Fix compilation errors on platforms with unsigned c_char

disk/src/superblock.rs

@@ -481,7 +481,7 @@ fn parse_label(raw_label: &[std::os::raw::c_char; 256]) -> String {
     let bytes: Vec<u8> = raw_label
         .iter()
         .take_while(|&&c| c != 0)
-        .map(|&c| c.cast_unsigned())
+        .map(|&c| c as u8)
         .collect();
     String::from_utf8_lossy(&bytes).into_owned()
 }
@@ -679,7 +679,7 @@ pub fn write_superblock_all_mirrors(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::{io::Cursor, mem};
+    use std::{io::Cursor, mem, os::raw::c_char};
 
     // --- super_mirror_offset ---
 
@@ -755,24 +755,24 @@ mod tests {
 
     #[test]
     fn parse_label_normal() {
-        let mut raw_label = [0i8; 256];
+        let mut raw_label = [c_char::default(); 256];
         for (i, &b) in b"my-volume".iter().enumerate() {
-            raw_label[i] = b as i8;
+            raw_label[i] = b as c_char;
         }
         assert_eq!(parse_label(&raw_label), "my-volume");
     }
 
     #[test]
     fn parse_label_empty() {
-        let raw_label = [0i8; 256];
+        let raw_label = [c_char::default(); 256];
         assert_eq!(parse_label(&raw_label), "");
     }
 
     #[test]
     fn parse_label_stops_at_nul() {
-        let mut raw_label = [0i8; 256];
+        let mut raw_label = [c_char::default(); 256];
         for (i, &b) in b"hello\0world".iter().enumerate() {
-            raw_label[i] = b as i8;
+            raw_label[i] = b as c_char;
         }
         assert_eq!(parse_label(&raw_label), "hello");
     }

uapi/src/filesystem.rs

@@ -103,7 +103,7 @@ pub fn wait_sync(fd: BorrowedFd, transid: u64) -> nix::Result<()> {
 ///
 /// Returns `Err` if the ioctl fails.
 pub fn label_get(fd: BorrowedFd) -> nix::Result<CString> {
-    let mut buf = [0i8; BTRFS_LABEL_SIZE as usize];
+    let mut buf = [c_char::default(); BTRFS_LABEL_SIZE as usize];
     unsafe { btrfs_ioc_get_fslabel(fd.as_raw_fd(), &raw mut buf) }?;
     let cstr = unsafe { CStr::from_ptr(buf.as_ptr()) };
     // CStr::to_owned() copies the bytes into a freshly allocated CString,
@@ -129,7 +129,7 @@ pub fn label_set(fd: BorrowedFd, label: &CStr) -> nix::Result<()> {
     if bytes.len() >= BTRFS_LABEL_SIZE as usize {
         return Err(nix::errno::Errno::EINVAL);
     }
-    let mut buf = [0i8; BTRFS_LABEL_SIZE as usize];
+    let mut buf = [c_char::default(); BTRFS_LABEL_SIZE as usize];
     for (i, &b) in bytes.iter().enumerate() {
         buf[i] = b as c_char;
     }

uapi/src/inode.rs

@@ -14,7 +14,10 @@ use crate::{
     },
     tree_search::{SearchFilter, tree_search},
 };
-use std::os::fd::{AsRawFd, BorrowedFd};
+use std::os::{
+    fd::{AsRawFd, BorrowedFd},
+    raw::c_char,
+};
 
 /// Look up the tree ID (root ID) of the subvolume containing the given file or directory.
 ///
@@ -104,7 +107,7 @@ pub fn ino_paths(fd: BorrowedFd<'_>, inum: u64) -> nix::Result<Vec<String>> {
 
         // The path string starts at the base of val array plus the offset
         let val_base = container.val.as_ptr() as usize;
-        let path_ptr = (val_base + val_offset) as *const i8;
+        let path_ptr = (val_base + val_offset) as *const c_char;
 
         // Convert C string to Rust String
         let c_str = unsafe { std::ffi::CStr::from_ptr(path_ptr) };