fix rewrite_string when a line feed is present in a sequence of whitespaces, resulting in strange formatting

src/string.rs

@@ -41,21 +41,43 @@ impl<'a> StringFormat<'a> {
             config,
         }
     }
+
+    /// Returns the maximum number of graphemes that is possible on a line while taking the
+    /// indentation into account.
+    ///
+    /// If we cannot put at least a single character per line, the rewrite won't succeed.
+    fn max_chars_with_indent(&self) -> Option<usize> {
+        Some(
+            self.shape
+                .width
+                .checked_sub(self.opener.len() + self.line_end.len() + 1)?
+                + 1;
+        )
+    }
+
+    /// Like max_chars_with_indent but the indentation is not substracted.
+    /// This allows to fit more graphemes from the string on a line when
+    /// SnippetState::Overflow.
+    fn max_chars_without_indent(&self) -> Option<usize> {
+        Some(self.config.max_width().checked_sub(self.line_end.len())?)
+    }
 }
 
-// FIXME: simplify this!
 pub fn rewrite_string<'a>(
     orig: &str,
     fmt: &StringFormat<'a>,
     max_width: Option<usize>,
 ) -> Option<String> {
+    let max_chars_with_indent = fmt.max_chars_with_indent()?;
+    let max_chars_without_indent = fmt.max_chars_without_indent()?;
+    let indent = fmt.shape.indent.to_string_with_newline(fmt.config);
+
     // Strip line breaks.
-    let re = Regex::new(r"([^\\](\\\\)*)\\[\n\r][[:space:]]*").unwrap();
-    let stripped_str = re.replace_all(orig, "$1");
+    // With this regex applied, all remaining whitespaces are significant
+    let strip_line_breaks_re = Regex::new(r"([^\\](\\\\)*)\\[\n\r][[:space:]]*").unwrap();
+    let stripped_str = strip_line_breaks_re.replace_all(orig, "$1");
 
     let graphemes = UnicodeSegmentation::graphemes(&*stripped_str, false).collect::<Vec<&str>>();
-    let shape = fmt.shape;
-    let indent = shape.indent.to_string_with_newline(fmt.config);
 
     // `cur_start` is the position in `orig` of the start of the current line.
     let mut cur_start = 0;
@@ -67,87 +89,131 @@ pub fn rewrite_string<'a>(
     );
     result.push_str(fmt.opener);
 
-    let ender_length = fmt.line_end.len();
-    // If we cannot put at least a single character per line, the rewrite won't
-    // succeed.
-    let mut max_chars = shape
-        .width
-        .checked_sub(fmt.opener.len() + ender_length + 1)?
-        + 1;
-
-    // Snip a line at a time from `orig` until it is used up. Push the snippet
+    // Snip a line at a time from `stripped_str` until it is used up. Push the snippet
     // onto result.
-    'outer: loop {
-        // `cur_end` will be where we break the line, as an offset into `orig`.
-        // Initialised to the maximum it could be (which may be beyond `orig`).
-        let mut cur_end = cur_start + max_chars;
-
-        // We can fit the rest of the string on this line, so we're done.
-        if cur_end >= graphemes.len() {
-            let line = &graphemes[cur_start..].join("");
-            result.push_str(line);
-            break 'outer;
+    let mut cur_max_chars = max_chars_with_indent;
+    loop {
+        // All the input starting at cur_start fits on the current line
+        if graphemes.len() - cur_start <= cur_max_chars {
+            result.push_str(&graphemes[cur_start..].join(""));
+            break;
         }
 
