Substitute usage information into the documentation

This will simplify keeping documentation in sync with the code.

cli/src/bin/usage-docs.rs

@@ -0,0 +1,50 @@
+use mdbook::book::{Book, BookItem};
+use mdbook::errors::Error;
+use mdbook::preprocess::{CmdPreprocessor, PreprocessorContext};
+use std::io;
+use std::process;
+use taskchampion_cli::Usage;
+
+/// This is a simple mdbook preprocessor designed to substitute information from the usage
+/// into the documentation.
+fn main() -> anyhow::Result<()> {
+    // cheap way to detect the "supports" arg
+    if std::env::args().len() > 1 {
+        // sure, whatever, we support it all
+        process::exit(0);
+    }
+
+    let (ctx, book) = CmdPreprocessor::parse_input(io::stdin())?;
+
+    if ctx.mdbook_version != mdbook::MDBOOK_VERSION {
+        eprintln!(
+            "Warning: This mdbook preprocessor was built against version {} of mdbook, \
+             but we're being called from version {}",
+            mdbook::MDBOOK_VERSION,
+            ctx.mdbook_version
+        );
+    }
+
+    let processed_book = process(&ctx, book)?;
+    serde_json::to_writer(io::stdout(), &processed_book)?;
+
+    Ok(())
+}
+
+fn process(_ctx: &PreprocessorContext, mut book: Book) -> Result<Book, Error> {
+    let usage = Usage::new();
+
+    book.for_each_mut(|sect| {
+        if let BookItem::Chapter(ref mut chapter) = sect {
+            let new_content = usage.substitute_docs(&chapter.content).unwrap();
+            if new_content != chapter.content {
+                eprintln!(
+                    "Substituting usage in {:?}",
+                    chapter.source_path.as_ref().unwrap()
+                );
+            }
+            chapter.content = new_content;
+        }
+    });
+    Ok(book)
+}