rust: block: use NullTerminatedFormatter

Use the new `NullTerminatedFormatter` to write the name of a `GenDisk` to
the name buffer. This new formatter automatically adds a trailing null
marker after the written characters, so we don't need to append that at the
call site any longer.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250902-rnull-up-v6-16-v7-8-b5212cc89b98@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

rust/kernel/block/mq/gen_disk.rs

@@ -7,9 +7,11 @@
 
 use crate::{
     bindings,
-    block::mq::{raw_writer::RawWriter, Operations, TagSet},
+    block::mq::{Operations, TagSet},
     error::{self, from_err_ptr, Result},
+    prelude::*,
     static_lock_class,
+    str::NullTerminatedFormatter,
     sync::Arc,
 };
 use core::fmt::{self, Write};
@@ -143,14 +145,14 @@ impl GenDiskBuilder {
         // SAFETY: `gendisk` is a valid pointer as we initialized it above
         unsafe { (*gendisk).fops = &TABLE };
 
-        let mut raw_writer = RawWriter::from_array(
+        let mut writer = NullTerminatedFormatter::new(
             // SAFETY: `gendisk` points to a valid and initialized instance. We
             // have exclusive access, since the disk is not added to the VFS
             // yet.
             unsafe { &mut (*gendisk).disk_name },
-        )?;
-        raw_writer.write_fmt(name)?;
-        raw_writer.write_char('\0')?;
+        )
+        .ok_or(EINVAL)?;
+        writer.write_fmt(name)?;
 
         // SAFETY: `gendisk` points to a valid and initialized instance of
         // `struct gendisk`. `set_capacity` takes a lock to synchronize this

rust/kernel/block/mq/raw_writer.rs

@@ -24,6 +24,7 @@ impl<'a> RawWriter<'a> {
         Ok(Self { buffer, pos: 0 })
     }
 
+    #[expect(dead_code)]
     pub(crate) fn from_array<const N: usize>(
         a: &'a mut [crate::ffi::c_char; N],
     ) -> Result<RawWriter<'a>> {

rust/kernel/str.rs

@@ -886,7 +886,6 @@ pub(crate) struct NullTerminatedFormatter<'a> {
 
 impl<'a> NullTerminatedFormatter<'a> {
     /// Create a new [`Self`] instance.
-    #[expect(dead_code)]
     pub(crate) fn new(buffer: &'a mut [u8]) -> Option<NullTerminatedFormatter<'a>> {
         *(buffer.first_mut()?) = 0;