rust: Add macro for passing static borrowed strings from Rust to C.

* ADD a new macro, tor_util::string::cstr!() which takes Rust strings,
   concatenates them together, appends a NUL byte, and converts it into a
   std::ffi::CStr for handing to C.

src/rust/tor_util/strings.rs

@@ -80,3 +80,139 @@ pub fn empty_static_cstr() -> &'static CStr {
 
     empty
 }
+
+/// Create a `CStr` from a literal byte slice, appending a NUL byte to it first.
+///
+/// # Warning
+///
+/// The literal byte slice which is taken as an argument *MUST NOT* have any NUL
+/// bytes (`b"\0"`) in it, anywhere, or else an empty string will be returned
+/// (`CStr::from_bytes_with_nul_unchecked(b"\0")`) so as to avoid `panic!()`ing.
+///
+/// # Examples
+///
+/// ```
+/// #[macro_use]
+/// extern crate tor_util;
+///
+/// use std::ffi::CStr;
+///
+/// # fn do_test() -> Result<&'static CStr, &'static str> {
+/// let message: &'static str = "This is a test of the tsunami warning system.";
+/// let tuesday: &'static CStr;
+/// let original: &str;
+///
+/// tuesday = cstr!("This is a test of the tsunami warning system.");
+/// original = tuesday.to_str().or(Err("Couldn't unwrap CStr!"))?;
+///
+/// assert!(original == message);
+/// #
+/// # Ok(tuesday)
+/// # }
+/// # fn main() {
+/// #     do_test();  // so that we can use the ? operator in the test
+/// # }
+/// ```
+/// It is also possible to pass several string literals to this macro.  They
+/// will be concatenated together in the order of the arguments, unmodified,
+/// before finally being suffixed with a NUL byte:
+///
+/// ```
+/// #[macro_use]
+/// extern crate tor_util;
+/// #
+/// # use std::ffi::CStr;
+/// #
+/// # fn do_test() -> Result<&'static CStr, &'static str> {
+///
+/// let quux: &'static CStr = cstr!("foo", "bar", "baz");
+/// let orig: &'static str = quux.to_str().or(Err("Couldn't unwrap CStr!"))?;
+///
+/// assert!(orig == "foobarbaz");
+/// # Ok(quux)
+/// # }
+/// # fn main() {
+/// #     do_test();  // so that we can use the ? operator in the test
+/// # }
+/// ```
+/// This is useful for passing static strings to C from Rust FFI code.  To do so
+/// so, use the `.as_ptr()` method on the resulting `&'static CStr` to convert
+/// it to the Rust equivalent of a C `const char*`:
+///
+/// ```
+/// #[macro_use]
+/// extern crate tor_util;
+///
+/// use std::ffi::CStr;
+/// use std::os::raw::c_char;
+///
+/// pub extern "C" fn give_static_borrowed_string_to_c() -> *const c_char {
+///     let hello: &'static CStr = cstr!("Hello, language my parents wrote.");
+///
+///     hello.as_ptr()
+/// }
+/// # fn main() {
+/// #     let greetings = give_static_borrowed_string_to_c();
+/// # }
+/// ```
+/// Note that the C code this static borrowed string is passed to *MUST NOT*
+/// attempt to free the memory for the string.
+///
+/// # Note
+///
+/// An unfortunate limitation of the rustc compiler (as of 1.25.0-nightly), is
+/// that the above code compiles, however if we were to change the assignment of
+/// `tuesday` as follows, it will fail to compile, because Rust macros are
+/// expanded at parse time, and at parse time there is no symbols table
+/// available.
+///
+/// ```ignore
+/// tuesday = cstr!(message);
+/// ```
+/// with the error message `error: expected a literal`.
+///
+/// # Returns
+///
+/// If the string literals passed as arguments contain no NUL bytes anywhere,
+/// then an `&'static CStr` containing the (concatenated) bytes of the string
+/// literal(s) passed as arguments, with a NUL byte appended, is returned.
+/// Otherwise, an `&'static CStr` containing a single NUL byte is returned (an
+/// "empty" string in C).
+#[macro_export]
+macro_rules! cstr {
+    ($($bytes:expr),*) => (
+        ::std::ffi::CStr::from_bytes_with_nul(
+            concat!($($bytes),*, "\0").as_bytes()
+        ).unwrap_or(
+            unsafe{
+                ::std::ffi::CStr::from_bytes_with_nul_unchecked(b"\0")
+            }
+        )
+    )
+}
+
+#[cfg(test)]
+mod test {
+    use std::ffi::CStr;
+
+    #[test]
+    fn cstr_macro() {
+        let _: &'static CStr = cstr!("boo");
+    }
+
+    #[test]
+    fn cstr_macro_multi_input() {
+        let quux: &'static CStr = cstr!("foo", "bar", "baz");
+
+        assert!(quux.to_str().unwrap() == "foobarbaz");
+    }
+
+    #[test]
+    fn cstr_macro_bad_input() {
+        let waving:   &'static CStr = cstr!("waving not drowning o/");
+        let drowning: &'static CStr = cstr!("\0 drowning not waving");
+
+        assert!(waving.to_str().unwrap()   == "waving not drowning o/");
+        assert!(drowning.to_str().unwrap() == "")
+    }
+}