[context] Allow building in #![no_std]

See issue #25

libs/context/Cargo.toml

@@ -20,14 +20,18 @@ slog = "2.7"
 [features]
 default = [
     "sync", # Support thread-safety by default
+    "std"
 ]
+# Use the standard library (required for `sync`)
+std = []
 # This will allow multiple threads to access the garbage collector
 # by creating a seperate context for each.
 #
 # Thread safe collectors can have increased overhead
 # by requiring communication between threads.
 sync = [
     "parking_lot",
-    "crossbeam-utils"
+    "crossbeam-utils",
+    "std"
 ]
 

libs/context/src/collector.rs

@@ -1,9 +1,10 @@
 //! The interface to a collector
 
-use std::fmt::{self, Debug, Formatter};
-use std::sync::Arc;
-use std::ptr::NonNull;
-use std::marker::PhantomData;
+use core::fmt::{self, Debug, Formatter};
+use core::ptr::NonNull;
+use core::marker::PhantomData;
+
+use alloc::sync::Arc;
 
 use slog::{Logger, o};
 
@@ -137,7 +138,7 @@ pub unsafe trait CollectorPtr<C: RawCollectorImpl<Ptr=Self>>: Copy + Eq
 /// This is implemented as a
 /// raw pointer via [Arc::into_raw]
 unsafe impl<C: RawCollectorImpl<Ptr=Self>> CollectorPtr<C> for NonNull<C> {
-    type Weak = std::sync::Weak<C>;
+    type Weak = alloc::sync::Weak<C>;
 
     #[inline]
     unsafe fn from_raw(ptr: *mut C) -> Self {
@@ -150,7 +151,7 @@ unsafe impl<C: RawCollectorImpl<Ptr=Self>> CollectorPtr<C> for NonNull<C> {
     unsafe fn clone_owned(&self) -> Self {
         let original = Arc::from_raw(self.as_ptr());
         let cloned = Arc::clone(&original);
-        std::mem::forget(original);
+        core::mem::forget(original);
         NonNull::new_unchecked(Arc::into_raw(cloned) as *mut _)
     }
 
@@ -189,7 +190,7 @@ unsafe impl<C: RawCollectorImpl<Ptr=Self>> CollectorPtr<C> for NonNull<C> {
     unsafe fn create_weak(&self) -> Self::Weak {
         let arc = Arc::from_raw(self.as_ptr());
         let weak = Arc::downgrade(&arc);
-        std::mem::forget(arc);
+        core::mem::forget(arc);
         weak
     }
 }
@@ -288,8 +289,8 @@ unsafe impl<C: RawCollectorImpl> ::zerogc::CollectorId for CollectorId<C> {
     unsafe fn assume_valid_system(&self) -> &Self::System {
         // TODO: Make the API nicer? (avoid borrowing and indirection)
         assert_eq!(
-            std::mem::size_of::<Self>(),
-            std::mem::size_of::<CollectorRef<C>>()
+            core::mem::size_of::<Self>(),
+            core::mem::size_of::<CollectorRef<C>>()
         );
         &*(self as *const CollectorId<C> as *const CollectorRef<C>)
     }

libs/context/src/handle.rs

@@ -1,9 +1,12 @@
 //! Implementation of [::zerogc::GcHandle]
 //!
 //! Inspired by [Mono's Lock free Gc Handles](https://www.mono-project.com/news/2016/08/16/lock-free-gc-handles/)
-use std::ptr::NonNull;
-use std::marker::PhantomData;
-use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
+use core::ptr::{self, NonNull};
+use core::marker::PhantomData;
+use core::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
+
+use alloc::boxed::Box;
+use alloc::vec::Vec;
 
