diff --git a/rustfmt-config/src/config_type.rs b/rustfmt-config/src/config_type.rs
index 51642570fdf..1f78b8af9d2 100644
--- a/rustfmt-config/src/config_type.rs
+++ b/rustfmt-config/src/config_type.rs
@@ -107,7 +107,7 @@ macro_rules! create_config {
 
         // Macro hygiene won't allow us to make `set_$i()` methods on Config
         // for each item, so this struct is used to give the API to set values:
-        // `config.get().option(false)`. It's pretty ugly. Consider replacing
+        // `config.set().option(false)`. It's pretty ugly. Consider replacing
         // with `config.set_option(false)` if we ever get a stable/usable
         // `concat_idents!()`.
         pub struct ConfigSetter<'a>(&'a mut Config);
diff --git a/rustfmt-config/src/options.rs b/rustfmt-config/src/options.rs
index bff6d2298d3..885d89ed752 100644
--- a/rustfmt-config/src/options.rs
+++ b/rustfmt-config/src/options.rs
@@ -176,6 +176,8 @@ configuration_option_enum! { WriteMode:
     Plain,
     // Outputs a checkstyle XML file.
     Checkstyle,
+    // Output the changed lines (for internal value only)
+    Modified,
 }
 
 configuration_option_enum! { Color:
diff --git a/rustfmt-core/src/filemap.rs b/rustfmt-core/src/filemap.rs
index 5986dbd6fa3..ee2253043a1 100644
--- a/rustfmt-core/src/filemap.rs
+++ b/rustfmt-core/src/filemap.rs
@@ -16,7 +16,7 @@ use std::path::Path;
 
 use checkstyle::{output_checkstyle_file, output_footer, output_header};
 use config::{Config, NewlineStyle, WriteMode};
-use rustfmt_diff::{make_diff, print_diff, Mismatch};
+use rustfmt_diff::{make_diff, output_modified, print_diff, Mismatch};
 use syntax::codemap::FileName;
 
 use FileRecord;
@@ -164,6 +164,15 @@ where
                 return Ok(has_diff);
             }
         }
+        WriteMode::Modified => {
+            let filename = filename_to_path();
+            if let Ok((ori, fmt)) = source_and_formatted_text(text, filename, config) {
+                let mismatch = make_diff(&ori, &fmt, 0);
+                let has_diff = !mismatch.is_empty();
+                output_modified(out, mismatch);
+                return Ok(has_diff);
+            }
+        }
         WriteMode::Checkstyle => {
             let filename = filename_to_path();
             let diff = create_diff(filename, text, config)?;
diff --git a/rustfmt-core/src/lib.rs b/rustfmt-core/src/lib.rs
index 6f471d0167f..d94f2a2c12c 100644
--- a/rustfmt-core/src/lib.rs
+++ b/rustfmt-core/src/lib.rs
@@ -26,7 +26,7 @@ extern crate unicode_segmentation;
 
 use std::collections::HashMap;
 use std::fmt;
-use std::io::{self, stdout, Write};
+use std::io::{self, stdout, BufRead, Write};
 use std::iter::repeat;
 use std::path::PathBuf;
 use std::rc::Rc;
@@ -704,6 +704,79 @@ pub fn format_input<T: Write>(
     }
 }
 
