New disable_all_formatting config option

src/config.rs

@@ -326,6 +326,7 @@ macro_rules! create_config {
 
 create_config! {
     verbose: bool, false, "Use verbose output";
+    disable_all_formatting: bool, false, "Don't reformat anything";
     skip_children: bool, false, "Don't reformat out of line modules";
     file_lines: FileLines, FileLines::all(),
         "Lines to format; this is not supported in rustfmt.toml, and can only be specified \

src/lib.rs

@@ -473,6 +473,9 @@ pub fn format_input<T: Write>(input: Input,
                               mut out: Option<&mut T>)
                               -> Result<(Summary, FileMap, FormatReport), (io::Error, Summary)> {
     let mut summary = Summary::new();
+    if config.disable_all_formatting {
+        return Ok((summary, FileMap::new(), FormatReport::new()));
+    }
     let codemap = Rc::new(CodeMap::new());
 
     let tty_handler =

tests/target/disable_all_formatting.rs

@@ -0,0 +1,4 @@
+// rustfmt-disable_all_formatting: true
+// Don't format anything.
+
+fn main() { println!("This should not be formatted."); }