Implement a rustdoc_include preprocessor (#1003)

* Allow underscores in the link type name

* Add some tests for include anchors

* Include parts of Rust files and hide the rest

Fixes #618.

* Increase min supported Rust version to 1.35

* Add a test for a behavior of rustdoc_include I want to depend on

At first I thought this was a bug, but then I looked at some use cases
we have in TRPL and decided this was a feature that I'd like to use.

.travis.yml

@@ -17,7 +17,7 @@ matrix:
       env: TARGET=x86_64-unknown-linux-gnu
     - rust: nightly
       env: TARGET=x86_64-unknown-linux-gnu
-    - rust: 1.34.0 # Minimum required Rust version
+    - rust: 1.35.0 # Minimum required Rust version
       env: TARGET=x86_64-unknown-linux-gnu
 
     - rust: stable

README.md

@@ -41,7 +41,7 @@ There are multiple ways to install mdBook.
 
 2. **From Crates.io**
 
-   This requires at least [Rust] 1.34 and Cargo to be installed. Once you have installed
+   This requires at least [Rust] 1.35 and Cargo to be installed. Once you have installed
    Rust, type the following in the terminal:
 
    ```

book-example/src/format/config.md

@@ -81,7 +81,7 @@ This controls the build process of your book.
 
 The following preprocessors are available and included by default:
 
-- `links`: Expand the `{{ #playpen }}` and `{{ #include }}` handlebars
+- `links`: Expand the `{{ #playpen }}`, `{{ #include }}`, and `{{ #rustdoc_include }}` handlebars
   helpers in a chapter to include the contents of a file.
 - `index`: Convert all chapter files named `README.md` into `index.md`. That is
   to say, all `README.md` would be rendered to an index file `index.html` in the

