clippy: remove some unnecessary mut

removing useless mutability mostly.

src/uu/cat/src/splice.rs

@@ -21,7 +21,7 @@ const BUF_SIZE: usize = 1024 * 16;
 /// copying or not. False means we don't have to.
 #[inline]
 pub(super) fn write_fast_using_splice<R: FdReadable>(
-    handle: &mut InputHandle<R>,
+    handle: &InputHandle<R>,
     write_fd: &impl AsRawFd,
 ) -> CatResult<bool> {
     let (pipe_rd, pipe_wr) = pipe()?;

src/uu/cksum/src/cksum.rs

@@ -207,8 +207,7 @@ fn digest_read<T: Read>(
         Ok((digest.result_str(), output_size))
     } else {
         // Assume it's SHAKE.  result_str() doesn't work with shake (as of 8/30/2016)
-        let mut bytes = Vec::new();
-        bytes.resize((output_bits + 7) / 8, 0);
+        let mut bytes = vec![0; (output_bits + 7) / 8];
         digest.hash_finalize(&mut bytes);
         Ok((encode(bytes), output_size))
     }

src/uu/env/src/env.rs

@@ -101,7 +101,7 @@ fn load_config_file(opts: &mut Options) -> UResult<()> {
 
 #[cfg(not(windows))]
 #[allow(clippy::ptr_arg)]
-fn build_command<'a, 'b>(args: &'a mut Vec<&'b str>) -> (Cow<'b, str>, &'a [&'b str]) {
+fn build_command<'a, 'b>(args: &'a Vec<&'b str>) -> (Cow<'b, str>, &'a [&'b str]) {
     let progname = Cow::from(args[0]);
     (progname, &args[1..])
 }
@@ -303,7 +303,10 @@ fn run_env(args: impl uucore::Args) -> UResult<()> {
         print_env(opts.line_ending);
     } else {
         // we need to execute a command
+        #[cfg(windows)]
         let (prog, args) = build_command(&mut opts.program);
+        #[cfg(not(windows))]
+        let (prog, args) = build_command(&opts.program);
 
         /*
          * On Unix-like systems Command::status either ends up calling either fork or posix_spawnp

src/uu/hashsum/src/hashsum.rs

@@ -803,8 +803,7 @@ fn digest_reader<T: Read>(
         Ok(digest.result_str())
     } else {
         // Assume it's SHAKE.  result_str() doesn't work with shake (as of 8/30/2016)
-        let mut bytes = Vec::new();
-        bytes.resize((output_bits + 7) / 8, 0);
+        let mut bytes = vec![0; (output_bits + 7) / 8];
         digest.hash_finalize(&mut bytes);
         Ok(encode(bytes))
     }

src/uu/sort/src/ext_sort.rs

@@ -224,7 +224,7 @@ fn read_write_loop<I: WriteableTmpFile>(
     let mut sender_option = Some(sender);
     let mut tmp_files = vec![];
     loop {
-        let mut chunk = match receiver.recv() {
+        let chunk = match receiver.recv() {
             Ok(it) => it,
             _ => {
                 return Ok(ReadResult::WroteChunksToFile { tmp_files });
@@ -232,7 +232,7 @@ fn read_write_loop<I: WriteableTmpFile>(
         };
 
         let tmp_file = write::<I>(
-            &mut chunk,
+            &chunk,
             tmp_dir.next_file()?,
             settings.compress_prog.as_deref(),
             separator,
@@ -262,7 +262,7 @@ fn read_write_loop<I: WriteableTmpFile>(
 /// Write the lines in `chunk` to `file`, separated by `separator`.
 /// `compress_prog` is used to optionally compress file contents.
 fn write<I: WriteableTmpFile>(
-    chunk: &mut Chunk,
+    chunk: &Chunk,
     file: (File, PathBuf),
     compress_prog: Option<&str>,
     separator: u8,