Test */ let a =0; let b=1;}
+
+fn foo(){ // Ends with }
+
+}

tests/target/comment.rs

@@ -59,8 +59,8 @@ fn issue_1086() {
 
 // random comment
 
-fn main() { // Test
-}
+// Test
+fn main() {}
 
 // #1643
 fn some_fn() // some comment

tests/target/issue-3255.rs

@@ -0,0 +1,60 @@
+// Sample comment
+fn foo() {
+    1
+}
+
+fn foo() {
+    // Sample comment
+    1
+}
+
+// Sample comment
+fn foo() {
+    1
+}
+
+fn bar() {
+    // Sample comment
+    fn foo() {
+        1
+    }
+}
+
+// Sample comment
+fn foo(
+    param: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA,
+    param2: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA,
+) {
+    1
+}
+
+fn foo() {} // Sample comment
+
+/* Sample comment */
+fn foo() {
+    1
+}
+
+/* Sample comment */
+fn foo() {}
+
+fn foo() {
+    /* Sample
+    comment */
+    1
+}
+
+// Some comment
+fn foo() {}
+
+// Test
+fn main() {}
+
+fn main() {
+    /* Test */
+    let a = 0;
+    let b = 1;
+}
+
+// Ends with }
+fn foo() {}