rust: str: add `Formatter` type

Add the `Formatter` type, which leverages `RawFormatter`,
but fails if callers attempt to write more than will fit
in the buffer.

In order to so, implement the `RawFormatter::from_buffer()`
constructor as well.
Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/str.rs

@@ -406,6 +406,23 @@ pub(crate) unsafe fn from_ptrs(pos: *mut u8, end: *mut u8) -> Self {
         }
     }
 
+    /// Creates a new instance of [`RawFormatter`] with the given buffer.
+    ///
+    /// # Safety
+    ///
+    /// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
+    /// for the lifetime of the returned [`RawFormatter`].
+    pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
+        let pos = buf as usize;
+        // INVARIANT: We ensure that `end` is never less then `buf`, and the safety requirements
+        // guarantees that the memory region is valid for writes.
+        Self {
+            pos,
+            beg: pos,
+            end: pos.saturating_add(len),
+        }
+    }
+
     /// Returns the current insert position.
     ///
     /// N.B. It may point to invalid memory.
@@ -439,3 +456,43 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
         Ok(())
     }
 }
+
+/// Allows formatting of [`fmt::Arguments`] into a raw buffer.
+///
+/// Fails if callers attempt to write more than will fit in the buffer.
+pub(crate) struct Formatter(RawFormatter);
+
+impl Formatter {
+    /// Creates a new instance of [`Formatter`] with the given buffer.
+    ///
+    /// # Safety
+    ///
+    /// The memory region starting at `buf` and extending for `len` bytes must be valid for writes
+    /// for the lifetime of the returned [`Formatter`].
+    #[allow(dead_code)]
+    pub(crate) unsafe fn from_buffer(buf: *mut u8, len: usize) -> Self {
+        // SAFETY: The safety requirements of this function satisfy those of the callee.
+        Self(unsafe { RawFormatter::from_buffer(buf, len) })
+    }
+}
+
+impl Deref for Formatter {
+    type Target = RawFormatter;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl fmt::Write for Formatter {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.0.write_str(s)?;
+
+        // Fail the request if we go past the end of the buffer.
+        if self.0.pos > self.0.end {
+            Err(fmt::Error)
+        } else {
+            Ok(())
+        }
+    }
+}