Fix build on aarch64

[mfrischknecht]

attach_to_tmux_session_outside_tmux uses raw pointers to c_char under the hood, but these were specified explicitly as *const i8 instead of *const c_char. As the documentation for CString::as_ptr ^1 points out, the method really returns a *const c_char, and the actual referenced byte type (i8 or u8) is architecture dependent:

The type of the returned pointer is const c_char, and whether it’s an alias for const i8 or *const u8 is platform-specific.

Changing the pointer type to *const c_char explicitly fixes the build on aarch64, where the underlying pointer type is actually *const u8 instead of *const i8.

[mfrischknecht]

@vinnymeller

I was sifting through failing Nixpkgs builds hoping to help fix a couple of Hydra build failures. This change was the result of investigating this build: https://hydra.nixos.org/build/242898606

Since you're also registered as the maintainer of the twm Nix package, I thought, simply posting the patch on Nixpkgs would be a little bit nonsensical.

The relevant log output from Hydra looks like this:

   Compiling twm v0.8.0 (/build/source)
error[E0277]: a value of type `Vec<*const i8>` cannot be built from an iterator over elements of type `*const u8`
   --> src/tmux.rs:108:10
    |
108 |         .collect();
    |          ^^^^^^^ value of type `Vec<*const i8>` cannot be built from `std::iter::Iterator<Item=*const u8>`
    |
    = help: the trait `FromIterator<*const u8>` is not implemented for `Vec<*const i8>`
    = help: the trait `FromIterator<T>` is implemented for `Vec<T>`
note: the method call chain might not have had the expected associated types
   --> src/tmux.rs:106:10
    |
97  |       let tmux_attach_args = vec![
    |  ____________________________-
98  | |         CString::new("tmux").unwrap(),
99  | |         CString::new("attach").unwrap(),
100 | |         CString::new("-t").unwrap(),
101 | |         CString::new(repo_name).with_context(|| "Unable to turn repo name to a cstring.")?,
102 | |     ];
    | |_____- this expression has type `Vec<CString>`
...
105 |           .iter()
    |            ------ `Iterator::Item` is `&CString` here
106 |           .map(|arg| arg.as_ptr())
    |            ^^^^^^^^^^^^^^^^^^^^^^^ `Iterator::Item` changed to `*const u8` here
107 |           .chain(std::iter::once(std::ptr::null()))
    |            ---------------------------------------- `Iterator::Item` remains `*const u8` here
note: required by a bound in `std::iter::Iterator::collect`
   --> /build/rustc-1.74.0-src/library/core/src/iter/traits/iterator.rs:2049:5

error[E0308]: mismatched types
   --> src/tmux.rs:111:38
    |
111 |         execvp(tmux_attach.as_ptr(), tmux_attach_args_ptrs.as_ptr());
    |         ------                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `*const *const u8`, found `*const *const i8`
    |         |
    |         arguments to this function are incorrect
    |
    = note: expected raw pointer `*const *const u8`
               found raw pointer `*const *const i8`
note: function defined here
   --> /build/twm-0.8.0-vendor.tar.gz/libc/src/unix/mod.rs:892:12
    |
892 |     pub fn execvp(c: *const c_char, argv: *const *const c_char) -> ::c_int;
    |            ^^^^^^

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `twm` (lib) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...

[vinnymeller]

@mfrischknecht thank you for this!