-        // Push cur_end left until we reach whitespace (or the line is too small).
-        while !is_whitespace(graphemes[cur_end - 1]) {
-            cur_end -= 1;
-            if cur_end < cur_start + MIN_STRING {
-                // We couldn't find whitespace before the string got too small.
-                // So start again at the max length and look for punctuation.
-                cur_end = cur_start + max_chars;
-                while !is_punctuation(graphemes[cur_end - 1]) {
-                    cur_end -= 1;
-
-                    // If we can't break at whitespace or punctuation, grow the string instead.
-                    if cur_end < cur_start + MIN_STRING {
-                        cur_end = cur_start + max_chars;
-                        while !(is_punctuation(graphemes[cur_end - 1])
-                            || is_whitespace(graphemes[cur_end - 1]))
-                        {
-                            cur_end += 1;
-                            if cur_end == graphemes.len() {
-                                let line = &graphemes[cur_start..].join("");
-                                result.push_str(line);
-                                break 'outer;
-                            }
-                        }
-                        break;
-                    }
-                }
+        // The input starting at cur_start needs to be broken
+        match break_string(cur_max_chars, fmt.trim_end, &graphemes[cur_start..]) {
+            SnippetState::LineEnd(line, len) => {
+                result.push_str(&line);
+                result.push_str(fmt.line_end);
+                result.push_str(&indent);
+                result.push_str(fmt.line_start);
+                cur_max_chars = max_chars_with_indent;
+                cur_start += len;
+            }
+            SnippetState::Overflow(line, len) => {
+                result.push_str(&line);
+                cur_max_chars = max_chars_without_indent;
+                cur_start += len;
+            }
+            SnippetState::EndOfInput(line) => {
+                result.push_str(&line);
                 break;
             }
         }
-        // Make sure there is no whitespace to the right of the break.
-        while cur_end < stripped_str.len() && is_whitespace(graphemes[cur_end]) {
-            cur_end += 1;
-        }
-
-        // Make the current line and add it on to result.
-        let raw_line = graphemes[cur_start..cur_end].join("");
-        let line = if fmt.trim_end {
-            raw_line.trim()
-        } else {
-            raw_line.as_str()
-        };
-
-        result.push_str(line);
-        result.push_str(fmt.line_end);
-        result.push_str(&indent);
-        result.push_str(fmt.line_start);
-
-        // The next line starts where the current line ends.
-        cur_start = cur_end;
-
-        if let Some(new_max_chars) = max_width {
-            max_chars = new_max_chars.checked_sub(fmt.opener.len() + ender_length + 1)? + 1;
-        }
     }
 
     result.push_str(fmt.closer);
     wrap_str(result, fmt.config.max_width(), fmt.shape)
 }
 
+/// Result of breaking a string so it fits in a line and the state it ended in.
+/// The state informs about what to do with the snippet and how to continue the breaking process.
+#[derive(Debug, PartialEq)]
+enum SnippetState {
+    /// The input could not be broken and so rewriting the string is finished.
+    EndOfInput(String),
+    /// The input could be broken and the returned snippet should be ended with a
+    /// `[StringFormat::line_end]`. The next snippet needs to be indented.
+    LineEnd(String, usize),
+    /// The input could be broken but the returned snippet should not be ended with a
+    /// `[StringFormat::line_end]` because the whitespace is significant. Therefore, the next
+    /// snippet should not be indented.
+    Overflow(String, usize),
+}
+
+/// Break the input string at a boundary character around the offset `max_chars`. A boundary
+/// character is either a punctuation or a whitespace.
+fn break_string(max_chars: usize, trim_end: bool, input: &[&str]) -> SnippetState {
+    let break_at = |index /* grapheme at index is included */| {
+        // Take in any whitespaces to the left/right of `input[index]` and
+        // check if there is a line feed, in which case whitespaces needs to be kept.
+        let mut index_minus_ws = index;
+        for (i, grapheme) in input[0..=index].iter().enumerate().rev() {
+            if !trim_end && is_line_feed(grapheme) {
+                return SnippetState::Overflow(input[0..=i].join("").to_string(), i + 1);
+            } else if !is_whitespace(grapheme) {
+                index_minus_ws = i;
+                break;
+            }
+        }
+        let mut index_plus_ws = index;
+        for (i, grapheme) in input[index + 1..].iter().enumerate() {
+            if !trim_end && is_line_feed(grapheme) {
+                return SnippetState::Overflow(
+                    input[0..=index + 1 + i].join("").to_string(),
+                    index + 2 + i,
+                );
+            } else if !is_whitespace(grapheme) {
+                index_plus_ws = index + i;
+                break;
+            }
+        }
+
+        if trim_end {
+            SnippetState::LineEnd(
+                input[0..=index_minus_ws].join("").to_string(),
+                index_plus_ws + 1,
+            )
+        } else {
+            SnippetState::LineEnd(
+                input[0..=index_plus_ws].join("").to_string(),
+                index_plus_ws + 1,
+            )
+        }
+    };
+
+    // Find the position in input for breaking the string
+    match input[0..max_chars]
+        .iter()
+        .rposition(|grapheme| is_whitespace(grapheme))
+    {
+        // Found a whitespace and what is on its left side is big enough.
+        Some(index) if index >= MIN_STRING => break_at(index),
+        // No whitespace found, try looking for a punctuation instead
+        _ => match input[0..max_chars]
+            .iter()
+            .rposition(|grapheme| is_punctuation(grapheme))
+        {
+            // Found a punctuation and what is on its left side is big enough.
+            Some(index) if index >= MIN_STRING => break_at(index),
+            // Either no boundary character was found to the left of `input[max_chars]`, or the line
+            // got too small. We try searching for a boundary character to the right.
+            _ => match input[max_chars..]
+                .iter()
+                .position(|grapheme| is_whitespace(grapheme) || is_punctuation(grapheme))
+            {
+                // A boundary was found after the line limit
+                Some(index) => break_at(max_chars + index),
+                // No boundary to the right, the input cannot be broken
+                None => SnippetState::EndOfInput(input.join("").to_string()),
+            },
+        },
+    }
+}
+
+fn is_line_feed(grapheme: &str) -> bool {
+    grapheme.as_bytes()[0] == b'\n'
+}
+
 fn is_whitespace(grapheme: &str) -> bool {
     grapheme.chars().all(|c| c.is_whitespace())
 }
@@ -161,8 +227,9 @@ fn is_punctuation(grapheme: &str) -> bool {
 
