Add warning for incompatibility with AtCoder 202004

proconio/Cargo.toml

@@ -59,3 +59,4 @@ assert_cli = "0.6.3"
 
 [features]
 derive = ["proconio-derive"]
+disable_compat_warning = []

proconio/README.md

@@ -5,7 +5,7 @@
 
 Easy IO library for competitive programming.
 
-`proconio` provides an easy way to read values from stdin (or other source).  The main is `input!` macro.
+`proconio` provides an easy way to read values from stdin (or other source). The main is `input!` macro.
 
 The macro's user interface is basically the same with [tanakh's input macro](https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8).
 
@@ -23,3 +23,14 @@ println!("{} {} {}", n, m, l);
 ```
 
 For more details, see documentation.
+
+- [Documentation for latest v0.3](https://docs.rs/proconio/0.3)  
+  v0.3.x is AtCoder 2020 compatible version.
+- [Documentation for latest version](https://docs.rs/proconio)  
+  The latest version containing new features and bug fixes.
+
+## AtCoder 2020 compatibility
+
+The latest version (v0.4.x) has some incompatible new features and bug fixes
+with the version in AtCoder 2020 judge server. We strongly recommend that you
+use v0.3.x in your local environment too.

proconio/src/lib.rs

@@ -503,8 +503,61 @@
 //! world
 //! ```
 //!
-//! If you don't like this behavior, you can remove #[fastout] from your `main()`.
+//! If you don't like this behavior, you can remove `#[fastout]` from your `main()`.
+//!
+//! # AtCoder compatibility warning
+//!
+//! You may see the following warning during compiling your program:
+//!
+//! ```console
+//! warning: use of deprecated constant `warning::_::WARN`: this version of proconio is not compatible
+//!          please use the v0.3.x version instead if you use this in the AtCoder 2020 update.
+//!          note: for uses outside of AtCoder, you can disable this warning via the `disable_compat_warning` feature.
+//!  --> proconio/src/warning.rs:6:6
+//!   |
+//! 6 |     (WARN, "please use proconio v0.3.x in AtCoder 2020");
+//!   |      ^^^^
+//!   |
+//!   = note: `#[warn(deprecated)]` on by default
+//! ```
+//!
+//! This is because the version you are using (v0.4.x) is not compatible with AtCoder 2020:
+//!
+//! - Some new features are added, so you may get Compile Error (CE) while your local test
+//!   successfully compiles.
+//! - Some bug fixes not compatible with v0.3.x are included, so you may encounter the unexpected
+//!   Runtime Error (RE) on server while your local test successfully passes.
+//!   One of the serious bugs is issue [#8](https://github.com/statiolake/proconio-rs/issues/8) or
+//!   [#14](https://github.com/statiolake/proconio-rs/issues/14).
+//!
+//! In order to avoid wasting time during contest to solve such a hard-to-notice problems, we
+//! strongly recommend that you use v0.3.x versions for AtCoder 2020 in your local environment too.
+//! The latest v0.3.x is almost same with the version used in the AtCoder, but guards to some bugs
+//! are added. For example, code that may cause above bug
+//! [#8](https://github.com/statiolake/proconio-rs/issues/8) in server is compile-time error in the
+//! latest v0.3.x version. We will add such warnings when we find another bug.
 //!
+//! ## Disable warning
+//!
+//! If you are not intended to use proconio with AtCoder 2020, you may disable the warning by
+//! enabling feature `disable_compat_warning`. Features can be enabled by modifying dependency
+//! section in Cargo.toml of your project to:
+//!
+//! ```toml
+//! [dependencies.proconio]
+//! version = "0.4"
+//! features = ["disable_compat_warning"] # Add this
+//! ```
+//!
+//! or
+//!
+//! ```toml
+//! [dependencies]
+//! proconio = { version = "0.4", features = ["disable_compat_warning"] }
+//! ```
+
+/// Module to emit warnings in unsupported environments.
+mod warning;
 
 #[cfg(feature = "derive")]
 pub use proconio_derive::*;

proconio/src/warning.rs

@@ -0,0 +1,15 @@
+#![allow(path_statements, clippy::no_effect)]
+
+#[cfg(not(feature = "disable_compat_warning"))]
+const _: () = {
+    #[rustfmt::skip]
+    (WARN, "please use proconio v0.3.x in AtCoder 2020");
+
+    #[deprecated(note = "this version of proconio is not compatible\n\
+        please use the v0.3.x version instead \
+        if you use this in the AtCoder 2020 update.\n\
+        note: for uses outside of AtCoder, \
+        you can disable this warning \
+        via the `disable_compat_warning` feature.")]
+    const WARN: () = ();
+};