Close #126

build.rs

@@ -1,11 +1,28 @@
 use rustc_version::{version, version_meta, Channel};
 
+fn allow_features() -> Option<Vec<String>> {
+    std::env::var("CARGO_ENCODED_RUSTFLAGS").ok().map(|args| {
+        args.split('\u{001f}')
+            .filter(|arg| arg.starts_with("-Zallow-features="))
+            .map(|arg| arg.split('=').nth(1).unwrap())
+            .flat_map(|features| features.split(','))
+            .map(|f| f.to_owned())
+            .collect()
+    })
+}
+
+fn can_enable_proc_macro_diagnostic() -> bool {
+    allow_features()
+        .map(|f| f.iter().any(|f| dbg!(f) == "proc_macro_diagnostic"))
+        .unwrap_or(true)
+}
+
 fn main() {
     let ver = version().unwrap();
     assert!(ver.major >= 1);
 
     match version_meta().unwrap().channel {
-        Channel::Nightly | Channel::Dev => {
+        Channel::Nightly | Channel::Dev if can_enable_proc_macro_diagnostic() => {
             println!("cargo:rustc-cfg=use_proc_macro_diagnostic");
         }
         _ => {}