Fix doc test when async fn in trait is natively supported

Author: dtolnay

build.rs

@@ -3,7 +3,7 @@ use std::process::Command;
 use std::str;
 
 fn main() {
-    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-env-changed=DOCS_RS");
 
     let compiler = match rustc_minor_version() {
         Some(compiler) => compiler,
@@ -17,6 +17,10 @@ fn main() {
     if compiler < 47 {
         println!("cargo:rustc-cfg=self_span_hack");
     }
+
+    if compiler >= 75 && env::var_os("DOCS_RS").is_none() {
+        println!("cargo:rustc-cfg=native_async_fn_in_trait");
+    }
 }
 
 fn rustc_minor_version() -> Option<u32> {

src/lib.rs

@@ -12,11 +12,12 @@
 //! Rust 1.39 did not include support for async fn in traits. Trying to include
 //! an async fn in a trait produces the following error:
 //!
-//! ```compile_fail
+#![cfg_attr(not(native_async_fn_in_trait), doc = "```compile_fail")]
+#![cfg_attr(native_async_fn_in_trait, doc = "```")]
 //! trait MyTrait {
 //!     async fn f() {}
 //! }
-//! ```
+#![doc = "```"]
 //!
 //! ```text
 //! error[E0706]: trait fns cannot be declared `async`