diff --git a/src/formatting/items.rs b/src/formatting/items.rs
index ade5e1e8b16..bd94f577ea6 100644
--- a/src/formatting/items.rs
+++ b/src/formatting/items.rs
@@ -349,12 +349,7 @@ impl<'a> FmtVisitor<'a> {
             self.last_pos = item.span.lo() + BytePos(brace_pos as u32 + 1);
             self.block_indent = self.block_indent.block_indent(self.config);
 
-            if item.body.is_empty() {
-                self.format_missing_no_indent(item.span.hi() - BytePos(1));
-                self.block_indent = self.block_indent.block_unindent(self.config);
-                let indent_str = self.block_indent.to_string(self.config);
-                self.push_str(&indent_str);
-            } else {
+            if !item.body.is_empty() {
                 let first_non_ws = item.body.first().map(|s| s.span().lo());
                 if let Some(opening_nls) = self.advance_to_first_block_item(first_non_ws) {
                     self.push_str(&opening_nls);
@@ -363,10 +358,11 @@ impl<'a> FmtVisitor<'a> {
                 for item in &item.body {
                     self.format_body_element(item);
                 }
-
-                self.block_indent = self.block_indent.block_unindent(self.config);
-                self.format_missing_with_indent(item.span.hi() - BytePos(1));
             }
+            self.format_missing_no_indent(item.span.hi() - BytePos(1));
+            self.block_indent = self.block_indent.block_unindent(self.config);
+            let indent_str = self.block_indent.to_string(self.config);
+            self.push_str(&indent_str);
 
             self.push_str("}");
         } else {
diff --git a/src/formatting/visitor.rs b/src/formatting/visitor.rs
index dea632f703a..06917a704cb 100644
--- a/src/formatting/visitor.rs
+++ b/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::{rewrite_comment, CodeCharKind, CommentCodeSlices},
+    comment::{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,
@@ -316,8 +316,17 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
             self.block_indent = self.block_indent.block_unindent(config);
         }
 
+        let comment_snippet = self.snippet(span);
+
+        let align_to_right = if unindent_comment && contains_comment(&comment_snippet) {
+            let first_lines = comment_snippet.splitn(2, '/').next().unwrap_or("");
+            last_line_width(first_lines) > last_line_width(&comment_snippet)
+        } else {
+            false
+        };
+
         let mut iter = CommentCodeSlices::with_offset(
-            self.snippet(span),
+            comment_snippet,
             last_line_offset,
             self.config.tab_spaces(),
         )
@@ -380,8 +389,13 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
                         _ => self.push_str("\n"),
                     }
                     newline_inserted = true;
-
+                    if unindent_comment && align_to_right {
+                        self.block_indent = self.block_indent.block_indent(self.config);
+                    }
                     self.push_str(&self.block_indent.to_string_with_newline(config));
+                    if unindent_comment && align_to_right {
+                        self.block_indent = self.block_indent.block_unindent(self.config);
+                    }
                 }
             }
             prev_kind = kind;
diff --git a/tests/source/issue-4120.rs b/tests/source/issue-4120.rs
new file mode 100644
index 00000000000..c9ce838c51a
--- /dev/null
+++ b/tests/source/issue-4120.rs
@@ -0,0 +1,85 @@
+fn main() {
+    let x = if true {
+        1
+            // In if
+    } else {
+        0
+            // In else
+    };
+
+    let x = if true {
+        1
+             /* In if */
+    } else {
+        0
+             /* In else */
+    };
+
+    let z = if true {
+        if true {
+            1
+
+                 // In if level 2
+        } else {
+            2
+        }
+    } else {
+        3
+    };
+
+    let a = if true {
+        1
+  // In if
+    } else {
+        0
+  // In else
+    };
+
+    let a = if true {
+        1
+
+    // In if
+    } else {
+        0
+    // In else
+    };
+
+    let b = if true {
+        1
+
+    // In if
+    } else {
+        0
+        // In else
+    };
+
+    let c = if true {
+        1
+
+        // In if
+    } else {
+        0
+        // In else
+    };
+    for i in 0..2 {
+        println!("Something");
+        // In for
+    }
+
+    for i in 0..2 {
+        println!("Something");
+        /* In for */
+    }
+
+    extern "C" {
+        fn first();
+
+        // In foreign mod
+    }
+
+    extern "C" {
+        fn first();
+
+        /* In foreign mod */
+    }
+}
diff --git a/tests/target/issue-4120.rs b/tests/target/issue-4120.rs
new file mode 100644
index 00000000000..a7d461dcfdb
--- /dev/null
+++ b/tests/target/issue-4120.rs
@@ -0,0 +1,85 @@
+fn main() {
+    let x = if true {
+        1
+        // In if
+    } else {
+        0
+        // In else
+    };
+
+    let x = if true {
+        1
+        /* In if */
+    } else {
+        0
+        /* In else */
+    };
+
+    let z = if true {
+        if true {
+            1
+
+            // In if level 2
+        } else {
+            2
+        }
+    } else {
+        3
+    };
+
+    let a = if true {
+        1
+    // In if
+    } else {
+        0
+        // In else
+    };
+
+    let a = if true {
+        1
+
+    // In if
+    } else {
+        0
+        // In else
+    };
+
+    let b = if true {
+        1
+
+    // In if
+    } else {
+        0
+        // In else
+    };
+
+    let c = if true {
+        1
+
+        // In if
+    } else {
+        0
+        // In else
+    };
+    for i in 0..2 {
+        println!("Something");
+        // In for
+    }
+
+    for i in 0..2 {
+        println!("Something");
+        /* In for */
+    }
+
+    extern "C" {
+        fn first();
+
+        // In foreign mod
+    }
+
+    extern "C" {
+        fn first();
+
+        /* In foreign mod */
+    }
+}