Implement new 'no-create' build flag.

Fixes https://github.com/azerupi/mdBook/issues/253.

src/bin/mdbook.rs

@@ -63,6 +63,7 @@ fn main() {
                         .about("Build the book from the markdown files")
                         .arg_from_usage("-o, --open 'Open the compiled book in a web browser'")
                         .arg_from_usage("-d, --dest-dir=[dest-dir] 'The output directory for your book{n}(Defaults to ./book when omitted)'")
+                        .arg_from_usage("--no-create 'Will not create non-existent files linked from SUMMARY.md'")
                         .arg_from_usage("[dir] 'A directory for your book{n}(Defaults to Current Directory when omitted)'"))
                     .subcommand(SubCommand::with_name("watch")
                         .about("Watch the files for changes")
@@ -174,6 +175,10 @@ fn build(args: &ArgMatches) -> Result<(), Box<Error>> {
         None => book
     };
 
+    if args.is_present("no-create") {
+        book.create_missing = false;
+    }
+
     try!(book.build());
 
     if args.is_present("open") {

src/book/mod.rs

@@ -32,6 +32,10 @@ pub struct MDBook {
     renderer: Box<Renderer>,
 
     livereload: Option<String>,
+
+    /// Should `mdbook build` create files referenced from SUMMARY.md if they
+    /// don't exist
+    pub create_missing: bool,
 }
 
 impl MDBook {
@@ -79,6 +83,7 @@ impl MDBook {
             renderer: Box::new(HtmlHandlebars::new()),
 
             livereload: None,
+            create_missing: true,
         }
     }
 
@@ -183,6 +188,9 @@ impl MDBook {
                         let path = self.src.join(&ch.path);
 
                         if !path.exists() {
+                            if !self.create_missing {
+                                return Err(format!("'{}' referenced from SUMMARY.md does not exist.", path.to_string_lossy()).into());
+                            }
                             debug!("[*]: {:?} does not exist, trying to create file", path);
                             try!(::std::fs::create_dir_all(path.parent().unwrap()));
                             let mut f = try!(File::create(path));