use the correct ParamEnv when checking future's return type in missing_errors_doc

tests/ui/crashes/ice-13862.rs

@@ -0,0 +1,19 @@
+#![crate_type = "lib"]
+#![no_std]
+
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+pub struct S<const N: u8>;
+
+impl<const N: u8> Future for S<N> {
+    type Output = ();
+    fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Self::Output> {
+        todo!()
+    }
+}
+
+pub fn f<const N: u8>() -> S<N> {
+    S
+}