Fix normalisation of multiline doc attributes

src/attr.rs

@@ -3,6 +3,7 @@
 use syntax::ast;
 use syntax::source_map::{BytePos, Span, DUMMY_SP};
 
+use self::doc_comment::DocCommentFormatter;
 use crate::comment::{contains_comment, rewrite_doc_comment, CommentStyle};
 use crate::config::lists::*;
 use crate::config::IndentStyle;
@@ -14,6 +15,8 @@ use crate::shape::Shape;
 use crate::types::{rewrite_path, PathContext};
 use crate::utils::{count_newlines, mk_sp};
 
+mod doc_comment;
+
 /// Returns attributes on the given statement.
 pub(crate) fn get_attrs_from_stmt(stmt: &ast::Stmt) -> &[ast::Attribute] {
     match stmt.node {
@@ -330,11 +333,9 @@ impl Rewrite for ast::Attribute {
                             ast::AttrStyle::Outer => CommentStyle::TripleSlash,
                         };
 
-                        // Remove possible whitespace from the `CommentStyle::opener()` so that
-                        // the literal itself has control over the comment's leading spaces.
-                        let opener = comment_style.opener().trim_end();
-
-                        let doc_comment = format!("{}{}", opener, literal);
+                        let doc_comment_formatter =
+                            DocCommentFormatter::new(literal.as_str().get(), comment_style);
+                        let doc_comment = format!("{}", doc_comment_formatter);
                         return rewrite_doc_comment(
                             &doc_comment,
                             shape.comment(context.config),

src/attr/doc_comment.rs

@@ -0,0 +1,77 @@
+use crate::comment::CommentStyle;
+use std::fmt::{self, Display};
+
+/// Formats a string as a doc comment using the given [`CommentStyle`].
+#[derive(new)]
+pub(super) struct DocCommentFormatter<'a> {
+    literal: &'a str,
+    style: CommentStyle<'a>,
+}
+
+impl Display for DocCommentFormatter<'_> {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let opener = self.style.opener().trim_end();
+        let mut lines = self.literal.lines().peekable();
+        while let Some(line) = lines.next() {
+            let is_last_line = lines.peek().is_none();
+            if is_last_line {
+                write!(formatter, "{}{}", opener, line)?;
+            } else {
+                writeln!(formatter, "{}{}", opener, line)?;
+            }
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn literal_controls_leading_spaces() {
+        test_doc_comment_is_formatted_correctly(
+            "    Lorem ipsum",
+            "///    Lorem ipsum",
+            CommentStyle::TripleSlash,
+        );
+    }
+
+    #[test]
+    fn single_line_doc_comment_is_formatted_correctly() {
+        test_doc_comment_is_formatted_correctly(
+            "Lorem ipsum",
+            "///Lorem ipsum",
+            CommentStyle::TripleSlash,
+        );
+    }
+
+    #[test]
+    fn multi_line_doc_comment_is_formatted_correctly() {
+        test_doc_comment_is_formatted_correctly(
+            "Lorem ipsum\nDolor sit amet",
+            "///Lorem ipsum\n///Dolor sit amet",
+            CommentStyle::TripleSlash,
+        );
+    }
+
+    #[test]
+    fn whitespace_within_lines_is_preserved() {
+        test_doc_comment_is_formatted_correctly(
+            " Lorem ipsum \n Dolor sit amet ",
+            "/// Lorem ipsum \n/// Dolor sit amet ",
+            CommentStyle::TripleSlash,
+        );
+    }
+
+    fn test_doc_comment_is_formatted_correctly(
+        literal: &str,
+        expected_comment: &str,
+        style: CommentStyle<'_>,
+    ) {
+        assert_eq!(
+            expected_comment,
+            format!("{}", DocCommentFormatter::new(&literal, style))
+        );
+    }
+}

tests/source/normalize_multiline_doc_attribute.rs

@@ -0,0 +1,7 @@
+// rustfmt-unstable: true
+// rustfmt-normalize_doc_attributes: true
+
+#[doc = "This comment
+is split
+on multiple lines"]
+fn foo() {}

tests/target/normalize_multiline_doc_attribute.rs

@@ -0,0 +1,7 @@
+// rustfmt-unstable: true
+// rustfmt-normalize_doc_attributes: true
+
+///This comment
+///is split
+///on multiple lines
+fn foo() {}