Flag defaults to false but is forced to true for new books

Author: Pierre Gradot

src/cmd/init.rs

@@ -1,8 +1,7 @@
 use crate::get_book_dir;
 use clap::{arg, ArgMatches, Command as ClapCommand};
-use mdbook::config;
 use mdbook::errors::Result;
-use mdbook::MDBook;
+use mdbook::{Config, MDBook};
 use std::io;
 use std::io::Write;
 use std::process::Command;
@@ -31,7 +30,12 @@ pub fn make_subcommand() -> ClapCommand {
 pub fn execute(args: &ArgMatches) -> Result<()> {
     let book_dir = get_book_dir(args);
     let mut builder = MDBook::init(&book_dir);
-    let mut config = config::Config::default();
+
+    let mut config = Config::default();
+    // We want new books to raise an error if a preprocessor is missing, but we want old books to simply emit a warning.
+    // This is why BuildConfig::default() sets it to false and why we force it to true here
+    config.build.error_on_missing_preprocessor = true;
+
     // If flag `--theme` is present, copy theme to src
     if args.get_flag("theme") {
         let theme_dir = book_dir.join("theme");

src/config.rs

@@ -481,9 +481,10 @@ pub struct BuildConfig {
     /// Extra directories to trigger rebuild when watching/serving
     pub extra_watch_dirs: Vec<PathBuf>,
     /// Should missing a preprocessor be considered an error?
-    /// By default, the application exits if a preprocessor is missing.
-    /// Set this flag to  ̀false` to raise a warning instead and continue generation,
+    /// By default, the application raises a warning instead and continue generation,
     /// even if the book may be generated incorrectly.
+    /// Set this flag to  ̀false` to consider this an error, and exits the application
+    /// if a preprocessor is missing.
     pub error_on_missing_preprocessor: bool,
 }
 
@@ -494,7 +495,7 @@ impl Default for BuildConfig {
             create_missing: true,
             use_default_preprocessors: true,
             extra_watch_dirs: Vec::new(),
-            error_on_missing_preprocessor: true,
+            error_on_missing_preprocessor: false,
         }
     }
 }
@@ -1075,7 +1076,8 @@ mod tests {
             create_missing: true,
             use_default_preprocessors: true,
             extra_watch_dirs: Vec::new(),
-            error_on_missing_preprocessor: true,
+            error_on_missing_preprocessor: false, // This flag is missing from "src" string,
+                                                  // so it should be false to ensure backward compatibility
         };
 
         let html_should_be = HtmlConfig {

tests/init.rs

@@ -94,7 +94,7 @@ fn run_mdbook_init_with_custom_book_and_src_locations() {
     let contents = fs::read_to_string(temp.path().join("book.toml")).unwrap();
     assert_eq!(
         contents,
-        "[book]\nauthors = []\nlanguage = \"en\"\nmultilingual = false\nsrc = \"in\"\n\n[build]\nbuild-dir = \"out\"\ncreate-missing = true\nerror-on-missing-preprocessor = true\nextra-watch-dirs = []\nuse-default-preprocessors = true\n"
+        "[book]\nauthors = []\nlanguage = \"en\"\nmultilingual = false\nsrc = \"in\"\n\n[build]\nbuild-dir = \"out\"\ncreate-missing = true\nerror-on-missing-preprocessor = false\nextra-watch-dirs = []\nuse-default-preprocessors = true\n"
     );
 }