 use zerogc::{Trace, GcSafe, GcErase, GcRebrand, GcVisitor, NullTrace, TraceImmutable, GcHandleSystem, GcBindHandle};
 use crate::{Gc, WeakCollectorRef, CollectorId, CollectorContext, CollectorRef, CollectionManager};
@@ -39,7 +42,7 @@ pub struct GcHandleList<C: RawHandleImpl> {
 }
 impl<C: RawHandleImpl> GcHandleList<C> {
     pub fn new() -> Self {
-        use std::ptr::null_mut;
+        use core::ptr::null_mut;
         GcHandleList {
             last_bucket: AtomicPtr::new(null_mut()),
             last_free_slot: AtomicPtr::new(null_mut()),
@@ -54,7 +57,7 @@ impl<C: RawHandleImpl> GcHandleList<C> {
         debug_assert_eq!(
             (*slot).valid.value
                 .load(Ordering::SeqCst),
-            std::ptr::null_mut()
+            ptr::null_mut()
         );
         let mut last_free = self.last_free_slot
             .load(Ordering::Acquire);
@@ -128,7 +131,7 @@ impl<C: RawHandleImpl> GcHandleList<C> {
                     debug_assert_eq!(
                         (*slot).valid.value
                             .load(Ordering::SeqCst),
-                        std::ptr::null_mut()
+                        ptr::null_mut()
                     );
                     /*
                      * We own the slot, initialize it to point to
@@ -297,7 +300,7 @@ impl<C: RawHandleImpl> GcHandleBucket<C> {
             .skip(last_alloc) {
             // TODO: All these fences must be horrible on ARM
             if slot.valid.value.compare_exchange(
-                std::ptr::null_mut(), value,
+                ptr::null_mut(), value,
                 Ordering::AcqRel,
                 Ordering::Relaxed
             ).is_ok() {
@@ -547,15 +550,18 @@ impl<T: GcSafe, C: RawHandleImpl> Drop for GcHandle<T, C> {
             debug_assert!(!inner.value
                 .load(Ordering::SeqCst)
                 .is_null(),
-                          "Pointer already invalid"
+                "Pointer already invalid"
             );
             let prev = inner.refcnt
                 .fetch_sub(1, Ordering::AcqRel);
             match prev {
                 0 => {
-                    // This should be impossible.
-                    eprintln!("GcHandle refcnt Underflow");
-                    std::process::abort();
+                    /*
+                     * This should be impossible.
+                     *
+                     * I believe it's undefined behavior!
+                     */
+                    panic!("UB: GcHandle refcnt overflow")
                 },
                 1 => {
                     // Free underlying memory
@@ -564,7 +570,7 @@ impl<T: GcSafe, C: RawHandleImpl> Drop for GcHandle<T, C> {
             }
             // Mark the value as freed
             inner.value.store(
-                std::ptr::null_mut(),
+                ptr::null_mut(),
                 Ordering::Release
             );
             unsafe {

libs/context/src/lib.rs

@@ -3,10 +3,27 @@
     untagged_unions, // I want to avoid ManuallyDrop in unions
     const_fn, // Apparently this feature is unstable???
 )]
+#![cfg_attr(not(feature = "std"), no_std)]
 //! The implementation of [::zerogc::CollectorContext] that is
 //! shared among both thread-safe and thread-unsafe code.
-use std::mem::ManuallyDrop;
-use std::fmt::{self, Debug, Formatter};
+
+/*
+ * NOTE: Allocation is still needed for internals
+ *
+ * Uses:
+ * 1. `Box` for each handle
+ * 2. `Vec` for listing buckets of handles
+ * 3. `Arc` and `Box` for boxing context state
+ * 
+ * TODO: Should we drop these uses entirely?
+ */
+extern crate alloc;
+
+use core::mem::ManuallyDrop;
+use core::fmt::{self, Debug, Formatter};
+
+use alloc::boxed::Box;
+use alloc::vec::Vec;
 
 use zerogc::prelude::*;
 
@@ -183,7 +200,7 @@ unsafe impl<C: RawCollectorImpl> GcContext for CollectorContext<C> {
             let result = func(&mut *sub_context, value);
             debug_assert!(!sub_context.root);
             // No need to run drop code on context.....
-            std::mem::forget(sub_context);
+            core::mem::forget(sub_context);
             debug_assert_eq!((*self.raw).state(), ContextState::Active);
             result
         })
@@ -233,7 +250,7 @@ impl<C: RawCollectorImpl> ShadowStack<C> {
     }
     #[inline]
     pub unsafe fn reverse_iter(&self) -> impl Iterator<Item=C::GcDynPointer> + '_ {
-        std::iter::successors(
+        core::iter::successors(
             self.last.as_ref(),
             |link| link.prev.as_ref()
         ).map(|link| link.element)

libs/context/src/state/mod.rs

@@ -1,7 +1,10 @@
 use crate::collector::RawCollectorImpl;
 use crate::{ContextState, ShadowStack, CollectorRef};
-use std::mem::ManuallyDrop;
-use std::fmt::Debug;
+
+use core::mem::ManuallyDrop;
+use core::fmt::Debug;
+
+use alloc::boxed::Box;
 
