Auto-detect newline style by default

Configurations.md

@@ -1338,8 +1338,8 @@ fn main() {
 
 Unix or Windows line endings
 
-- **Default value**: `"Native"`
-- **Possible values**: `"Native"`, `"Unix"`, `"Windows"`
+- **Default value**: `"Auto"`
+- **Possible values**: `"Auto"`, `"Native"`, `"Unix"`, `"Windows"`
 - **Stable**: Yes
 
 ## `normalize_comments`

src/config/mod.rs

@@ -39,7 +39,7 @@ create_config! {
     max_width: usize, 100, true, "Maximum width of each line";
     hard_tabs: bool, false, true, "Use tab characters for indentation, spaces for alignment";
     tab_spaces: usize, 4, true, "Number of spaces per tab";
-    newline_style: NewlineStyle, NewlineStyle::Native, true, "Unix or Windows line endings";
+    newline_style: NewlineStyle, NewlineStyle::Auto, true, "Unix or Windows line endings";
     use_small_heuristics: Heuristics, Heuristics::Default, true, "Whether to use different \
         formatting for items and expressions if they satisfy a heuristic notion of 'small'";
     indent_style: IndentStyle, IndentStyle::Block, false, "How do we indent expressions or items";

src/config/options.rs

@@ -105,11 +105,68 @@ macro_rules! configuration_option_enum{
 }
 
 configuration_option_enum! { NewlineStyle:
+    Auto, // Auto-detect based on the raw source input
     Windows, // \r\n
     Unix, // \n
     Native, // \r\n in Windows, \n on other platforms
 }
 
+impl NewlineStyle {
+    fn auto_detect(raw_input_text: &str) -> NewlineStyle {
+        if let Some(pos) = raw_input_text.find('\n') {
+            let pos = pos.saturating_sub(1);
+            if let Some('\r') = raw_input_text.chars().nth(pos) {
+                NewlineStyle::Windows
+            } else {
+                NewlineStyle::Unix
+            }
+        } else {
+            NewlineStyle::Native
+        }
+    }
+
+    fn native() -> NewlineStyle {
+        if cfg!(windows) {
+            NewlineStyle::Windows
+        } else {
+            NewlineStyle::Unix
+        }
+    }
+
+    /// Apply this newline style to the formatted text. When the style is set
+    /// to `Auto`, the `raw_input_text` is used to detect the existing line
+    /// endings.
+    ///
+    /// If the style is set to `Auto` and `raw_input_text` contains no
+    /// newlines, the `Native` style will be used.
+    pub(crate) fn apply(self, formatted_text: &mut String, raw_input_text: &str) {
+        use NewlineStyle::*;
+        let mut style = self;
+        if style == Auto {
+            style = Self::auto_detect(raw_input_text);
+        }
+        if style == Native {
+            style = Self::native();
+        }
+        match style {
+            Windows => {
+                let mut transformed = String::with_capacity(formatted_text.capacity());
+                for c in formatted_text.chars() {
+                    match c {
+                        '\n' => transformed.push_str("\r\n"),
+                        '\r' => continue,
+                        c => transformed.push(c),
+                    }
+                }
+                *formatted_text = transformed;
+            }
+            Unix => return,
+            Native => unreachable!("NewlineStyle::Native"),
+            Auto => unreachable!("NewlineStyle::Auto"),
+        }
+    }
+}
+
 configuration_option_enum! { BraceStyle:
     AlwaysNextLine,
     PreferSameLine,
@@ -367,3 +424,59 @@ impl Edition {
         }
     }
 }
+
+#[test]
+fn test_newline_style_auto_detect() {
+    let lf = "One\nTwo\nThree";
+    let crlf = "One\r\nTwo\r\nThree";
+    let none = "One Two Three";
+
+    assert_eq!(NewlineStyle::Unix, NewlineStyle::auto_detect(lf));
+    assert_eq!(NewlineStyle::Windows, NewlineStyle::auto_detect(crlf));
+    assert_eq!(NewlineStyle::Native, NewlineStyle::auto_detect(none));
+}
+
+#[test]
+fn test_newline_style_auto_apply() {
+    let auto = NewlineStyle::Auto;
+
+    let formatted_text = "One\nTwo\nThree";
+    let raw_input_text = "One\nTwo\nThree";
+
+    let mut out = String::from(formatted_text);
+    auto.apply(&mut out, raw_input_text);
+    assert_eq!("One\nTwo\nThree", &out, "auto should detect 'lf'");
+
+    let formatted_text = "One\nTwo\nThree";
+    let raw_input_text = "One\r\nTwo\r\nThree";
+
+    let mut out = String::from(formatted_text);
+    auto.apply(&mut out, raw_input_text);
+    assert_eq!("One\r\nTwo\r\nThree", &out, "auto should detect 'crlf'");
+
+    #[cfg(not(windows))]
+    {
+        let formatted_text = "One\nTwo\nThree";
+        let raw_input_text = "One Two Three";
+
+        let mut out = String::from(formatted_text);
+        auto.apply(&mut out, raw_input_text);
+        assert_eq!(
+            "One\nTwo\nThree", &out,
+            "auto-native-unix should detect 'lf'"
+        );
+    }
+
+    #[cfg(windows)]
+    {
+        let formatted_text = "One\nTwo\nThree";
+        let raw_input_text = "One Two Three";
+
+        let mut out = String::from(formatted_text);
+        auto.apply(&mut out, raw_input_text);
+        assert_eq!(
+            "One\r\nTwo\r\nThree", &out,
+            "auto-native-windows should detect 'crlf'"
+        );
+    }
+}

src/formatting.rs

@@ -13,7 +13,7 @@ use syntax::errors::Handler;
 use syntax::parse::{self, ParseSess};
 
 use comment::{CharClasses, FullCodeCharKind};
-use config::{Config, FileName, NewlineStyle, Verbosity};
+use config::{Config, FileName, Verbosity};
 use issues::BadIssueSeeker;
 use visitor::{FmtVisitor, SnippetProvider};
 use {filemap, modules, ErrorKind, FormatReport, Input, Session};
@@ -166,7 +166,9 @@ impl<'a, T: FormatHandler + 'a> FormatContext<'a, T> {
             &self.config,
             &self.report,
         );
-        replace_with_system_newlines(&mut visitor.buffer, &self.config);
+        self.config
+            .newline_style()
+            .apply(&mut visitor.buffer, &big_snippet);
 
         if visitor.macro_rewrite_failure {
             self.report.add_macro_format_failure();
@@ -645,34 +647,6 @@ fn make_parse_sess(codemap: Rc<CodeMap>, config: &Config) -> ParseSess {
     ParseSess::with_span_handler(tty_handler, codemap)
 }
 
-fn replace_with_system_newlines(text: &mut String, config: &Config) -> () {
-    let style = if config.newline_style() == NewlineStyle::Native {
-        if cfg!(windows) {
-            NewlineStyle::Windows
-        } else {
-            NewlineStyle::Unix
-        }
-    } else {
-        config.newline_style()
-    };
-
-    match style {
-        NewlineStyle::Unix => return,
-        NewlineStyle::Windows => {
-            let mut transformed = String::with_capacity(text.capacity());
-            for c in text.chars() {
-                match c {
-                    '\n' => transformed.push_str("\r\n"),
-                    '\r' => continue,
-                    c => transformed.push(c),
-                }
-            }
-            *text = transformed;
-        }
-        NewlineStyle::Native => unreachable!(),
-    }
-}
-
 fn should_emit_verbose<F>(is_stdin: bool, config: &Config, f: F)
 where
     F: Fn(),