rust: Refactor protover::compute_for_old_tor().

During code review and discussion with Chelsea Komlo, she pointed out
that protover::compute_for_old_tor() was a public function whose
return type was `&'static CStr`.  We both agree that C-like parts of
APIs should:

1. not be exposed publicly (to other Rust crates),
2. only be called in the appropriate FFI code,
3. not expose types which are meant for FFI code (e.g. `*mut char`,
   `CString`, `*const c_int`, etc.) to the pure-Rust code of other
   crates.
4. FFI code (e.g. things in `ffi.rs` modules) should _never_ be called
   from pure-Rust, not even from other modules in its own crate
   (i.e. do not call `protover::ffi::*` from anywhere in
   `protover::protoset::*`, etc).

With that in mind, this commit makes the following changes:

 * CHANGE `protover::compute_for_old_tor()` to be
   visible only at the `pub(crate)` level.
 * RENAME `protover::compute_for_old_tor()` to
   `protover::compute_for_old_tor_cstr()` to reflect the last change.
 * ADD a new `protover::compute_for_old_tor()` function wrapper which
   is public and intended for other Rust code to use, which returns a
   `&str`.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/rust/protover/ffi.rs b/src/rust/protover/ffi.rs
index 3632f5de8f..a40353eb13 100644
--- a/src/rust/protover/ffi.rs
+++ b/src/rust/protover/ffi.rs
@@ -246,7 +246,7 @@ pub extern "C" fn protover_compute_for_old_tor(version: *const c_char) -> *const
         Err(_) => return empty.as_ptr(),
     };
 
-    elder_protocols = compute_for_old_tor(&version);
+    elder_protocols = compute_for_old_tor_cstr(&version);
 
     // If we're going to pass it to C, there cannot be any intermediate NUL
     // bytes.  An assert is okay here, since changing the const byte slice