diff --git a/Cargo.lock b/Cargo.lock
index 162e9160f85..2d0495bc6cb 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -5,7 +5,9 @@ dependencies = [
  "diff 0.1.9 (registry+https://github.com/rust-lang/crates.io-index)",
  "env_logger 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "getopts 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)",
+ "itertools 0.4.12 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
+ "multimap 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "regex 0.1.63 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",
  "strings 0.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -47,6 +49,11 @@ name = "getopts"
 version = "0.2.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "itertools"
+version = "0.4.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "kernel32-sys"
 version = "0.2.1"
@@ -79,6 +86,11 @@ name = "mempool"
 version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "multimap"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "regex"
 version = "0.1.63"
diff --git a/Cargo.toml b/Cargo.toml
index 6fe6bc05d56..ca4ec71a4a2 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -25,3 +25,5 @@ syntex_syntax = "0.32"
 log = "0.3"
 env_logger = "0.3"
 getopts = "0.2"
+itertools = "0.4"
+multimap = "0.3"
diff --git a/src/config.rs b/src/config.rs
index e4d0e4517c2..45bd9a6f7c5 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -10,6 +10,7 @@
 
 extern crate toml;
 
+use file_lines::FileLines;
 use lists::{SeparatorTactic, ListTactic};
 use std::io::Write;
 