 /// The internal state of the collector
 ///

libs/context/src/state/nosync.rs

@@ -2,11 +2,15 @@
 //! that doesn't support multiple threads/contexts.
 //!
 //! In exchange, there is no locking :)
+//!
+//! Also, there is `#![no_std]` support
+
+use core::cell::{Cell, UnsafeCell, RefCell};
+use core::mem::ManuallyDrop;
+use core::fmt::{self, Debug, Formatter};
+use core::marker::PhantomData;
 
-use std::cell::{Cell, UnsafeCell, RefCell};
-use std::mem::ManuallyDrop;
-use std::fmt::{self, Debug, Formatter};
-use std::marker::PhantomData;
+use alloc::boxed::Box;
 
 use slog::{Logger, FnValue, trace, o};
 
@@ -125,7 +129,7 @@ unsafe impl<C> super::RawContext<C> for RawContext<C>
         let context = ManuallyDrop::new(Box::new(RawContext {
             logger: logger.clone(), collector,
             shadow_stack: UnsafeCell::new(ShadowStack {
-                last: std::ptr::null_mut(),
+                last: core::ptr::null_mut(),
             }),
             state: Cell::new(ContextState::Active)
         }));
@@ -165,7 +169,7 @@ unsafe impl<C> super::RawContext<C> for RawContext<C>
         trace!(
             self.logger, "Beginning collection";
             "ptr" => ?ptr,
-            "shadow_stack" => FnValue(|_| format!("{:?}", shadow_stack.as_vec())),
+            "shadow_stack" => FnValue(|_| alloc::format!("{:?}", shadow_stack.as_vec())),
             "state" => ?self.state,
             "collection_id" => collection_id,
             "original_size" => %self.collector.as_raw().allocated_size(),

libs/context/src/state/sync.rs

@@ -1,4 +1,7 @@
-/// Thread safe state
+//! Thread safe state
+//!
+//! Note that this module has full permission to use
+//! the standard library in all its glory.
 use std::cell::{Cell, UnsafeCell};
 use std::sync::atomic::{Ordering, AtomicBool};
 use std::mem::ManuallyDrop;

libs/context/src/utils.rs

@@ -1,9 +1,9 @@
 //! Utilities for the context library
 //!
 //! Also used by some collector implementations.
-use std::fmt::{self, Debug, Formatter, Display};
+use core::fmt::{self, Debug, Formatter, Display};
 #[cfg(not(feature = "sync"))]
-use std::cell::Cell;
+use core::cell::Cell;
 
 #[cfg(feature = "sync")]
 pub type AtomicCell<T> = ::crossbeam_utils::atomic::AtomicCell<T>;
@@ -39,21 +39,38 @@ impl<T: Copy> AtomicCell<T> {
 pub enum ThreadId {
     #[allow(unused)]
     Nop,
+    #[cfg(feature = "std")]
     Enabled {
         id: std::thread::ThreadId,
         name: Option<String>
     }
 }
 impl ThreadId {
+    #[cfg(feature = "std")]
     pub fn current() -> ThreadId {
+        // NOTE: It's okay: `sync` requires std
         let thread = std::thread::current();
         ThreadId::Enabled {
             id: thread.id(),
             name: thread.name().map(String::from)
         }
     }
+    #[cfg(not(feature = "std"))]
+    #[inline]
+    pub fn current() -> ThreadId {
+        ThreadId::Nop
+    }
 }
 impl slog::Value for ThreadId {
+    #[cfg(not(feature = "std"))]
+    fn serialize(
+        &self, _record: &slog::Record,
+        _key: &'static str,
+        _serializer: &mut dyn slog::Serializer
+    ) -> slog::Result<()> {
+        Ok(()) // Nop
+    }
+    #[cfg(feature = "std")]
     fn serialize(
         &self, _record: &slog::Record,
         key: &'static str,
@@ -81,9 +98,11 @@ impl Debug for ThreadId {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         match *self {
             ThreadId::Nop => f.write_str("ThreadId(??)"),
+            #[cfg(feature = "std")]
             ThreadId::Enabled { id, name: None } => {
                 write!(f, "{:?}", id)
             },
+            #[cfg(feature = "std")]
             ThreadId::Enabled { id, name: Some(ref name) } => {
                 f.debug_tuple("ThreadId")
                     .field(&id)