Add build.rs example

README.md

@@ -126,6 +126,29 @@ fn duration_as_days(dur: Duration) -> u64 {
 }
 ```
 
+Emitting Cargo cfg directives from a build script. Note that this requires
+listing `rustversion` under `[build-dependencies]` in Cargo.toml, not
+`[dependencies]`.
+
+```rust
+// build.rs
+
+fn main() {
+    if rustversion::cfg!(since(1.36)) {
+        println!("cargo:rustc-cfg=no_std");
+    }
+}
+```
+
+```rust
+// src/lib.rs
+
+#![cfg_attr(no_std, no_std)]
+
+#[cfg(no_std)]
+extern crate alloc;
+```
+
 <br>
 
 #### License

src/lib.rs

@@ -150,6 +150,29 @@
 //! }
 //! ```
 //!
+//! Emitting Cargo cfg directives from a build script. Note that this requires
+//! listing `rustversion` under `[build-dependencies]` in Cargo.toml, not
+//! `[dependencies]`.
+//!
+//! ```
+//! // build.rs
+//!
+//! fn main() {
+//!     if rustversion::cfg!(since(1.36)) {
+//!         println!("cargo:rustc-cfg=no_std");
+//!     }
+//! }
+//! ```
+//!
+//! ```
+//! // src/lib.rs
+//!
+//! #![cfg_attr(no_std, no_std)]
+//!
+//! #[cfg(no_std)]
+//! extern crate alloc;
+//! ```
+//!
 //! <br>
 
 #![doc(html_root_url = "https://docs.rs/rustversion/1.0.18")]