Move utils bridged methods into their own ffi module

Discovered that we can use `Pin<&mut ...>` to allow mutation of Rust
types defined in another cxx bridge module, based on:
- https://cxx.rs/extern-c++.html#opaque-c-types
  > Unlike extern Rust types and shared types, an extern C++ type is not
  > permitted to be passed by plain mutable reference &mut MyType across the
  > FFI bridge. For mutation support, the bridge is required to use Pin<&mut
  > MyType>. This is to safeguard against things like mem::swap-ing the
  > contents of two mutable references, given that Rust doesn't have
  > information about the size of the underlying object and couldn't invoke
  > an appropriate C++ move constructor anyway.
- dtolnay/cxx#942

include/filepath.h

@@ -36,7 +36,7 @@ class Filepath {
 	{
 	}
 
-	Filepath(rust::Box<filepath::bridged::PathBuf> rs_object)
+	Filepath(rust::Box<filepath::bridged::PathBuf>&& rs_object)
 		: rs_object(std::move(rs_object))
 	{
 	}

rust/libnewsboat-ffi/src/filepath.rs

@@ -27,13 +27,6 @@ mod bridged {
         fn set_extension(filepath: &mut PathBuf, extension: Vec<u8>) -> bool;
         fn starts_with(filepath: &PathBuf, str: Vec<u8>) -> bool;
         fn file_name(filepath: &PathBuf) -> Vec<u8>;
-
-        // These functions are actually in utils.rs, but I couldn't find a way to return
-        // `Box<PathBuf>` from libnewsboat-ffi/src/utils.rs, so I moved the bindings here
-        fn get_default_browser() -> Box<PathBuf>;
-        fn resolve_tilde(path: &PathBuf) -> Box<PathBuf>;
-        fn resolve_relative(reference: &PathBuf, path: &PathBuf) -> Box<PathBuf>;
-        fn getcwd() -> Box<PathBuf>;
     }
 }
 
@@ -67,26 +60,6 @@ fn clone(filepath: &PathBuf) -> Box<PathBuf> {
     Box::new(PathBuf(filepath.0.clone()))
 }
 
-fn get_default_browser() -> Box<PathBuf> {
-    Box::new(PathBuf(libnewsboat::utils::get_default_browser()))
-}
-
-fn resolve_tilde(path: &PathBuf) -> Box<PathBuf> {
-    Box::new(PathBuf(libnewsboat::utils::resolve_tilde(path.0.clone())))
-}
-
-fn resolve_relative(reference: &PathBuf, path: &PathBuf) -> Box<PathBuf> {
-    Box::new(PathBuf(libnewsboat::utils::resolve_relative(
-        &reference.0,
-        &path.0,
-    )))
-}
-
-fn getcwd() -> Box<PathBuf> {
-    let result = libnewsboat::utils::getcwd().unwrap_or_else(|_| std::path::PathBuf::new());
-    Box::new(PathBuf(result))
-}
-
 fn is_absolute(filepath: &PathBuf) -> bool {
     filepath.0.is_absolute()
 }

rust/libnewsboat-ffi/src/utils.rs

@@ -2,6 +2,7 @@ use crate::filepath::PathBuf;
 use libc::{c_char, c_ulong};
 use libnewsboat::utils::{self, *};
 use std::ffi::{CStr, CString};
+use std::pin::Pin;
 
 #[cxx::bridge(namespace = "newsboat::utils")]
 mod ffi {
@@ -79,6 +80,10 @@ mod bridged {
         fn tokenize_quoted(line: &str, delimiters: &str) -> Vec<String>;
         fn is_valid_podcast_type(mimetype: &str) -> bool;
 
+        fn get_default_browser(mut path: Pin<&mut PathBuf>);
+        fn resolve_tilde(path: &PathBuf, mut output: Pin<&mut PathBuf>);
+        fn resolve_relative(reference: &PathBuf, path: &PathBuf, mut output: Pin<&mut PathBuf>);
+        fn getcwd(mut path: Pin<&mut PathBuf>);
         fn mkdir_parents(path: &PathBuf, mode: u32) -> isize;
 
         fn unescape_url(url: String, success: &mut bool) -> String;
@@ -189,6 +194,22 @@ fn extract_filter(line: &str) -> ffi::FilterUrlParts {
     }
 }
 
+fn get_default_browser(mut path: Pin<&mut PathBuf>) {
+    path.0 = utils::get_default_browser();
+}
+
+fn resolve_tilde(path: &PathBuf, mut output: Pin<&mut PathBuf>) {
+    output.0 = utils::resolve_tilde(path.0.clone());
+}
+
+fn resolve_relative(reference: &PathBuf, path: &PathBuf, mut output: Pin<&mut PathBuf>) {
+    output.0 = utils::resolve_relative(&reference.0, &path.0);
+}
+
+fn getcwd(mut path: Pin<&mut PathBuf>) {
+    path.0 = utils::getcwd().unwrap_or_else(|_| std::path::PathBuf::new());
+}
+
 fn mkdir_parents(path: &PathBuf, mode: u32) -> isize {
     match utils::mkdir_parents(&path.0, mode) {
         Ok(_) => 0,

src/utils.cpp

@@ -340,17 +340,17 @@ std::string utils::run_program(const char* argv[], const std::string& input)
 
 Filepath utils::resolve_tilde(const Filepath& path)
 {
-	// The function is in utils.rs, but its binding is in filepath.rs because
-	// I can't make it return `Box<PathBuf>` any other way
-	return filepath::bridged::resolve_tilde(path);
+	auto output = filepath::bridged::create_empty();
+	utils::bridged::resolve_tilde(path, *output);
+	return output;
 }
 
 Filepath utils::resolve_relative(const Filepath& reference,
 	const Filepath& fname)
 {
-	// The function is in utils.rs, but its binding is in filepath.rs because
-	// I can't make it return `Box<PathBuf>` any other way
-	return filepath::bridged::resolve_relative(reference, fname);
+	auto output = filepath::bridged::create_empty();
+	utils::bridged::resolve_relative(reference, fname, *output);
+	return output;
 }
 
 std::string utils::replace_all(std::string str,
@@ -719,9 +719,9 @@ nonstd::optional<std::uint8_t> utils::run_non_interactively(
 
 Filepath utils::getcwd()
 {
-	// The function is in utils.rs, but its binding is in filepath.rs because
-	// I can't make it return `Box<PathBuf>` any other way
-	return filepath::bridged::getcwd();
+	auto path = filepath::bridged::create_empty();
+	utils::bridged::getcwd(*path);
+	return path;
 }
 
 nonstd::expected<std::vector<std::string>, utils::ReadTextFileError> utils::read_text_file(
@@ -842,9 +842,9 @@ void utils::initialize_ssl_implementation(void)
 
 Filepath utils::get_default_browser()
 {
-	// The function is in utils.rs, but its binding is in filepath.rs because
-	// I can't make it return `Box<PathBuf>` any other way
-	return filepath::bridged::get_default_browser();
+	auto path = filepath::bridged::create_empty();
+	utils::bridged::get_default_browser(*path);
+	return path;
 }
 
 std::string utils::md5hash(const std::string& input)