Avoid unecessary imports from alloc for prelude types

Types like `Vec` and `Box` are already in the std prelude,
so don't need to be imported when cfg(feature = "std") is
enabled.

Author: Techcable

src/lib.rs

@@ -45,6 +45,8 @@ extern crate self as zerogc;
  * I want this library to use 'mostly' stable features,
  * unless there's good justification to use an unstable feature.
  */
+#[cfg(all(not(feature = "std"), feature = "alloc"))]
+use alloc::vec::Vec;
 use core::cmp::Ordering;
 use core::fmt::{self, Debug, Display, Formatter};
 use core::hash::{Hash, Hasher};
@@ -524,7 +526,7 @@ pub unsafe trait GcSimpleAlloc: GcContext {
     /// Allocate an array, taking ownership of the values in
     /// the specified vec.
     #[inline]
-    #[cfg(feature = "alloc")]
+    #[cfg(any(feature = "alloc", feature = "std"))]
     fn alloc_array_from_vec<'gc, T>(&'gc self, mut src: Vec<T>) -> GcArray<'gc, T, Self::Id>
     where
         T: GcSafe<'gc, Self::Id>,

src/manually_traced/stdalloc.rs

@@ -2,10 +2,13 @@
 //!
 //! These can be used in `#![no_std]` crates without requiring
 //! the entire standard library.
+#[cfg(not(feature = "std"))]
 use alloc::boxed::Box;
 use alloc::rc::Rc;
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 use alloc::sync::Arc;
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
 use crate::prelude::*;

src/vec.rs

@@ -103,8 +103,9 @@
 //! It has been called the [stretchy vector problem](https://www.ravenbrook.com/project/mps/master/manual/html/guide/vector.html)
 //! by some. This is less of a problem for `zerogc`, because collections can only
 //! happen at explicit safepoints.
+#[cfg(all(not(feature = "std"), feature = "alloc"))]
+use alloc::vec::Vec;
 use core::cell::UnsafeCell;
-use core::convert::{AsMut, AsRef};
 use core::fmt::{self, Debug, Formatter};
 use core::marker::PhantomData;
 use core::ops::{Deref, DerefMut, Index, IndexMut, RangeBounds};
@@ -383,6 +384,7 @@ unsafe impl<'gc, T: GcSafe<'gc, Id>, Id: CollectorId> IGcVec<'gc, T> for GcVec<'
     pub fn copy_from_slice(src: &[T], ctx: &'gc Id::Context) -> Self
     where
         T: Copy;
+    #[cfg(any(feature = "alloc", feature = "std"))]
     pub fn from_vec(src: Vec<T>, ctx: &'gc Id::Context) -> Self;
 
     /*

src/vec/cell.rs

@@ -1,4 +1,6 @@
 //! The implementation of [GcVecCell]
+#[cfg(all(not(feature = "std"), feature = "alloc"))]
+use alloc::vec::Vec;
 use core::cell::RefCell;
 
 use inherent::inherent;
@@ -135,6 +137,7 @@ unsafe impl<'gc, T: GcSafe<'gc, Id>, Id: SimpleAllocCollectorId> IGcVec<'gc, T>
     pub fn copy_from_slice(src: &[T], ctx: &'gc <Id as CollectorId>::Context) -> Self
     where
         T: Copy;
+    #[cfg(feature = "alloc")]
     pub fn from_vec(src: Vec<T>, ctx: &'gc <Id as CollectorId>::Context) -> Self;
     pub fn get(&mut self, index: usize) -> Option<T>
     where

src/vec/raw.rs

@@ -1,6 +1,7 @@
 //! The underlying representation of a [GcVec](`crate::vec::GcVec`)
 
-use core::iter::Iterator;
+#[cfg(all(not(feature = "std"), feature = "alloc"))]
+use alloc::vec::Vec;
 use core::marker::PhantomData;
 
 use crate::{CollectorId, GcRebrand, GcSafe};
@@ -40,6 +41,7 @@ pub unsafe trait IGcVec<'gc, T: GcSafe<'gc, Self::Id>>: Sized + Extend<T> {
     ///
     /// This consumes ownership of the original vector.
     #[inline]
+    #[cfg(any(feature = "alloc", feature = "std"))]
     fn from_vec(mut src: Vec<T>, ctx: &'gc <Self::Id as CollectorId>::Context) -> Self {
         let len = src.len();
         let mut res = Self::with_capacity_in(len, ctx);