 #[cfg(test)]
 mod test {
-    use super::{rewrite_string, StringFormat};
+    use super::{break_string, rewrite_string, SnippetState, StringFormat};
     use shape::{Indent, Shape};
+    use unicode_segmentation::UnicodeSegmentation;
 
     #[test]
     fn issue343() {
@@ -170,4 +237,89 @@ mod test {
         let fmt = StringFormat::new(Shape::legacy(2, Indent::empty()), &config);
         rewrite_string("eq_", &fmt, None);
     }
+
+    #[test]
+    fn should_break_on_whitespace() {
+        let string = "Placerat felis. Mauris porta ante sagittis purus.";
+        let graphemes = UnicodeSegmentation::graphemes(&*string, false).collect::<Vec<&str>>();
+        assert_eq!(
+            break_string(20, false, &graphemes[..]),
+            SnippetState::LineEnd("Placerat felis. ".to_string(), 16)
+        );
+        assert_eq!(
+            break_string(20, true, &graphemes[..]),
+            SnippetState::LineEnd("Placerat felis.".to_string(), 16)
+        );
+    }
+
+    #[test]
+    fn should_break_on_punctuation() {
+        let string = "Placerat_felis._Mauris_porta_ante_sagittis_purus.";
+        let graphemes = UnicodeSegmentation::graphemes(&*string, false).collect::<Vec<&str>>();
+        assert_eq!(
+            break_string(20, false, &graphemes[..]),
+            SnippetState::LineEnd("Placerat_felis.".to_string(), 15)
+        );
+    }
+
+    #[test]
+    fn should_break_forward() {
+        let string = "Venenatis_tellus_vel_tellus. Aliquam aliquam dolor at justo.";
+        let graphemes = UnicodeSegmentation::graphemes(&*string, false).collect::<Vec<&str>>();
+        assert_eq!(
+            break_string(20, false, &graphemes[..]),
+            SnippetState::LineEnd("Venenatis_tellus_vel_tellus. ".to_string(), 29)
+        );
+        assert_eq!(
+            break_string(20, true, &graphemes[..]),
+            SnippetState::LineEnd("Venenatis_tellus_vel_tellus.".to_string(), 29)
+        );
+    }
+
+    #[test]
+    fn nothing_to_break() {
+        let string = "Venenatis_tellus_vel_tellus";
+        let graphemes = UnicodeSegmentation::graphemes(&*string, false).collect::<Vec<&str>>();
+        assert_eq!(
+            break_string(20, false, &graphemes[..]),
+            SnippetState::EndOfInput("Venenatis_tellus_vel_tellus".to_string())
+        );
+    }
+
+    #[test]
+    fn significant_whitespaces() {
+        let string = "Neque in sem.      \n      Pellentesque tellus augue.";
+        let graphemes = UnicodeSegmentation::graphemes(&*string, false).collect::<Vec<&str>>();
+        assert_eq!(
+            break_string(15, false, &graphemes[..]),
+            SnippetState::Overflow("Neque in sem.      \n".to_string(), 20)
+        );
+        assert_eq!(
+            break_string(25, false, &graphemes[..]),
+            SnippetState::Overflow("Neque in sem.      \n".to_string(), 20)
+        );
+        // if `StringFormat::line_end` is true, then the line feed does not matter anymore
+        assert_eq!(
+            break_string(15, true, &graphemes[..]),
+            SnippetState::LineEnd("Neque in sem.".to_string(), 26)
+        );
+        assert_eq!(
+            break_string(25, true, &graphemes[..]),
+            SnippetState::LineEnd("Neque in sem.".to_string(), 26)
+        );
+    }
+
+    #[test]
+    fn big_whitespace() {
+        let string = "Neque in sem.            Pellentesque tellus augue.";
+        let graphemes = UnicodeSegmentation::graphemes(&*string, false).collect::<Vec<&str>>();
+        assert_eq!(
+            break_string(20, false, &graphemes[..]),
+            SnippetState::LineEnd("Neque in sem.            ".to_string(), 25)
+        );
+        assert_eq!(
+            break_string(20, true, &graphemes[..]),
+            SnippetState::LineEnd("Neque in sem.".to_string(), 25)
+        );
+    }
 }