+/// A single span of changed lines, with 0 or more removed lines
+/// and a vector of 0 or more inserted lines.
+#[derive(Debug, PartialEq, Eq)]
+pub struct ModifiedChunk {
+    /// The first to be removed from the original text
+    pub line_number_orig: u32,
+    /// The number of lines which have been replaced
+    pub lines_removed: u32,
+    /// The new lines
+    pub lines: Vec<String>,
+}
+
+/// Set of changed sections of a file.
+#[derive(Debug, PartialEq, Eq)]
+pub struct ModifiedLines {
+    /// The set of changed chunks.
+    pub chunks: Vec<ModifiedChunk>,
+}
+
+/// The successful result of formatting via get_modified_lines().
+pub struct ModifiedLinesResult {
+    /// The high level summary details
+    pub summary: Summary,
+    /// The result Filemap
+    pub filemap: FileMap,
+    /// Map of formatting errors
+    pub report: FormatReport,
+    /// The sets of updated lines.
+    pub modified_lines: ModifiedLines,
+}
+
+/// Format a file and return a `ModifiedLines` data structure describing
+/// the changed ranges of lines.
+pub fn get_modified_lines(
+    input: Input,
+    config: &Config,
+) -> Result<ModifiedLinesResult, (io::Error, Summary)> {
+    let mut data = Vec::new();
+
+    let mut config = config.clone();
+    config.set().write_mode(config::WriteMode::Modified);
+    let (summary, filemap, formatreport) = format_input(input, &config, Some(&mut data))?;
+
+    let mut lines = data.lines();
+    let mut chunks = Vec::new();
+    while let Some(Ok(header)) = lines.next() {
+        // Parse the header line
+        let values: Vec<_> = header
+            .split(' ')
+            .map(|s| s.parse::<u32>().unwrap())
+            .collect();
+        assert_eq!(values.len(), 3);
+        let line_number_orig = values[0];
+        let lines_removed = values[1];
+        let num_added = values[2];
+        let mut added_lines = Vec::new();
+        for _ in 0..num_added {
+            added_lines.push(lines.next().unwrap().unwrap());
+        }
+        chunks.push(ModifiedChunk {
+            line_number_orig,
+            lines_removed,
+            lines: added_lines,
+        });
+    }
+    Ok(ModifiedLinesResult {
+        summary: summary,
+        filemap: filemap,
+        report: formatreport,
+        modified_lines: ModifiedLines { chunks },
+    })
+}
+
 #[derive(Debug)]
 pub enum Input {
     File(PathBuf),
diff --git a/rustfmt-core/src/rustfmt_diff.rs b/rustfmt-core/src/rustfmt_diff.rs
index 1a2f570f89e..f9919157620 100644
--- a/rustfmt-core/src/rustfmt_diff.rs
+++ b/rustfmt-core/src/rustfmt_diff.rs
@@ -13,6 +13,7 @@ use diff;
 use std::collections::VecDeque;
 use std::io;
 use term;
+use std::io::Write;
 use utils::use_colored_tty;
 
 #[derive(Debug, PartialEq)]
@@ -24,14 +25,19 @@ pub enum DiffLine {
 
 #[derive(Debug, PartialEq)]
 pub struct Mismatch {
+    /// The line number in the formatted version.
     pub line_number: u32,
+    /// The line number in the original version.
+    pub line_number_orig: u32,
+    /// The set of lines (context and old/new) in the mismatch.
     pub lines: Vec<DiffLine>,
 }
 
 impl Mismatch {
-    fn new(line_number: u32) -> Mismatch {
+    fn new(line_number: u32, line_number_orig: u32) -> Mismatch {
         Mismatch {
             line_number,
+            line_number_orig,
             lines: Vec::new(),
         }
     }
@@ -77,17 +83,21 @@ impl OutputWriter {
 // Produces a diff between the expected output and actual output of rustfmt.
 pub fn make_diff(expected: &str, actual: &str, context_size: usize) -> Vec<Mismatch> {
     let mut line_number = 1;
+    let mut line_number_orig = 1;
     let mut context_queue: VecDeque<&str> = VecDeque::with_capacity(context_size);
     let mut lines_since_mismatch = context_size + 1;
     let mut results = Vec::new();
-    let mut mismatch = Mismatch::new(0);
+    let mut mismatch = Mismatch::new(0, 0);
 
     for result in diff::lines(expected, actual) {
         match result {
             diff::Result::Left(str) => {
                 if lines_since_mismatch >= context_size && lines_since_mismatch > 0 {
                     results.push(mismatch);
-                    mismatch = Mismatch::new(line_number - context_queue.len() as u32);
+                    mismatch = Mismatch::new(
+                        line_number - context_queue.len() as u32,
+                        line_number_orig - context_queue.len() as u32,
+                    );
                 }
 
                 while let Some(line) = context_queue.pop_front() {
@@ -95,12 +105,16 @@ pub fn make_diff(expected: &str, actual: &str, context_size: usize) -> Vec<Misma
                 }
 
                 mismatch.lines.push(DiffLine::Resulting(str.to_owned()));
+                line_number_orig += 1;
                 lines_since_mismatch = 0;
             }
             diff::Result::Right(str) => {
                 if lines_since_mismatch >= context_size && lines_since_mismatch > 0 {
                     results.push(mismatch);
-                    mismatch = Mismatch::new(line_number - context_queue.len() as u32);
+                    mismatch = Mismatch::new(
+                        line_number - context_queue.len() as u32,
+                        line_number_orig - context_queue.len() as u32,
+                    );
                 }
 
                 while let Some(line) = context_queue.pop_front() {
@@ -123,6 +137,7 @@ pub fn make_diff(expected: &str, actual: &str, context_size: usize) -> Vec<Misma
                 }
 
                 line_number += 1;
+                line_number_orig += 1;
                 lines_since_mismatch += 1;
             }
         }
@@ -158,6 +173,42 @@ where
     }
 }
 
