Rollup merge of #59739 - cramertj:stabilize, r=withoutboats

Stabilize futures_api

cc #59725.
Based on #59733 and #59119 -- only the last two commits here are relevant.

r? @withoutboats , @oli-obk for the introduction of `rustc_allow_const_fn_ptr`.

Author: Centril

src/liballoc/boxed.rs

@@ -911,7 +911,7 @@ impl<G: ?Sized + Generator> Generator for Pin<Box<G>> {
     }
 }
 
-#[unstable(feature = "futures_api", issue = "50547")]
+#[stable(feature = "futures_api", since = "1.36.0")]
 impl<F: ?Sized + Future + Unpin> Future for Box<F> {
     type Output = F::Output;
 

src/liballoc/lib.rs

@@ -85,7 +85,6 @@
 #![feature(fmt_internals)]
 #![feature(fn_traits)]
 #![feature(fundamental)]
-#![feature(futures_api)]
 #![feature(lang_items)]
 #![feature(libc)]
 #![feature(needs_allocator)]