Preserve comments in empty statements (rust-lang#4180)

* Preserve comments in empty statements

Closes rust-lang#4018

* fixup! Preserve comments in empty statements

rustfmt-core/rustfmt-lib/src/formatting/stmt.rs

@@ -53,6 +53,10 @@ impl<'a> Stmt<'a> {
         result
     }
 
+    pub(crate) fn is_empty(&self) -> bool {
+        matches!(self.inner.kind, ast::StmtKind::Empty)
+    }
+
     fn is_last_expr(&self) -> bool {
         if !self.is_last {
             return false;

rustfmt-core/rustfmt-lib/src/formatting/visitor.rs

@@ -25,7 +25,7 @@ use crate::formatting::{
     utils::{
         self, contains_skip, count_newlines, depr_skip_annotation, inner_attributes,
         last_line_contains_single_line_comment, last_line_width, mk_sp, ptr_vec_to_ref_vec,
-        rewrite_ident, stmt_expr,
+        rewrite_ident, starts_with_newline, stmt_expr,
     },
 };
 use crate::result::{ErrorKind, FormatError};
@@ -120,10 +120,22 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
             self.parse_sess.span_to_debug_info(stmt.span())
         );
 
-        // https://github.com/rust-lang/rust/issues/63679.
-        let is_all_semicolons =
-            |snippet: &str| snippet.chars().all(|c| c.is_whitespace() || c == ';');
-        if is_all_semicolons(&self.snippet(stmt.span())) {
+        if stmt.is_empty() {
+            // If the statement is empty, just skip over it. Before that, make sure any comment
+            // snippet preceding the semicolon is picked up.
+            let snippet = self.snippet(mk_sp(self.last_pos, stmt.span().lo()));
+            let original_starts_with_newline = snippet
+                .find(|c| c != ' ')
+                .map_or(false, |i| starts_with_newline(&snippet[i..]));
+            let snippet = snippet.trim();
+            if !snippet.is_empty() {
+                if original_starts_with_newline {
+                    self.push_str("\n");
+                }
+                self.push_str(&self.block_indent.to_string(self.config));
+                self.push_str(snippet);
+            }
+
             self.last_pos = stmt.span().hi();
             return;
         }

rustfmt-core/rustfmt-lib/tests/source/issue-4018.rs

@@ -0,0 +1,13 @@
+fn main() {
+    ;
+           /* extra comment */            ;
+}
+
+fn main() {
+    println!("");
+    // comment 1
+    // comment 2
+    // comment 3
+    // comment 4
+    ;
+}

rustfmt-core/rustfmt-lib/tests/target/issue-4018.rs

@@ -0,0 +1,11 @@
+fn main() {
+    /* extra comment */
+}
+
+fn main() {
+    println!("");
+    // comment 1
+    // comment 2
+    // comment 3
+    // comment 4
+}