Make tests pass on stable

tests/project.rs

@@ -2,6 +2,16 @@
 #![warn(rust_2018_idioms, single_use_lifetimes)]
 #![allow(dead_code)]
 
+// This hack is needed until https://github.com/rust-lang/rust/pull/69201
+// makes it way into stable.
+// Ceurrently, `#[attr] if true {}` doesn't even *parse* on stable,
+// which means that it will error even behind a `#[rustversion::nightly]`
+//
+// This trick makes sure that we don't even attempt to parse
+// the `#[project] if let _` test on stable.
+#[rustversion::nightly]
+include!("project_if_attr.rs.in");
+
 use pin_project::{pin_project, project};
 use std::pin::Pin;
 
@@ -86,33 +96,6 @@ fn project_stmt_expr() {
     assert_eq!(val, true);
 }
 
-#[rustversion::nightly]
-#[test]
-#[project]
-fn project_if_let() {
-    #[pin_project]
-    enum Foo<A, B> {
-        Variant1(#[pin] A),
-        Variant2(u8),
-        Variant3 {
-            #[pin] field: B
-        }
-    }
-
-    let mut foo: Foo<bool, f32> = Foo::Variant1(true);
-    let foo = Pin::new(&mut foo).project();
-
-    #[project]
-    if let Foo::Variant1(a) = foo {
-        let a: Pin<&mut bool> = a;
-        assert_eq!(*a, true);
-    } else if let Foo::Variant2(_) = foo {
-        unreachable!();
-    } else if let Foo::Variant3 { .. } = foo {
-        unreachable!();
-    }
-}
-
 #[test]
 fn project_impl() {
     #[pin_project]

tests/project_if_attr.rs.in

@@ -0,0 +1,29 @@
+// FIXME: Once https://github.com/rust-lang/rust/pull/69201 makes its
+// way into stable, move this back into `project.rs
+
+#[test]
+#[project]
+fn project_if_let() {
+    #[pin_project]
+    enum Foo<A, B> {
+        Variant1(#[pin] A),
+        Variant2(u8),
+        Variant3 {
+            #[pin] field: B
+        }
+    }
+
+    let mut foo: Foo<bool, f32> = Foo::Variant1(true);
+    let foo = Pin::new(&mut foo).project();
+
+    #[project]
+    if let Foo::Variant1(a) = foo {
+        let a: Pin<&mut bool> = a;
+        assert_eq!(*a, true);
+    } else if let Foo::Variant2(_) = foo {
+        unreachable!();
+    } else if let Foo::Variant3 { .. } = foo {
+        unreachable!();
+    }
+}
+