Add a cargo feature to automatically detect usage of a nightly compiler.

Cargo.toml

@@ -32,9 +32,13 @@ trybuild = { version = "1.0.99", features = ["diff"] }
 default = ["syn-error"]
 syn-error = ["dep:syn"]
 nightly = []
+detect-nightly = []
 
 [lints.rust]
-unexpected_cfgs = { level = "warn", check-cfg = ['cfg(run_ui_tests)'] }
+unexpected_cfgs = { level = "warn", check-cfg = [
+    'cfg(run_ui_tests)',
+    'cfg(detected_nightly)',
+] }
 
 [lints.clippy]
 pedantic = { level = "warn", priority = -1 }

build.rs

@@ -0,0 +1,20 @@
+#[cfg(all(feature = "detect-nightly", not(feature = "nightly")))]
+pub fn main() {
+    let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".to_string());
+
+    let rustc_version = std::process::Command::new(rustc)
+        .arg("--version")
+        .output()
+        .map(|c| c.stdout)
+        .unwrap_or_default();
+
+    let rustc_version = String::from_utf8(rustc_version).unwrap_or_default();
+    if rustc_version.contains("nightly") {
+        println!("cargo:rustc-cfg=detected_nightly");
+    }
+}
+
+#[cfg(not(feature = "detect-nightly"))]
+pub fn main() {
+    // do nothing if the 'detect-nightly' feature is not enabled
+}

src/lib.rs

@@ -270,8 +270,10 @@
 //! [`proc-macro2::Span`]: https://docs.rs/proc-macro2/1.0.10/proc_macro2/struct.Span.html
 //! [`ToTokens`]: https://docs.rs/quote/1.0.3/quote/trait.ToTokens.html
 //!
-
-#![cfg_attr(feature = "nightly", feature(proc_macro_diagnostic))]
+#![cfg_attr(
+    any(feature = "nightly", detected_nightly),
+    feature(proc_macro_diagnostic)
+)]
 #![forbid(unsafe_code)]
 
 extern crate proc_macro;
@@ -294,11 +296,11 @@ mod diagnostic;
 mod macros;
 mod sealed;
 
-#[cfg(not(feature = "nightly"))]
+#[cfg(not(any(feature = "nightly", detected_nightly)))]
 #[path = "imp/fallback.rs"]
 mod imp;
 
-#[cfg(feature = "nightly")]
+#[cfg(any(feature = "nightly", detected_nightly))]
 #[path = "imp/delegate.rs"]
 mod imp;