@@ -200,6 +201,12 @@ impl ConfigType for String {
     }
 }
 
+impl ConfigType for FileLines {
+    fn doc_hint() -> String {
+        String::from("<json>")
+    }
+}
+
 pub struct ConfigHelpItem {
     option_name: &'static str,
     doc_string: &'static str,
@@ -327,6 +334,8 @@ macro_rules! create_config {
 create_config! {
     verbose: bool, false, "Use verbose output";
     skip_children: bool, false, "Don't reformat out of line modules";
+    file_lines: FileLines, FileLines::all(),
+        "Lines to format";
     max_width: usize, 100, "Maximum width of each line";
     ideal_width: usize, 80, "Ideal width of each line";
     tab_spaces: usize, 4, "Number of spaces per tab";
diff --git a/src/file_lines.rs b/src/file_lines.rs
new file mode 100644
index 00000000000..e437202360d
--- /dev/null
+++ b/src/file_lines.rs
@@ -0,0 +1,221 @@
+//! This module contains types and functions to support formatting specific line ranges.
+use std::{cmp, iter, str};
+
+use itertools::Itertools;
+use multimap::MultiMap;
+use rustc_serialize::{self, json};
+
+use codemap::LineRange;
+
+/// A range that is inclusive of both ends.
+#[derive(Clone, Copy, Debug, Eq, PartialEq, RustcDecodable)]
+struct Range {
+    pub lo: usize,
+    pub hi: usize,
+}
+
+impl<'a> From<&'a LineRange> for Range {
+    fn from(range: &'a LineRange) -> Range {
+        Range::new(range.lo, range.hi)
+    }
+}
+
+impl Range {
+    fn new(lo: usize, hi: usize) -> Range {
+        Range { lo: lo, hi: hi }
+    }
+
+    fn is_empty(self) -> bool {
+        self.lo > self.hi
+    }
+
+    fn contains(self, other: Range) -> bool {
+        if other.is_empty() {
+            true
+        } else {
+            !self.is_empty() && self.lo <= other.lo && self.hi >= other.hi
+        }
+    }
+
+    fn intersects(self, other: Range) -> bool {
+        if self.is_empty() || other.is_empty() {
+            false
+        } else {
+            (self.lo <= other.hi && other.hi <= self.hi) ||
+            (other.lo <= self.hi && self.hi <= other.hi)
+        }
+    }
+
+    fn adjacent_to(self, other: Range) -> bool {
+        if self.is_empty() || other.is_empty() {
+            false
+        } else {
+            self.hi + 1 == other.lo || other.hi + 1 == self.lo
+        }
+    }
+
+    /// Returns a new `Range` with lines from `self` and `other` if they were adjacent or
+    /// intersect; returns `None` otherwise.
+    fn merge(self, other: Range) -> Option<Range> {
+        if self.adjacent_to(other) || self.intersects(other) {
+            Some(Range::new(cmp::min(self.lo, other.lo), cmp::max(self.hi, other.hi)))
+        } else {
+            None
+        }
+    }
+}
+
+/// A set of lines in files.
+///
+/// It is represented as a multimap keyed on file names, with values a collection of
+/// non-overlapping ranges sorted by their start point. An inner `None` is interpreted to mean all
+/// lines in all files.
+#[derive(Clone, Debug, Default)]
+pub struct FileLines(Option<MultiMap<String, Range>>);
+
+/// Normalizes the ranges so that the invariants for `FileLines` hold: ranges are non-overlapping,
+/// and ordered by their start point.
+fn normalize_ranges(map: &mut MultiMap<String, Range>) {
+    for (_, ranges) in map.iter_all_mut() {
+        ranges.sort_by_key(|x| x.lo);
+        let merged = ranges.drain(..).coalesce(|x, y| x.merge(y).ok_or((x, y))).collect();
+        *ranges = merged;
+    }
+}
+
+impl FileLines {
+    /// Creates a `FileLines` that contains all lines in all files.
+    pub fn all() -> FileLines {
+        FileLines(None)
+    }
+
+    /// Creates a `FileLines` from a `MultiMap`, ensuring that the invariants hold.
+    fn from_multimap(map: MultiMap<String, Range>) -> FileLines {
+        let mut map = map;
+        normalize_ranges(&mut map);
+        FileLines(Some(map))
+    }
+
+    /// Returns an iterator over the files contained in `self`.
+    pub fn files(&self) -> Files {
+        Files(self.0.as_ref().map(MultiMap::keys))
+    }
+
+    /// Returns true if `range` is fully contained in `self`.
+    pub fn contains(&self, range: &LineRange) -> bool {
+        let map = match self.0 {
+            // `None` means "all lines in all files".
+            None => return true,
+            Some(ref map) => map,
+        };
+
+        match map.get_vec(range.file_name()) {
+            None => false,
+            Some(ranges) => ranges.iter().any(|r| r.contains(Range::from(range))),
+        }
+    }
+
+    /// Returns true if any lines in `range` are in `self`.
+    pub fn intersects(&self, range: &LineRange) -> bool {
+        let map = match self.0 {
+            // `None` means "all lines in all files".
+            None => return true,
+            Some(ref map) => map,
+        };
+
+        match map.get_vec(range.file_name()) {
+            None => false,
+            Some(ranges) => ranges.iter().any(|r| r.intersects(Range::from(range))),
+        }
+    }
+}
+
+/// FileLines files iterator.
+pub struct Files<'a>(Option<::std::collections::hash_map::Keys<'a, String, Vec<Range>>>);
+
+impl<'a> iter::Iterator for Files<'a> {
+    type Item = &'a String;
+
+    fn next(&mut self) -> Option<&'a String> {
+        self.0.as_mut().and_then(Iterator::next)
+    }
+}
+
+// This impl is needed for `Config::override_value` to work for use in tests.
+impl str::FromStr for FileLines {
+    type Err = String;
+
+    fn from_str(s: &str) -> Result<FileLines, String> {
+        let v: Vec<JsonSpan> = try!(json::decode(s).map_err(|e| e.to_string()));
+        let m = v.into_iter().map(JsonSpan::into_tuple).collect();
+        Ok(FileLines::from_multimap(m))
+    }
+}
+
+// For JSON decoding.
+#[derive(Clone, Debug, RustcDecodable)]
+struct JsonSpan {
+    file: String,
+    range: (usize, usize),
+}
+
+impl JsonSpan {
+    // To allow `collect()`ing into a `MultiMap`.
+    fn into_tuple(self) -> (String, Range) {
+        let (lo, hi) = self.range;
+        (self.file, Range::new(lo, hi))
+    }
+}
+
+// This impl is needed for inclusion in the `Config` struct. We don't have a toml representation
+// for `FileLines`, so it will just panic instead.
+impl rustc_serialize::Decodable for FileLines {
+    fn decode<D: rustc_serialize::Decoder>(_: &mut D) -> Result<Self, D::Error> {
+        unimplemented!();
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::Range;
+
+    #[test]
+    fn test_range_intersects() {
+        assert!(Range::new(1, 2).intersects(Range::new(1, 1)));
+        assert!(Range::new(1, 2).intersects(Range::new(2, 2)));
+        assert!(!Range::new(1, 2).intersects(Range::new(0, 0)));
+        assert!(!Range::new(1, 2).intersects(Range::new(3, 10)));
+        assert!(!Range::new(1, 3).intersects(Range::new(5, 5)));
+    }
+
+    #[test]
+    fn test_range_adjacent_to() {
+        assert!(!Range::new(1, 2).adjacent_to(Range::new(1, 1)));
+        assert!(!Range::new(1, 2).adjacent_to(Range::new(2, 2)));
+        assert!(Range::new(1, 2).adjacent_to(Range::new(0, 0)));
+        assert!(Range::new(1, 2).adjacent_to(Range::new(3, 10)));
+        assert!(!Range::new(1, 3).adjacent_to(Range::new(5, 5)));
+    }
+
+    #[test]
+    fn test_range_contains() {
+        assert!(Range::new(1, 2).contains(Range::new(1, 1)));
+        assert!(Range::new(1, 2).contains(Range::new(2, 2)));
+        assert!(!Range::new(1, 2).contains(Range::new(0, 0)));
+        assert!(!Range::new(1, 2).contains(Range::new(3, 10)));
+    }
+
+    #[test]
+    fn test_range_merge() {
+        assert_eq!(None, Range::new(1, 3).merge(Range::new(5, 5)));
+        assert_eq!(None, Range::new(4, 7).merge(Range::new(0, 1)));
+        assert_eq!(Some(Range::new(3, 7)),
+                   Range::new(3, 5).merge(Range::new(4, 7)));
+        assert_eq!(Some(Range::new(3, 7)),
+                   Range::new(3, 5).merge(Range::new(5, 7)));
+        assert_eq!(Some(Range::new(3, 7)),
+                   Range::new(3, 5).merge(Range::new(6, 7)));
+        assert_eq!(Some(Range::new(3, 7)),
+                   Range::new(3, 7).merge(Range::new(4, 5)));
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 12bfc6ef2da..322eed374de 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -24,6 +24,8 @@ extern crate unicode_segmentation;
 extern crate regex;
 extern crate diff;
 extern crate term;
+extern crate itertools;
+extern crate multimap;
 
 use syntax::ast;
 use syntax::codemap::{mk_sp, CodeMap, Span};
@@ -53,6 +55,7 @@ mod utils;
 pub mod config;
 pub mod codemap;
 pub mod filemap;
+pub mod file_lines;
 pub mod visitor;
 mod checkstyle;
 mod items;