Rollup merge of #41572 - frewsxcv:bump-mdbook, r=steveklabnik

Bump mdbook dep to pick up new 'create missing' toggle feature.

This will avoid obscure Travis CI error messages:

* https://github.com/rust-lang/rust/pull/40290#issuecomment-294137045

Original mdbook issue:

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

mdbook PR:

* https://github.com/azerupi/mdBook/pull/254

3 files changed, 19 insertions, 8 deletions

diff --git a/src/Cargo.lock b/src/Cargo.lock
index 0a584ac6b01..659feef80a4 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -269,7 +269,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "mdbook"
-version = "0.0.19"
+version = "0.0.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "clap 2.22.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -425,7 +425,7 @@ name = "rustbook"
 version = "0.1.0"
 dependencies = [
  "clap 2.22.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "mdbook 0.0.19 (registry+https://github.com/rust-lang/crates.io-index)",
+ "mdbook 0.0.21 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -1001,7 +1001,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum lazy_static 0.2.5 (registry+https://github.com/rust-lang/crates.io-index)" = "4732c563b9a21a406565c4747daa7b46742f082911ae4753f390dc9ec7ee1a97"
 "checksum libc 0.2.21 (registry+https://github.com/rust-lang/crates.io-index)" = "88ee81885f9f04bff991e306fea7c1c60a5f0f9e409e99f6b40e3311a3363135"
 "checksum log 0.3.7 (registry+https://github.com/rust-lang/crates.io-index)" = "5141eca02775a762cc6cd564d8d2c50f67c0ea3a372cbf1c51592b3e029e10ad"
-"checksum mdbook 0.0.19 (registry+https://github.com/rust-lang/crates.io-index)" = "2598843aeda0c5bb2e8e4d714564f1c3fc40f7844157e34563bf96ae3866b56e"
+"checksum mdbook 0.0.21 (registry+https://github.com/rust-lang/crates.io-index)" = "f1e2e9d848514dcfad4195788d0d42ae5153a477c191d75d5b84fab10f222fbd"
 "checksum memchr 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "1dbccc0e46f1ea47b9f17e6d67c5a96bd27030519c519c9c91327e31275a47b4"
 "checksum num-traits 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)" = "e1cbfa3781f3fe73dc05321bed52a06d2d491eaa764c52335cf4399f046ece99"
 "checksum num_cpus 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ca313f1862c7ec3e0dfe8ace9fa91b1d9cb5c84ace3d00f5ec4216238e93c167"
diff --git a/src/tools/rustbook/Cargo.toml b/src/tools/rustbook/Cargo.toml
index e3209330672..2d5d163d403 100644
--- a/src/tools/rustbook/Cargo.toml
+++ b/src/tools/rustbook/Cargo.toml
@@ -8,5 +8,5 @@ license = "MIT/Apache-2.0"
 clap = "2.19.3"
 
 [dependencies.mdbook]
-version = "0.0.19"
+version = "0.0.21"
 default-features = false
diff --git a/src/tools/rustbook/src/main.rs b/src/tools/rustbook/src/main.rs
index b77baed5326..33326de9c1c 100644
--- a/src/tools/rustbook/src/main.rs
+++ b/src/tools/rustbook/src/main.rs
@@ -53,8 +53,7 @@ fn main() {
 
 // Build command implementation
 fn build(args: &ArgMatches) -> Result<(), Box<Error>> {
-    let book_dir = get_book_dir(args);
-    let book = MDBook::new(&book_dir).read_config();
+    let book = build_mdbook_struct(args);
 
     let mut book = match args.value_of("dest-dir") {
         Some(dest_dir) => book.set_dest(Path::new(dest_dir)),
@@ -67,14 +66,26 @@ fn build(args: &ArgMatches) -> Result<(), Box<Error>> {
 }
 
 fn test(args: &ArgMatches) -> Result<(), Box<Error>> {
-    let book_dir = get_book_dir(args);
-    let mut book = MDBook::new(&book_dir).read_config();
+    let mut book = build_mdbook_struct(args);
 
     try!(book.test());
 
     Ok(())
 }
 
+fn build_mdbook_struct(args: &ArgMatches) -> mdbook::MDBook {
+    let book_dir = get_book_dir(args);
+    let mut book = MDBook::new(&book_dir).read_config();
+
+    // By default mdbook will attempt to create non-existent files referenced
+    // from SUMMARY.md files. This is problematic on CI where we mount the
+    // source directory as readonly. To avoid any issues, we'll disabled
+    // mdbook's implicit file creation feature.
+    book.create_missing = false;
+
+    book
+}
+
 fn get_book_dir(args: &ArgMatches) -> PathBuf {
     if let Some(dir) = args.value_of("dir") {
         // Check if path is relative from current dir, or absolute...