path: use std::io::Error instead of raw ints

The functions we use already give us errors from `std`, so we might as
well use them, instead of converting them to raw libc errors.

Part of #12365

src/path.rs

@@ -88,14 +88,14 @@ pub fn path_get_config_remoteness() -> DirRemoteness {
 /// Use the given environment stack to ensure this only occurs once.
 pub fn path_emit_config_directory_messages(vars: &EnvStack) {
     let data = get_data_directory();
-    if !data.success() {
+    if let Some(error) = &data.err {
         maybe_issue_path_warning(
             L!("data"),
             wgettext!("can not save history"),
             data.used_xdg,
             L!("XDG_DATA_HOME"),
             &data.path,
-            data.err,
+            error,
             vars,
         );
     }
@@ -104,14 +104,14 @@ pub fn path_emit_config_directory_messages(vars: &EnvStack) {
     }
 
     let config = get_config_directory();
-    if !config.success() {
+    if let Some(error) = &data.err {
         maybe_issue_path_warning(
             L!("config"),
             wgettext!("can not save universal variables or functions"),
             config.used_xdg,
             L!("XDG_CONFIG_HOME"),
             &config.path,
-            config.err,
+            error,
             vars,
         );
     }
@@ -130,7 +130,7 @@ fn maybe_issue_path_warning(
     using_xdg: bool,
     xdg_var: &wstr,
     path: &wstr,
-    saved_errno: libc::c_int,
+    error: &std::io::Error,
     vars: &EnvStack,
 ) {
     let warning_var_name = L!("_FISH_WARNED_").to_owned() + which_dir;
@@ -170,7 +170,7 @@ fn maybe_issue_path_warning(
         );
         flog!(
             warning_path,
-            wgettext_fmt!("The error was '%s'.", Errno(saved_errno).to_string())
+            wgettext_fmt!("The error was '%s'.", error.to_string())
         );
         flog!(
             warning_path,
@@ -562,14 +562,14 @@ struct BaseDirectory {
     /// whether the dir is remote
     remoteness: DirRemoteness,
     /// the error code if creating the directory failed, or 0 on success.
-    err: libc::c_int,
+    err: Option<std::io::Error>,
     /// whether an XDG variable was used in resolving the directory.
     used_xdg: bool,
 }
 
 impl BaseDirectory {
     fn success(&self) -> bool {
-        self.err == 0
+        self.err.is_none()
     }
 }
 
@@ -589,12 +589,7 @@ fn make_base_directory(xdg_var: &wstr, non_xdg_homepath: &wstr) -> BaseDirectory
         let mut build_dir = PathBuf::from(BUILD_DIR);
         build_dir.push("fish-test-home");
 
-        let err = match std::fs::create_dir_all(&build_dir) {
-            Ok(_) => 0,
-            Err(e) => e
-                .raw_os_error()
-                .expect("Failed to create fish base directory, but it wasn't an OS error!"),
-        };
+        let err = std::fs::create_dir_all(&build_dir).err();
 
         return BaseDirectory {
             path: bytes2wcstring(build_dir.as_os_str().as_bytes()),
@@ -621,20 +616,21 @@ fn make_base_directory(xdg_var: &wstr, non_xdg_homepath: &wstr) -> BaseDirectory
         used_xdg = false;
     }
 
-    set_errno(Errno(0));
-    let err;
     let mut remoteness = DirRemoteness::Unknown;
-    if path.is_empty() {
-        err = ENOENT;
+    let err = if path.is_empty() {
+        Some(std::io::Error::new(
+            std::io::ErrorKind::NotFound,
+            "Path is empty",
+        ))
     } else if let Err(io_error) = create_dir_all_with_mode(wcs2osstring(&path), 0o700) {
-        err = io_error.raw_os_error().unwrap_or_default();
+        Some(io_error)
     } else {
-        err = 0;
         // Need to append a trailing slash to check the contents of the directory, not its parent.
         let mut tmp = path.clone();
         tmp.push('/');
         remoteness = path_remoteness(&tmp);
-    }
+        None
+    };
 
     BaseDirectory {
         path,