+/// Convert a Mismatch into a serialised form which just includes
+/// enough information to modify the original file.
+/// Each section starts with a line with three integers, space separated:
+///     lineno num_removed num_added
+/// followed by (num_added) lines of added text.  The line numbers are
+/// relative to the original file.
+pub fn output_modified<W>(mut out: W, diff: Vec<Mismatch>)
+where
+    W: Write,
+{
+    for mismatch in diff {
+        let (num_removed, num_added) = mismatch.lines.iter().fold((0, 0), |(rem, add), line| {
+            match *line {
+                DiffLine::Context(_) => panic!("No Context expected"),
+                DiffLine::Expected(_) => (rem, add + 1),
+                DiffLine::Resulting(_) => (rem + 1, add),
+            }
+        });
+        // Write a header with enough information to separate the modified lines.
+        writeln!(
+            out,
+            "{} {} {}",
+            mismatch.line_number_orig, num_removed, num_added
+        ).unwrap();
+
+        for line in mismatch.lines {
+            match line {
+                DiffLine::Context(_) | DiffLine::Resulting(_) => (),
+                DiffLine::Expected(ref str) => {
+                    writeln!(out, "{}", str).unwrap();
+                }
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::{make_diff, Mismatch};
@@ -173,6 +224,7 @@ mod test {
             vec![
                 Mismatch {
                     line_number: 2,
+                    line_number_orig: 2,
                     lines: vec![
                         Context("two".to_owned()),
                         Resulting("three".to_owned()),
@@ -194,6 +246,7 @@ mod test {
             vec![
                 Mismatch {
                     line_number: 2,
+                    line_number_orig: 2,
                     lines: vec![
                         Context("two".to_owned()),
                         Resulting("three".to_owned()),
@@ -203,6 +256,7 @@ mod test {
                 },
                 Mismatch {
                     line_number: 5,
+                    line_number_orig: 5,
                     lines: vec![
                         Resulting("five".to_owned()),
                         Expected("cinq".to_owned()),
@@ -223,6 +277,7 @@ mod test {
             vec![
                 Mismatch {
                     line_number: 3,
+                    line_number_orig: 3,
                     lines: vec![Resulting("three".to_owned()), Expected("trois".to_owned())],
                 },
             ]
@@ -239,6 +294,7 @@ mod test {
             vec![
                 Mismatch {
                     line_number: 5,
+                    line_number_orig: 5,
                     lines: vec![Context("five".to_owned()), Expected("".to_owned())],
                 },
             ]
diff --git a/rustfmt-core/tests/lib.rs b/rustfmt-core/tests/lib.rs
index c618ab9c760..3a990ebe88c 100644
--- a/rustfmt-core/tests/lib.rs
+++ b/rustfmt-core/tests/lib.rs
@@ -143,6 +143,30 @@ fn checkstyle_test() {
     assert_output(Path::new(filename), Path::new(expected_filename));
 }
 
+#[test]
+fn modified_test() {
+    // Test "modified" output
+    let filename = "tests/writemode/source/modified.rs";
+    let result = get_modified_lines(Input::File(filename.into()), &Config::default()).unwrap();
+    assert_eq!(
+        result.modified_lines,
+        ModifiedLines {
+            chunks: vec![
+                ModifiedChunk {
+                    line_number_orig: 4,
+                    lines_removed: 4,
+                    lines: vec!["fn blah() {}".into()],
+                },
+                ModifiedChunk {
+                    line_number_orig: 9,
+                    lines_removed: 6,
+                    lines: vec!["#[cfg(a, b)]".into(), "fn main() {}".into()],
+                },
+            ],
+        }
+    );
+}
+
 // Helper function for comparing the results of rustfmt
 // to a known output file generated by one of the write modes.
 fn assert_output(source: &Path, expected_filename: &Path) {
@@ -529,6 +553,7 @@ fn rustfmt_diff_make_diff_tests() {
         vec![
             Mismatch {
                 line_number: 1,
+                line_number_orig: 1,
                 lines: vec![
                     DiffLine::Context("a".into()),
                     DiffLine::Resulting("b".into()),
diff --git a/rustfmt-core/tests/writemode/source/modified.rs b/rustfmt-core/tests/writemode/source/modified.rs
new file mode 100644
index 00000000000..948beb348db
--- /dev/null
+++ b/rustfmt-core/tests/writemode/source/modified.rs
@@ -0,0 +1,14 @@
+// rustfmt-write_mode: modified
+// Test "modified" output
+
+fn
+blah
+()
+{ }
+
+
+#[cfg
+(  a , b
+)]
+fn
+main() {}
diff --git a/rustfmt-core/tests/writemode/target/modified.txt b/rustfmt-core/tests/writemode/target/modified.txt
new file mode 100644
index 00000000000..5c0539a665e
--- /dev/null
+++ b/rustfmt-core/tests/writemode/target/modified.txt
@@ -0,0 +1,5 @@
+4 4 1
+fn blah() {}
+10 5 2
+#[cfg(a, b)]
+fn main() {}