front_matter: fix traversal of content directory

The previous approach that only looked for markdown files in content/
and content/inside-rust/ stopped working when we started using Zola's
feature of colocating a blog post with its assets. In those cases, the
markdown file might be located at content/<slug>/index.md.

Author: senekor

Cargo.lock

@@ -7,3 +7,6 @@ edition = "2024"
 eyre = "=0.6.12"
 serde = { version = "=1.0.219", features = ["derive"] }
 toml = "=0.8.20"
+
+[dev-dependencies]
+walkdir = "2.5.0"

front_matter/Cargo.toml

@@ -253,11 +253,14 @@ The post {post} has abnormal front matter.
     }
 
     fn all_posts() -> impl Iterator<Item = PathBuf> {
-        let repo_root = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("..");
-        fs::read_dir(repo_root.join("content"))
-            .unwrap()
-            .chain(fs::read_dir(repo_root.join("content/inside-rust")).unwrap())
-            .map(|p| p.unwrap().path())
-            .filter(|p| p.is_file() && p.file_name() != Some("_index.md".as_ref()))
+        walkdir::WalkDir::new(concat!(env!("CARGO_MANIFEST_DIR"), "/../content"))
+            .into_iter()
+            .filter_map(|e| e.ok().map(|e| e.into_path()))
+            .filter(|p| {
+                p.is_file()
+                    && p.extension() == Some("md".as_ref())
+                    && p.file_name() != Some("_index.md".as_ref())
+                    && p.file_name() != Some("latest.md".as_ref())
+            })
     }
 }