src/utils.rs

@@ -320,7 +320,7 @@ macro_rules! skip_out_of_file_lines_range_visitor {
 }
 
 // Wraps String in an Option. Returns Some when the string adheres to the
-// Rewrite constraints defined for the Rewrite trait and else otherwise.
+// Rewrite constraints defined for the Rewrite trait and None otherwise.
 pub fn wrap_str(s: String, max_width: usize, shape: Shape) -> Option<String> {
     if is_valid_str(&s, max_width, shape) {
         Some(s)

tests/source/issue-1210/a.rs

@@ -0,0 +1,12 @@
+// rustfmt-format_strings: true
+// rustfmt-max_width: 50
+
+impl Foo {
+    fn cxx(&self, target: &str) -> &Path {
+        match self.cxx.get(target) {
+            Some(p) => p.path(),
+            None => panic!("\n\ntarget `{}` is not configured as a host,
+                            only as a target\n\n", target),
+        }
+    }
+}

tests/source/issue-1210/b.rs

@@ -0,0 +1,12 @@
+// rustfmt-format_strings: true
+// rustfmt-max_width: 50
+
+impl Foo {
+    fn cxx(&self, target: &str) -> &Path {
+        match self.cxx.get(target) {
+            Some(p) => p.path(),
+            None => panic!("\ntarget `{}`: is not, configured as a host,
+                            only as a target\n\n", target),
+        }
+    }
+}

tests/source/issue-1210/c.rs

@@ -0,0 +1,5 @@
+// rustfmt-format_strings: true
+// rustfmt-max_width: 50
+
+const foo: String = "trailing_spaces!!            
+                keep them! Amet neque. Praesent rhoncus eros non velit.";

tests/source/issue-1210/d.rs

@@ -0,0 +1,4 @@
+// rustfmt-wrap_comments: true
+
+// trailing_spaces_in_comment!!                   
+//                remove those from above

tests/source/issue-1210/e.rs

@@ -0,0 +1,8 @@
+// rustfmt-format_strings: true
+// rustfmt-max_width: 50
+
+// explicit line breaks should be kept in order to preserve the layout
+
+const foo: String = "Suspendisse vel augue at felis tincidunt sollicitudin. Fusce arcu.
+               Duis et odio et leo
+        sollicitudin consequat. Aliquam lobortis.  Phasellus condimentum.";

tests/target/issue-1210/a.rs

@@ -0,0 +1,16 @@
+// rustfmt-format_strings: true
+// rustfmt-max_width: 50
+
+impl Foo {
+    fn cxx(&self, target: &str) -> &Path {
+        match self.cxx.get(target) {
+            Some(p) => p.path(),
+            None => panic!(
+                "\n\ntarget `{}` is not \
+                 configured as a host,
+                            only as a target\n\n",
+                target
+            ),
+        }
+    }
+}

tests/target/issue-1210/b.rs

@@ -0,0 +1,16 @@
+// rustfmt-format_strings: true
+// rustfmt-max_width: 50
+
+impl Foo {
+    fn cxx(&self, target: &str) -> &Path {
+        match self.cxx.get(target) {
+            Some(p) => p.path(),
+            None => panic!(
+                "\ntarget `{}`: is not, \
+                 configured as a host,
+                            only as a target\n\n",
+                target
+            ),
+        }
+    }
+}

tests/target/issue-1210/c.rs

@@ -0,0 +1,7 @@
+// rustfmt-format_strings: true
+// rustfmt-max_width: 50
+
+const foo: String =
+    "trailing_spaces!!            
+                keep them! Amet neque. Praesent \
+     rhoncus eros non velit.";

tests/target/issue-1210/d.rs

@@ -0,0 +1,4 @@
+// rustfmt-wrap_comments: true
+
+// trailing_spaces_in_comment!!
+//                remove those from above

tests/target/issue-1210/e.rs

@@ -0,0 +1,11 @@
+// rustfmt-format_strings: true
+// rustfmt-max_width: 50
+
+// explicit line breaks should be kept in order to preserve the layout
+
+const foo: String =
+    "Suspendisse vel augue at felis tincidunt \
+     sollicitudin. Fusce arcu.
+               Duis et odio et leo
+        sollicitudin consequat. Aliquam \
+     lobortis.  Phasellus condimentum.";