Implement a markdown renderer (#1018)

Use case: when trying to `mdbook test` a file that has many `include`
directives, and a test fails, the line numbers in the `rustdoc` output
don't match the line numbers in the original markdown file.

Turning on the markdown renderer implemented here lets you see what is
being passed to `rustdoc` by saving the markdown after the preprocessors
have run.

This renderer could be helpful for debugging many preprocessors, but
it's probably not useful in the general case, so it's turned off by
default.

README.md

@@ -169,6 +169,9 @@ format, however there's nothing stopping a renderer from doing static analysis
 of a book in order to validate links or run tests. Some existing renderers are:
 
 - `html` - the built-in renderer which will generate a HTML version of the book
+- `markdown` - the built-in renderer (disabled by default) which will run
+  preprocessors then output the resulting Markdown. Useful for debugging
+  preprocessors.
 - [`linkcheck`] - a backend which will check that all links are valid
 - [`epub`] - an experimental EPUB generator
 

book-example/src/format/config.md

@@ -242,6 +242,26 @@ heading-split-level = 3
 copy-js = true
 ```
 
+### Markdown Renderer
+
+The Markdown renderer will run preprocessors and then output the resulting
+Markdown. This is mostly useful for debugging preprocessors, especially in
+conjunction with `mdbook test` to see the Markdown that `mdbook` is passing
+to `rustdoc`.
+
+The Markdown renderer is included with `mdbook` but disabled by default.
+Enable it by adding an emtpy table to your `book.toml` as follows:
+
+```toml
+[output.markdown]
+```
+
+There are no configuration options for the Markdown renderer at this time;
+only whether it is enabled or disabled.
+
+See [the preprocessors documentation](#configuring-preprocessors) for how to
+specify which preprocessors should run before the Markdown renderer.
+
 ### Custom Renderers
 
 A custom renderer can be enabled by adding a `[output.foo]` table to your

src/book/mod.rs

@@ -24,7 +24,7 @@ use crate::errors::*;
 use crate::preprocess::{
     CmdPreprocessor, IndexPreprocessor, LinkPreprocessor, Preprocessor, PreprocessorContext,
 };
-use crate::renderer::{CmdRenderer, HtmlHandlebars, RenderContext, Renderer};
+use crate::renderer::{CmdRenderer, HtmlHandlebars, MarkdownRenderer, RenderContext, Renderer};
 use crate::utils;
 
 use crate::config::Config;
@@ -336,6 +336,8 @@ fn determine_renderers(config: &Config) -> Vec<Box<dyn Renderer>> {
         renderers.extend(output_table.iter().map(|(key, table)| {
             if key == "html" {
                 Box::new(HtmlHandlebars::new()) as Box<dyn Renderer>
+            } else if key == "markdown" {
+                Box::new(MarkdownRenderer::new()) as Box<dyn Renderer>
             } else {
                 interpret_custom_renderer(key, table)
             }

src/renderer/markdown_renderer.rs

@@ -0,0 +1,46 @@
+use crate::book::BookItem;
+use crate::errors::*;
+use crate::renderer::{RenderContext, Renderer};
+use crate::utils;
+
+use std::fs;
+
+#[derive(Default)]
+/// A renderer to output the Markdown after the preprocessors have run. Mostly useful
+/// when debugging preprocessors.
+pub struct MarkdownRenderer;
+
+impl MarkdownRenderer {
+    /// Create a new `MarkdownRenderer` instance.
+    pub fn new() -> Self {
+        MarkdownRenderer
+    }
+}
+
+impl Renderer for MarkdownRenderer {
+    fn name(&self) -> &str {
+        "markdown"
+    }
+
+    fn render(&self, ctx: &RenderContext) -> Result<()> {
+        let destination = &ctx.destination;
+        let book = &ctx.book;
+
+        if destination.exists() {
+            utils::fs::remove_dir_content(destination)
+                .chain_err(|| "Unable to remove stale Markdown output")?;
+        }
+
+        trace!("markdown render");
+        for item in book.iter() {
+            if let BookItem::Chapter(ref ch) = *item {
+                utils::fs::write_file(&ctx.destination, &ch.path, ch.content.as_bytes())?;
+            }
+        }
+
+        fs::create_dir_all(&destination)
+            .chain_err(|| "Unexpected error when constructing destination path")?;
+
+        Ok(())
+    }
+}

src/renderer/mod.rs

@@ -12,8 +12,10 @@
 //! [RenderContext]: struct.RenderContext.html
 
 pub use self::html_handlebars::HtmlHandlebars;
+pub use self::markdown_renderer::MarkdownRenderer;
 
 mod html_handlebars;
+mod markdown_renderer;
 
 use shlex::Shlex;
 use std::fs;