From 2bcae6b0a956ddda6dd5795ff657e168df169fc1 Mon Sep 17 00:00:00 2001
From: Eric Huss <eric@huss.org>
Date: Wed, 10 Dec 2025 12:51:19 -0800
Subject: [PATCH 1/2] Add a test with a custom preprocessor

This test illustrates that the test function is not working correctly
when there is a custom preprocessor.
---
 tests/testsuite/preprocessor.rs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tests/testsuite/preprocessor.rs b/tests/testsuite/preprocessor.rs
index cd8b9efad1..7aa02bd55e 100644
--- a/tests/testsuite/preprocessor.rs
+++ b/tests/testsuite/preprocessor.rs
@@ -44,6 +44,20 @@ fn runs_preprocessors() {
     assert_eq!(inner.rendered_with, ["html"]);
 }
 
+// Run tests with a custom preprocessor.
+#[test]
+fn test_with_custom_preprocessor() {
+    let test = BookTest::init(|_| {});
+    let spy: Arc<Mutex<Inner>> = Default::default();
+    let mut book = test.load_book();
+    book.with_preprocessor(Spy(Arc::clone(&spy)));
+    book.test(vec![]).unwrap();
+
+    let inner = spy.lock().unwrap();
+    assert_eq!(inner.run_count, 0);
+    assert_eq!(inner.rendered_with, Vec::<String>::new());
+}
+
 // No-op preprocessor works.
 #[test]
 fn nop_preprocessor() {

From 7bdea7c0852e2ed765c25091f97b684ffdc9ba25 Mon Sep 17 00:00:00 2001
From: Eric Huss <eric@huss.org>
Date: Wed, 10 Dec 2025 13:42:15 -0800
Subject: [PATCH 2/2] Don't rebuild preprocessor map during test

This fixes a problem where custom preprocessors were not being
registered when running tests. This was caused by the test function
rebuilding the preprocessor map.

This removes the code that was rebuilding the preprocessors and removing
the IndexPreprocessor when running tests. Skipping IndexPreprocessor was
added back in https://github.com/rust-lang/mdBook/pull/741 to fix
https://github.com/rust-lang/mdBook/issues/724 which was caused by
https://github.com/rust-lang/mdBook/pull/685 which added the
IndexPreprocessor.

Additionally, https://github.com/rust-lang/mdBook/pull/1986 added
running *all* preprocessors.

The IndexPreprocessor was removed because in the past the code was
testing against the source directly, and the path from `chapter.path` is
the converted `index.md` file, and that filename does not exist in the
source. This isn't a problem anymore because due to
https://github.com/rust-lang/mdBook/pull/891 it is not reading from the
`src` directory.

Note that this results in a minor change where the chapter path changes
from `README.md` to `index.md` in the output and the `--chapter` option.
I think I'm ok with that change, though it would be easy to switch it
back if that's an issue.
---
 crates/mdbook-driver/src/mdbook.rs | 5 -----
 tests/testsuite/preprocessor.rs    | 4 ++--
 2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/crates/mdbook-driver/src/mdbook.rs b/crates/mdbook-driver/src/mdbook.rs
index e5dac9027f..03ebcb8c30 100644
--- a/crates/mdbook-driver/src/mdbook.rs
+++ b/crates/mdbook-driver/src/mdbook.rs
@@ -263,11 +263,6 @@ impl MDBook {
             }
         }
 
-        // Index Preprocessor is disabled so that chapter paths
-        // continue to point to the actual markdown files.
-        self.preprocessors = determine_preprocessors(&self.config, &self.root)?;
-        self.preprocessors
-            .shift_remove_entry(IndexPreprocessor::NAME);
         let (book, _) = self.preprocess_book(&TestRenderer)?;
 
         let color_output = std::io::stderr().is_terminal();
diff --git a/tests/testsuite/preprocessor.rs b/tests/testsuite/preprocessor.rs
index 7aa02bd55e..33c320785e 100644
--- a/tests/testsuite/preprocessor.rs
+++ b/tests/testsuite/preprocessor.rs
@@ -54,8 +54,8 @@ fn test_with_custom_preprocessor() {
     book.test(vec![]).unwrap();
 
     let inner = spy.lock().unwrap();
-    assert_eq!(inner.run_count, 0);
-    assert_eq!(inner.rendered_with, Vec::<String>::new());
+    assert_eq!(inner.run_count, 1);
+    assert_eq!(inner.rendered_with, ["test"]);
 }
 
 // No-op preprocessor works.