book-example/src/format/mdbook.md

@@ -3,7 +3,9 @@
 ## Hiding code lines
 
 There is a feature in mdBook that lets you hide code lines by prepending them
-with a `#`.
+with a `#` [in the same way that Rustdoc does][rustdoc-hide].
+
+[rustdoc-hide]: https://doc.rust-lang.org/stable/rustdoc/documentation-tests.html#hiding-portions-of-the-example
 
 ```bash
 # fn main() {
@@ -107,6 +109,70 @@ This is the full file.
 
 Lines containing anchor patterns inside the included anchor are ignored.
 
+## Including a file but initially hiding all except specified lines
+
+The `rustdoc_include` helper is for including code from external Rust files that contain complete
+examples, but only initially showing particular lines specified with line numbers or anchors in the
+same way as with `include`.
+
+The lines not in the line number range or between the anchors will still be included, but they will
+be prefaced with `#`. This way, a reader can expand the snippet to see the complete example, and
+Rustdoc will use the complete example when you run `mdbook test`.
+
+For example, consider a file named `file.rs` that contains this Rust program:
+
+```rust
+fn main() {
+    let x = add_one(2);
+    assert_eq!(x, 3);
+}
+
+fn add_one(num: i32) -> i32 {
+    num + 1
+}
+```
+
+We can include a snippet that initially shows only line 2 by using this syntax:
+
+````hbs
+To call the `add_one` function, we pass it an `i32` and bind the returned value to `x`:
+
+```rust
+\{{#rustdoc_include file.rs:2}}
+```
+````
+
+This would have the same effect as if we had manually inserted the code and hidden all but line 2
+using `#`:
+
+````hbs
+To call the `add_one` function, we pass it an `i32` and bind the returned value to `x`:
+
+```rust
+# fn main() {
+    let x = add_one(2);
+#     assert_eq!(x, 3);
+# }
+#
+# fn add_one(num: i32) -> i32 {
+#     num + 1
+#}
+```
+````
+
+That is, it looks like this (click the "expand" icon to see the rest of the file):
+
+```rust
+# fn main() {
+    let x = add_one(2);
+#     assert_eq!(x, 3);
+# }
+#
+# fn add_one(num: i32) -> i32 {
+#     num + 1
+#}
+```
+
 ## Inserting runnable Rust files
 
 With the following syntax, you can insert runnable Rust files into your book:

src/preprocess/links.rs

@@ -1,5 +1,8 @@
 use crate::errors::*;
-use crate::utils::{take_anchored_lines, take_lines};
+use crate::utils::{
+    take_anchored_lines, take_lines, take_rustdoc_include_anchored_lines,
+    take_rustdoc_include_lines,
+};
 use regex::{CaptureMatches, Captures, Regex};
 use std::fs;
 use std::ops::{Bound, Range, RangeBounds, RangeFrom, RangeFull, RangeTo};
@@ -11,8 +14,15 @@ use crate::book::{Book, BookItem};
 const ESCAPE_CHAR: char = '\\';
 const MAX_LINK_NESTED_DEPTH: usize = 10;
 
-/// A preprocessor for expanding the `{{# playpen}}` and `{{# include}}`
-/// helpers in a chapter.
+/// A preprocessor for expanding helpers in a chapter. Supported helpers are:
+///
+/// - `{{# include}}` - Insert an external file of any type. Include the whole file, only particular
+///.  lines, or only between the specified anchors.
+/// - `{{# rustdoc_include}}` - Insert an external Rust file, showing the particular lines
+///.  specified or the lines between specified anchors, and include the rest of the file behind `#`.
+///   This hides the lines from initial display but shows them when the reader expands the code
+///   block and provides them to Rustdoc for testing.
+/// - `{{# playpen}}` - Insert runnable Rust files
 #[derive(Default)]
 pub struct LinkPreprocessor;
 
@@ -104,6 +114,7 @@ enum LinkType<'a> {
     Escaped,
     Include(PathBuf, RangeOrAnchor),
     Playpen(PathBuf, Vec<&'a str>),
+    RustdocInclude(PathBuf, RangeOrAnchor),
 }
 
 #[derive(PartialEq, Debug, Clone)]
@@ -172,6 +183,7 @@ impl<'a> LinkType<'a> {
             LinkType::Escaped => None,
             LinkType::Include(p, _) => Some(return_relative_path(base, &p)),
             LinkType::Playpen(p, _) => Some(return_relative_path(base, &p)),
+            LinkType::RustdocInclude(p, _) => Some(return_relative_path(base, &p)),
         }
     }
 }
@@ -222,6 +234,15 @@ fn parse_include_path(path: &str) -> LinkType<'static> {
     LinkType::Include(path, range_or_anchor)
 }
 
+fn parse_rustdoc_include_path(path: &str) -> LinkType<'static> {
+    let mut parts = path.splitn(2, ':');
+
+    let path = parts.next().unwrap().into();
+    let range_or_anchor = parse_range_or_anchor(parts.next());
+
+    LinkType::RustdocInclude(path, range_or_anchor)
+}
+
 #[derive(PartialEq, Debug, Clone)]
 struct Link<'a> {
     start_index: usize,
@@ -241,6 +262,7 @@ impl<'a> Link<'a> {
                 match (typ.as_str(), file_arg) {
                     ("include", Some(pth)) => Some(parse_include_path(pth)),
                     ("playpen", Some(pth)) => Some(LinkType::Playpen(pth.into(), props)),
+                    ("rustdoc_include", Some(pth)) => Some(parse_rustdoc_include_path(pth)),
                     _ => None,
                 }
             }
@@ -281,6 +303,26 @@ impl<'a> Link<'a> {
                         )
                     })
             }
+            LinkType::RustdocInclude(ref pat, ref range_or_anchor) => {
+                let target = base.join(pat);
+
+                fs::read_to_string(&target)
+                    .map(|s| match range_or_anchor {
+                        RangeOrAnchor::Range(range) => {
+                            take_rustdoc_include_lines(&s, range.clone())
+                        }
+                        RangeOrAnchor::Anchor(anchor) => {
+                            take_rustdoc_include_anchored_lines(&s, anchor)
+                        }
+                    })
+                    .chain_err(|| {
+                        format!(
+                            "Could not read file for link {} ({})",
+                            self.link_text,
+                            target.display(),
+                        )
+                    })
+            }
             LinkType::Playpen(ref pat, ref attrs) => {
                 let target = base.join(pat);
 
@@ -326,7 +368,7 @@ fn find_links(contents: &str) -> LinkIter<'_> {
             \\\{\{\#.*\}\}             # match escaped link
             |                          # or
             \{\{\s*                    # link opening parens and whitespace
-            \#([a-zA-Z0-9]+)           # link type
+            \#([a-zA-Z0-9_]+)          # link type
             \s+                        # separating whitespace
             ([a-zA-Z0-9\s_.\-:/\\]+)   # link target path and space separated properties
             \s*\}\}                    # whitespace and link closing parens"

src/utils/mod.rs

@@ -11,7 +11,10 @@ use std::borrow::Cow;
 use std::fmt::Write;
 use std::path::Path;
 
-pub use self::string::{take_anchored_lines, take_lines};
+pub use self::string::{
+    take_anchored_lines, take_lines, take_rustdoc_include_anchored_lines,
+    take_rustdoc_include_lines,
+};
 
 /// Replaces multiple consecutive whitespace characters with a single space character.
 pub fn collapse_whitespace(text: &str) -> Cow<'_, str> {