bail! in render() if specified theme directory does not exist

src/renderer/html_handlebars/hbs_renderer.rs

@@ -481,7 +481,13 @@ impl Renderer for HtmlHandlebars {
         let mut handlebars = Handlebars::new();
 
         let theme_dir = match html_config.theme {
-            Some(ref theme) => ctx.root.join(theme),
+            Some(ref theme) => {
+                let dir = ctx.root.join(theme);
+                if !dir.is_dir() {
+                    bail!("theme dir {} does not exist", dir.display());
+                }
+                dir
+            }
             None => ctx.root.join("theme"),
         };
 

tests/rendered_output.rs

@@ -657,6 +657,57 @@ fn summary_with_markdown_formatting() {
     );
 }
 
+/// Ensure building fails if `[output.html].theme` points to a non-existent directory
+#[test]
+fn failure_on_missing_theme_directory() {
+    // 1. Using default theme should work
+    let temp = DummyBook::new().build().unwrap();
+    let book_toml = r#"
+        [book]
+        title = "implicit"
+        src = "src"
+        "#;
+
+    write_file(temp.path(), "book.toml", book_toml.as_bytes()).unwrap();
+    let md = MDBook::load(temp.path()).unwrap();
+    let got = md.build();
+    assert!(got.is_ok());
+
+    // 2. Pointing to a normal directory should work
+    let temp = DummyBook::new().build().unwrap();
+    let created = fs::create_dir(temp.path().join("theme-directory"));
+    assert!(created.is_ok());
+    let book_toml = r#"
+        [book]
+        title = "implicit"
+        src = "src"
+
+        [output.html]
+        theme = "./theme-directory"
+        "#;
+
+    write_file(temp.path(), "book.toml", book_toml.as_bytes()).unwrap();
+    let md = MDBook::load(temp.path()).unwrap();
+    let got = md.build();
+    assert!(got.is_ok());
+
+    // 3. Pointing to a non-existent directory should fail
+    let temp = DummyBook::new().build().unwrap();
+    let book_toml = r#"
+        [book]
+        title = "implicit"
+        src = "src"
+
+        [output.html]
+        theme = "./non-existent-directory"
+        "#;
+
+    write_file(temp.path(), "book.toml", book_toml.as_bytes()).unwrap();
+    let md = MDBook::load(temp.path()).unwrap();
+    let got = md.build();
+    assert!(got.is_err());
+}
+
 #[cfg(feature = "search")]
 mod search {
     use crate::dummy_book::DummyBook;