Add a lifetime annotation to the Postprocesor type

This lets the compiler reason about the lifetimes of objects used by the
postprocessor, if the callback captures variables.

See #175

src/lib.rs

@@ -133,8 +133,8 @@ pub type MarkdownEvents<'a> = Vec<Event<'a>>;
 /// # exporter.run().unwrap();
 /// ```
 
-pub type Postprocessor =
-    dyn Fn(&mut Context, &mut MarkdownEvents) -> PostprocessorResult + Send + Sync;
+pub type Postprocessor<'f> =
+    dyn Fn(&mut Context, &mut MarkdownEvents) -> PostprocessorResult + Send + Sync + 'f;
 type Result<T, E = ExportError> = std::result::Result<T, E>;
 
 const PERCENTENCODE_CHARS: &AsciiSet = &CONTROLS.add(b' ').add(b'(').add(b')').add(b'%').add(b'?');
@@ -231,8 +231,8 @@ pub struct Exporter<'a> {
     vault_contents: Option<Vec<PathBuf>>,
     walk_options: WalkOptions<'a>,
     process_embeds_recursively: bool,
-    postprocessors: Vec<&'a Postprocessor>,
-    embed_postprocessors: Vec<&'a Postprocessor>,
+    postprocessors: Vec<&'a Postprocessor<'a>>,
+    embed_postprocessors: Vec<&'a Postprocessor<'a>>,
 }
 
 impl<'a> fmt::Debug for Exporter<'a> {

tests/postprocessors_test.rs

@@ -3,8 +3,10 @@ use obsidian_export::{Context, Exporter, MarkdownEvents, PostprocessorResult};
 use pretty_assertions::assert_eq;
 use pulldown_cmark::{CowStr, Event};
 use serde_yaml::Value;
+use std::collections::HashSet;
 use std::fs::{read_to_string, remove_file};
 use std::path::PathBuf;
+use std::sync::Mutex;
 use tempfile::TempDir;
 
 /// This postprocessor replaces any instance of "foo" with "bar" in the note body.
@@ -105,6 +107,38 @@ fn test_postprocessor_change_destination() {
     assert!(new_note_path.exists());
 }
 
+// Ensure postprocessor type definition has proper lifetimes to allow state (here: `parents`)
+// to be passed in. Otherwise, this fails with an error like:
+//     error[E0597]: `parents` does not live long enough
+//     cast requires that `parents` is borrowed for `'static`
+#[test]
+fn test_postprocessor_stateful_callback() {
+    let tmp_dir = TempDir::new().expect("failed to make tempdir");
+    let mut exporter = Exporter::new(
+        PathBuf::from("tests/testdata/input/postprocessors"),
+        tmp_dir.path().to_path_buf(),
+    );
+
+    let parents: Mutex<HashSet<PathBuf>> = Default::default();
+    let callback = |ctx: &mut Context, _mdevents: &mut MarkdownEvents| -> PostprocessorResult {
+        parents
+            .lock()
+            .unwrap()
+            .insert(ctx.destination.parent().unwrap().to_path_buf());
+        PostprocessorResult::Continue
+    };
+    exporter.add_postprocessor(&callback);
+
+    exporter.run().unwrap();
+
+    let expected = tmp_dir.path().clone();
+
+    let parents = parents.lock().unwrap();
+    println!("{:?}", parents);
+    assert_eq!(1, parents.len());
+    assert!(parents.contains(expected));
+}
+
 // The purpose of this test to verify the `append_frontmatter` postprocessor is called to extend
 // the frontmatter, and the `foo_to_bar` postprocessor is called to replace instances of "foo" with
 // "bar" (only in the note body).