rust: add MSRV

- make a little change to ensure MSRV 1.40.0
- show MSRV (Minimum Supported Rust Version) in README
- show up-to-date state of crate dependencies in README
- update comments

rust/README.md

@@ -2,3 +2,5 @@
 
 [![Crate](https://img.shields.io/crates/v/libnotcurses-sys.svg)](https://crates.io/crates/libnotcurses-sys)
 [![API](https://docs.rs/libnotcurses-sys/badge.svg)](https://docs.rs/libnotcurses-sys)
+[![dependency status](https://deps.rs/crate/libnotcurses-sys/2.0.7/status.svg)](https://deps.rs/crate/libnotcurses-sys/2.0.7)
+[![MSRV: 1.40.0](https://flat.badgen.net/badge/MSRV/1.40.0/purple)](https://blog.rust-lang.org/2019/12/19/Rust-1.40.0.html)

rust/src/cells.rs

@@ -143,9 +143,8 @@ pub unsafe fn cells_load_box(
                     gclu = unsafe { gclu.offset(ulen as isize) };
                     ulen = unsafe { cell_prime(plane, hl, gcluster, style, channels) };
 
-                    #[allow(unused_assignments)] // gclu is not read
                     if ulen > 0 {
-                        gclu = unsafe { gclu.offset(ulen as isize) };
+                        let _gclu = unsafe { gclu.offset(ulen as isize) };
                         ulen = unsafe { cell_prime(plane, vl, gcluster, style, channels) };
 
                         if ulen > 0 {

rust/src/lib.rs

@@ -1,8 +1,9 @@
 //! `libnotcurses-sys` is an unsafe Rust wrapper for the notcurses C library
 //!
-//! It follows very closely the C API, adapted to the Rust API Guidelines.
+//! The API is mostly the original one, while following the Rust API Guidelines.
 //!
-//! For a higher lever API, safer and more idiomatic, take a look at
+//! For a safer wrapper with a more idiomatic API for Rust, you can use
+//! [](https://crates.io/crates/notcurses)
 //! [notcurses-rs](https://github.com/dankamongmen/notcurses-rs)
 //!
 //! ### notcurses C API docs:

rust/src/types/cell.rs

@@ -138,6 +138,8 @@ pub const NCCELL_WIDEASIAN_MASK: u64 = crate::bindings::CELL_WIDEASIAN_MASK as u
 //
 /// Extended Grapheme Cluster. A 32-bit `Char` type
 ///
+/// - https://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries
+///
 /// This 32 bit char, together with the associated plane's associated egcpool,
 /// completely define this cell's `NcChar`. Unless the `NcChar` requires more than
 /// four bytes to encode as UTF-8, it will be inlined here: