diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f7541ec5..277dbf89 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -85,12 +85,6 @@ jobs:
           cargo test --all --all-features
         env:
           RUSTFLAGS: -Dwarnings --cfg pin_project_show_unpin_struct
-      - name: cargo check -Zallow-features
-        if: matrix.rust == 'nightly'
-        run: |
-          cargo check --all --all-features
-        env:
-          RUSTFLAGS: -Dwarnings --cfg pin_project_show_unpin_struct -Zallow-features=proc_macro_hygiene,stmt_expr_attributes
       # Refs: https://github.com/rust-lang/cargo/issues/5657
       - name: cargo check -Zminimal-versions
         if: matrix.rust == 'nightly'
diff --git a/pin-project-internal/build.rs b/pin-project-internal/build.rs
deleted file mode 100644
index 953ca3ad..00000000
--- a/pin-project-internal/build.rs
+++ /dev/null
@@ -1,37 +0,0 @@
-use std::env;
-
-fn main() {
-    // While this crate supports stable Rust, it currently requires
-    // nightly Rust in order for rustdoc to correctly document auto-generated
-    // `Unpin` impls. This does not affect the runtime functionality of this crate,
-    // nor does it affect the safety of the api provided by this crate.
-    //
-    // This is disabled by default and can be enabled using
-    // `--cfg pin_project_show_unpin_struct` in RUSTFLAGS.
-    //
-    // Refs:
-    // * https://github.com/taiki-e/pin-project/pull/53#issuecomment-525906867
-    // * https://github.com/taiki-e/pin-project/pull/70
-    // * https://github.com/rust-lang/rust/issues/63281
-    if cfg!(pin_project_show_unpin_struct) && 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
-}
diff --git a/pin-project-internal/src/lib.rs b/pin-project-internal/src/lib.rs
index da4cd9b4..9a678f07 100644
--- a/pin-project-internal/src/lib.rs
+++ b/pin-project-internal/src/lib.rs
@@ -10,7 +10,19 @@
 #![warn(rust_2018_idioms, single_use_lifetimes, unreachable_pub)]
 #![warn(clippy::all, clippy::pedantic)]
 #![allow(clippy::use_self, clippy::needless_doctest_main)]
-#![cfg_attr(proc_macro_def_site, feature(proc_macro_def_site))]
+// While this crate supports stable Rust, it currently requires
+// nightly Rust in order for rustdoc to correctly document auto-generated
+// `Unpin` impls. This does not affect the runtime functionality of this crate,
+// nor does it affect the safety of the api provided by this crate.
+//
+// This is disabled by default and can be enabled using
+// `--cfg pin_project_show_unpin_struct` in RUSTFLAGS.
+//
+// Refs:
+// * https://github.com/taiki-e/pin-project/pull/53#issuecomment-525906867
+// * https://github.com/taiki-e/pin-project/pull/70
+// * https://github.com/rust-lang/rust/issues/63281
+#![cfg_attr(pin_project_show_unpin_struct, feature(proc_macro_def_site))]
 
 extern crate proc_macro;
 
diff --git a/pin-project-internal/src/pin_project/derive.rs b/pin-project-internal/src/pin_project/derive.rs
index 004399fe..9c249190 100644
--- a/pin-project-internal/src/pin_project/derive.rs
+++ b/pin-project-internal/src/pin_project/derive.rs
@@ -86,11 +86,11 @@ impl DeriveContext {
         let orig_ident = &self.ident;
 
         let make_span = || {
-            #[cfg(proc_macro_def_site)]
+            #[cfg(pin_project_show_unpin_struct)]
             {
                 proc_macro::Span::def_site().into()
             }
-            #[cfg(not(proc_macro_def_site))]
+            #[cfg(not(pin_project_show_unpin_struct))]
             {
                 proc_macro2::Span::call_site()
             }
@@ -159,7 +159,8 @@ impl DeriveContext {
         };
         full_where_clause.predicates.push(unpin_clause);
 
-        let attrs = if cfg!(proc_macro_def_site) { quote!() } else { quote!(#[doc(hidden)]) };
+        let attrs =
+            if cfg!(pin_project_show_unpin_struct) { quote!() } else { quote!(#[doc(hidden)]) };
 
         let inner_data = quote! {
             // This needs to have the same visibility as the original type,
@@ -186,7 +187,7 @@ impl DeriveContext {
             impl #impl_generics ::core::marker::Unpin for #orig_ident #ty_generics #full_where_clause {}
         };
 
-        if cfg!(proc_macro_def_site) {
+        if cfg!(pin_project_show_unpin_struct) {
             // 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