Import from alloc to make remaining std dependencies clear

src/lib.rs

@@ -118,6 +118,8 @@ compile_error! {"\
     build script as well.
 "}
 
+extern crate alloc;
+
 #[cfg(feature = "proc-macro")]
 extern crate proc_macro;
 

src/marker.rs

@@ -1,6 +1,6 @@
+use alloc::rc::Rc;
 use core::marker::PhantomData;
-use std::panic::{RefUnwindSafe, UnwindSafe};
-use std::rc::Rc;
+use core::panic::{RefUnwindSafe, UnwindSafe};
 
 // Zero sized marker with the correct set of autotrait impls we want all proc
 // macro types to have.

src/rcvec.rs

@@ -1,8 +1,8 @@
+use alloc::rc::Rc;
+use alloc::vec;
 use core::mem;
+use core::panic::RefUnwindSafe;
 use core::slice;
-use std::panic::RefUnwindSafe;
-use std::rc::Rc;
-use std::vec;
 
 pub(crate) struct RcVec<T> {
     inner: Rc<Vec<T>>,