Use different numbers of #s when expanding documentation comments

Any documentation comments that contain raw-string-looking sequences may
pretty-print invalid code when expanding them, as the current logic
always uses the `r"literal"` form, without appending any `#`s.

This commit calculates the minimum number of `#`s required to wrap a
comment correctly and appends `#`s appropriately.

Fixes #27489.

src/libstd/macros.rs

@@ -365,6 +365,9 @@ pub mod builtin {
     /// stringification of all the tokens passed to the macro. No restrictions
     /// are placed on the syntax of the macro invocation itself.
     ///
+    /// Note that the expanded results of the input tokens may change in the
+    /// future. You should be careful if you rely on the output.
+    ///
     /// # Examples
     ///
     /// ```

src/libsyntax/ast.rs

@@ -1174,14 +1174,28 @@ impl TokenTree {
             }
             (&TokenTree::Token(sp, token::DocComment(name)), _) => {
                 let stripped = strip_doc_comment_decoration(&name.as_str());
+
+                // Searches for the occurrences of `"#*` and returns the minimum number of `#`s
+                // required to wrap the text.
+                let num_of_hashes = stripped.chars().scan(0, |cnt, x| {
+                    *cnt = if x == '"' {
+                        1
+                    } else if *cnt != 0 && x == '#' {
+                        *cnt + 1
+                    } else {
+                        0
+                    };
+                    Some(*cnt)
+                }).max().unwrap_or(0);
+
                 TokenTree::Delimited(sp, Rc::new(Delimited {
                     delim: token::Bracket,
                     open_span: sp,
                     tts: vec![TokenTree::Token(sp, token::Ident(token::str_to_ident("doc"),
                                                                 token::Plain)),
                               TokenTree::Token(sp, token::Eq),
                               TokenTree::Token(sp, token::Literal(
-                                  token::StrRaw(token::intern(&stripped), 0), None))],
+                                  token::StrRaw(token::intern(&stripped), num_of_hashes), None))],
                     close_span: sp,
                 }))
             }

src/test/run-pass/macro-doc-raw-str-hashes.rs

@@ -0,0 +1,39 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// The number of `#`s used to wrap the documentation comment should differ regarding the content.
+//
+// Related issue: #27489
+
+macro_rules! homura {
+    ($x:expr, #[$y:meta]) => (assert_eq!($x, stringify!($y)))
+}
+
+fn main() {
+    homura! {
+        r#"doc = r" Madoka""#,
+        /// Madoka
+    };
+
+    homura! {
+        r##"doc = r#" One quote mark: ["]"#"##,
+        /// One quote mark: ["]
+    };
+
+    homura! {
+        r##"doc = r#" Two quote marks: [""]"#"##,
+        /// Two quote marks: [""]
+    };
+
+    homura! {
+        r#####"doc = r####" Raw string ending sequences: ["###]"####"#####,
+        /// Raw string ending sequences: ["###]
+    };
+}