rust: str: add conversion from CStr to CString

These methods can be used to copy the data in a temporary c string into
a separate allocation, so that it can be accessed later even if the
original is deallocated.

The API in this change mirrors the standard library API for the `&str`
and `String` types. The `ToOwned` trait is not implemented because it
assumes that allocations are infallible.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230503141016.683634-1-aliceryhl@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/str.rs

@@ -2,6 +2,7 @@
 
 //! String representations.
 
+use alloc::alloc::AllocError;
 use alloc::vec::Vec;
 use core::fmt::{self, Write};
 use core::ops::{self, Deref, Index};
@@ -199,6 +200,12 @@ pub fn to_str(&self) -> Result<&str, core::str::Utf8Error> {
     pub unsafe fn as_str_unchecked(&self) -> &str {
         unsafe { core::str::from_utf8_unchecked(self.as_bytes()) }
     }
+
+    /// Convert this [`CStr`] into a [`CString`] by allocating memory and
+    /// copying over the string data.
+    pub fn to_cstring(&self) -> Result<CString, AllocError> {
+        CString::try_from(self)
+    }
 }
 
 impl fmt::Display for CStr {
@@ -584,6 +591,21 @@ fn deref(&self) -> &Self::Target {
     }
 }
 
+impl<'a> TryFrom<&'a CStr> for CString {
+    type Error = AllocError;
+
+    fn try_from(cstr: &'a CStr) -> Result<CString, AllocError> {
+        let mut buf = Vec::new();
+
+        buf.try_extend_from_slice(cstr.as_bytes_with_nul())
+            .map_err(|_| AllocError)?;
+
+        // INVARIANT: The `CStr` and `CString` types have the same invariants for
+        // the string data, and we copied it over without changes.
+        Ok(CString { buf })
+    }
+}
+
 /// A convenience alias for [`core::format_args`].
 #[macro_export]
 macro_rules! fmt {