diff --git a/rustfmt-core/src/comment.rs b/rustfmt-core/src/comment.rs
index 9c0322bcaeb1..dc0c59a617e9 100644
--- a/rustfmt-core/src/comment.rs
+++ b/rustfmt-core/src/comment.rs
@@ -214,11 +214,25 @@ pub fn combine_strs_with_missing_comments(
     ))
 }
 
+pub fn rewrite_doc_comment(orig: &str, shape: Shape, config: &Config) -> Option<String> {
+    _rewrite_comment(orig, false, shape, config, true)
+}
+
 pub fn rewrite_comment(
     orig: &str,
     block_style: bool,
     shape: Shape,
     config: &Config,
+) -> Option<String> {
+    _rewrite_comment(orig, block_style, shape, config, false)
+}
+
+fn _rewrite_comment(
+    orig: &str,
+    block_style: bool,
+    shape: Shape,
+    config: &Config,
+    is_doc_comment: bool,
 ) -> Option<String> {
     // If there are lines without a starting sigil, we won't format them correctly
     // so in that case we won't even re-align (if !config.normalize_comments()) and
@@ -231,7 +245,7 @@ pub fn rewrite_comment(
         return Some(orig.to_owned());
     }
     if !config.normalize_comments() && !config.wrap_comments() {
-        return light_rewrite_comment(orig, shape.indent, config);
+        return light_rewrite_comment(orig, shape.indent, config, is_doc_comment);
     }
 
     identify_comment(orig, block_style, shape, config)
@@ -494,15 +508,29 @@ pub fn recover_missing_comment_in_span(
     }
 }
 
+/// Trim trailing whitespaces unless they consist of two or more whitespaces.
+fn trim_right_unless_two_whitespaces(s: &str, is_doc_comment: bool) -> &str {
+    if is_doc_comment && s.ends_with("  ") {
+        s
+    } else {
+        s.trim_right()
+    }
+}
+
 /// Trims whitespace and aligns to indent, but otherwise does not change comments.
-fn light_rewrite_comment(orig: &str, offset: Indent, config: &Config) -> Option<String> {
+fn light_rewrite_comment(
+    orig: &str,
+    offset: Indent,
+    config: &Config,
+    is_doc_comment: bool,
+) -> Option<String> {
     let lines: Vec<&str> = orig.lines()
         .map(|l| {
             // This is basically just l.trim(), but in the case that a line starts
             // with `*` we want to leave one space before it, so it aligns with the
             // `*` in `/*`.
             let first_non_whitespace = l.find(|c| !char::is_whitespace(c));
-            if let Some(fnw) = first_non_whitespace {
+            let left_trimmed = if let Some(fnw) = first_non_whitespace {
                 if l.as_bytes()[fnw] == b'*' && fnw > 0 {
                     &l[fnw - 1..]
                 } else {
@@ -510,7 +538,9 @@ fn light_rewrite_comment(orig: &str, offset: Indent, config: &Config) -> Option<
                 }
             } else {
                 ""
-            }.trim_right()
+            };
+            // Preserve markdown's double-space line break syntax in doc comment.
+            trim_right_unless_two_whitespaces(left_trimmed, is_doc_comment)
         })
         .collect();
     Some(lines.join(&format!("\n{}", offset.to_string(config))))
diff --git a/rustfmt-core/src/visitor.rs b/rustfmt-core/src/visitor.rs
index 0a0d59d0f02c..2d10be39d092 100644
--- a/rustfmt-core/src/visitor.rs
+++ b/rustfmt-core/src/visitor.rs
@@ -19,7 +19,7 @@ use syntax::parse::ParseSess;
 use codemap::{LineRangeUtils, SpanUtils};
 use comment::{combine_strs_with_missing_comments, contains_comment, CodeCharKind,
               CommentCodeSlices, FindUncommented};
-use comment::rewrite_comment;
+use comment::rewrite_doc_comment;
 use config::{BraceStyle, Config};
 use expr::rewrite_literal;
 use items::{format_impl, format_trait, format_trait_alias, rewrite_associated_impl_type,
@@ -892,7 +892,7 @@ impl Rewrite for ast::Attribute {
                     .unwrap_or(0),
                 ..shape
             };
-            rewrite_comment(snippet, false, doc_shape, context.config)
+            rewrite_doc_comment(snippet, doc_shape, context.config)
         } else {
             if contains_comment(snippet) {
                 return Some(snippet.to_owned());
@@ -957,7 +957,7 @@ fn rewrite_first_group_attrs(
             .join("\n");
         return Some((
             sugared_docs.len(),
-            rewrite_comment(&snippet, false, shape, context.config)?,
+            rewrite_doc_comment(&snippet, shape, context.config)?,
         ));
     }
     // Rewrite `#[derive(..)]`s.
diff --git a/rustfmt-core/tests/source/markdown-comment.rs b/rustfmt-core/tests/source/markdown-comment.rs
new file mode 100644
index 000000000000..1ec26562fe28
--- /dev/null
+++ b/rustfmt-core/tests/source/markdown-comment.rs
@@ -0,0 +1,15 @@
+// Preserve two trailing whitespaces in doc comment,
+// but trim any whitespaces in normal comment.
+
+//! hello world  
+//! hello world 
+
+/// hello world    
+/// hello world 
+/// hello world  
+fn foo() {
+    // hello world  
+    // hello world 
+    let x = 3;
+    println!("x = {}", x);
+}
diff --git a/rustfmt-core/tests/target/markdown-comment.rs b/rustfmt-core/tests/target/markdown-comment.rs
new file mode 100644
index 000000000000..71a9921d2862
--- /dev/null
+++ b/rustfmt-core/tests/target/markdown-comment.rs
@@ -0,0 +1,15 @@
+// Preserve two trailing whitespaces in doc comment,
+// but trim any whitespaces in normal comment.
+
+//! hello world  
+//! hello world
+
+/// hello world    
+/// hello world
+/// hello world  
+fn foo() {
+    // hello world
+    // hello world
+    let x = 3;
+    println!("x = {}", x);
+}