Detect if the proc_macro_def_site feature is not disallowed by -Zallow-features

.gitignore

@@ -1,3 +1,4 @@
+.DS_Store
 .idea/
 .vscode/
 target/

pin-project-internal/build.rs

@@ -1,15 +1,36 @@
 // Based on https://stackoverflow.com/a/49250753/1290530
 
+use std::env;
+
 use rustc_version::{version_meta, Channel};
 
 fn main() {
     // Set cfg flags depending on release channel
     match version_meta().unwrap().channel {
         // Enable our feature on nightly, or when using a
-        // locally build rustc
-        Channel::Nightly | Channel::Dev => {
-            println!("cargo:rustc-cfg=feature=\"RUSTC_IS_NIGHTLY\"");
+        // locally build rustc, unless `-Zallow-features`
+        // in RUSTFLAGS disallows unstable features.
+        Channel::Nightly | Channel::Dev if feature_allowed("proc_macro_def_site") => {
+            println!("cargo:rustc-cfg=proc_macro_def_site");
         }
         _ => {}
     }
 }
+
+// Based on https://github.com/alexcrichton/proc-macro2/pull/176
+fn feature_allowed(feature: &str) -> bool {
+    if let Some(rustflags) = env::var_os("RUSTFLAGS") {
+        for mut flag in rustflags.to_string_lossy().split(' ') {
+            if flag.starts_with("-Z") {
+                flag = &flag["-Z".len()..];
+            }
+            if flag.starts_with("allow-features=") {
+                flag = &flag["allow-features=".len()..];
+                return flag.split(',').any(|allowed| allowed == feature);
+            }
+        }
+    }
+
+    // No allow-features= flag, allowed by default.
+    true
+}

pin-project-internal/src/lib.rs

@@ -8,7 +8,7 @@
 #![warn(single_use_lifetimes)]
 #![warn(clippy::all, clippy::pedantic)]
 #![allow(clippy::use_self)]
-#![cfg_attr(feature = "RUSTC_IS_NIGHTLY", feature(proc_macro_def_site))]
+#![cfg_attr(proc_macro_def_site, feature(proc_macro_def_site))]
 
 extern crate proc_macro;
 

pin-project-internal/src/pin_project/mod.rs

@@ -129,12 +129,12 @@ impl Context {
             }
         } else {
             let make_span = || {
-                #[cfg(feature = "RUSTC_IS_NIGHTLY")]
+                #[cfg(proc_macro_def_site)]
                 {
                     proc_macro::Span::def_site().into()
                 }
 
-                #[cfg(not(feature = "RUSTC_IS_NIGHTLY"))]
+                #[cfg(not(proc_macro_def_site))]
                 {
                     Span::call_site()
                 }
@@ -229,7 +229,7 @@ impl Context {
                 impl #impl_generics ::core::marker::Unpin for #orig_ident #ty_generics #full_where_clause {}
             };
 
-            if cfg!(feature = "RUSTC_IS_NIGHTLY") {
+            if cfg!(proc_macro_def_site) {
                 // On nightly, we use def-site hygiene to make it impossible
                 // for user code to refer to any of the types we define.
                 // This allows us to omit wrapping the generated types