alloc: Add unstable Cfg feature no-global_oom_handling

For certain sorts of systems, programming, it's deemed essential that
all allocation failures be explicitly handled where they occur. For
example, see Linus Torvald's opinion in [1]. Merely not calling global
panic handlers, or always `try_reserving` first (for vectors), is not
deemed good enough, because the mere presence of the global OOM handlers
is burdens static analysis.

One option for these projects to use rust would just be to skip `alloc`,
rolling their own allocation abstractions.  But this would, in my
opinion be a real shame. `alloc` has a few `try_*` methods already, and
we could easily have more. Features like custom allocator support also
demonstrate and existing to support diverse use-cases with the same
abstractions.

A natural way to add such a feature flag would a Cargo feature, but
there are currently uncertainties around how std library crate's Cargo
features may or not be stable, so to avoid any risk of stabilizing by
mistake we are going with a more low-level "raw cfg" token, which
cannot be interacted with via Cargo alone.

Note also that since there is no notion of "default cfg tokens" outside
of Cargo features, we have to invert the condition from
`global_oom_handling` to to `not(no_global_oom_handling)`. This breaks
the monotonicity that would be important for a Cargo feature (i.e.
turning on more features should never break compatibility), but it
doesn't matter for raw cfg tokens which are not intended to be
"constraint solved" by Cargo or anything else.

To support this use-case we create a new feature, "global-oom-handling",
on by default, and put the global OOM handler infra and everything else
it that depends on it behind it. By default, nothing is changed, but
users concerned about global handling can make sure it is disabled, and
be confident that all OOM handling is local and explicit.

For this first iteration, non-flat collections are outright disabled.
`Vec` and `String` don't yet have `try_*` allocation methods, but are
kept anyways since they can be oom-safely created "from parts", and we
hope to add those `try_` methods in the future.

[1]: https://lore.kernel.org/lkml/CAHk-=wh_sNLoz84AUUzuqXEsYH35u=8HV3vK-jbRbJ_B-JjGrg@mail.gmail.com/

library/alloc/src/alloc.rs

@@ -308,7 +308,7 @@ unsafe impl Allocator for Global {
 
 /// The allocator for unique pointers.
 // This function must not unwind. If it does, MIR codegen will fail.
-#[cfg(not(test))]
+#[cfg(all(not(no_global_oom_handling), not(test)))]
 #[lang = "exchange_malloc"]
 #[inline]
 unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
@@ -337,6 +337,7 @@ pub(crate) unsafe fn box_free<T: ?Sized, A: Allocator>(ptr: Unique<T>, alloc: A)
 
 // # Allocation error handler
 
+#[cfg(not(no_global_oom_handling))]
 extern "Rust" {
     // This is the magic symbol to call the global alloc error handler.  rustc generates
     // it to call `__rg_oom` if there is a `#[alloc_error_handler]`, or to call the
@@ -358,7 +359,7 @@ extern "Rust" {
 /// [`set_alloc_error_hook`]: ../../std/alloc/fn.set_alloc_error_hook.html
 /// [`take_alloc_error_hook`]: ../../std/alloc/fn.take_alloc_error_hook.html
 #[stable(feature = "global_alloc", since = "1.28.0")]
-#[cfg(not(test))]
+#[cfg(all(not(no_global_oom_handling), not(test)))]
 #[rustc_allocator_nounwind]
 #[cold]
 pub fn handle_alloc_error(layout: Layout) -> ! {
@@ -368,10 +369,10 @@ pub fn handle_alloc_error(layout: Layout) -> ! {
 }
 
 // For alloc test `std::alloc::handle_alloc_error` can be used directly.
-#[cfg(test)]
+#[cfg(all(not(no_global_oom_handling), test))]
 pub use std::alloc::handle_alloc_error;
 
-#[cfg(not(any(target_os = "hermit", test)))]
+#[cfg(all(not(no_global_oom_handling), not(any(target_os = "hermit", test))))]
 #[doc(hidden)]
 #[allow(unused_attributes)]
 #[unstable(feature = "alloc_internals", issue = "none")]

library/alloc/src/borrow.rs

@@ -4,12 +4,15 @@
 
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
-use core::ops::{Add, AddAssign, Deref};
+use core::ops::Deref;
+#[cfg(not(no_global_oom_handling))]
+use core::ops::{Add, AddAssign};
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::borrow::{Borrow, BorrowMut};
 
 use crate::fmt;
+#[cfg(not(no_global_oom_handling))]
 use crate::string::String;
 
 use Cow::*;
@@ -429,6 +432,7 @@ impl<T: ?Sized + ToOwned> AsRef<T> for Cow<'_, T> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "cow_add", since = "1.14.0")]
 impl<'a> Add<&'a str> for Cow<'a, str> {
     type Output = Cow<'a, str>;
@@ -440,6 +444,7 @@ impl<'a> Add<&'a str> for Cow<'a, str> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "cow_add", since = "1.14.0")]
 impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
     type Output = Cow<'a, str>;
@@ -451,6 +456,7 @@ impl<'a> Add<Cow<'a, str>> for Cow<'a, str> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "cow_add", since = "1.14.0")]
 impl<'a> AddAssign<&'a str> for Cow<'a, str> {
     fn add_assign(&mut self, rhs: &'a str) {
@@ -467,6 +473,7 @@ impl<'a> AddAssign<&'a str> for Cow<'a, str> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "cow_add", since = "1.14.0")]
 impl<'a> AddAssign<Cow<'a, str>> for Cow<'a, str> {
     fn add_assign(&mut self, rhs: Cow<'a, str>) {

library/alloc/src/boxed.rs

@@ -139,7 +139,9 @@ use core::convert::{From, TryFrom};
 use core::fmt;
 use core::future::Future;
 use core::hash::{Hash, Hasher};
-use core::iter::{FromIterator, FusedIterator, Iterator};
+#[cfg(not(no_global_oom_handling))]
+use core::iter::FromIterator;
+use core::iter::{FusedIterator, Iterator};
 use core::marker::{Unpin, Unsize};
 use core::mem;
 use core::ops::{
@@ -150,10 +152,16 @@ use core::ptr::{self, Unique};
 use core::stream::Stream;
 use core::task::{Context, Poll};
 
-use crate::alloc::{handle_alloc_error, AllocError, Allocator, Global, Layout, WriteCloneIntoRaw};
+#[cfg(not(no_global_oom_handling))]
+use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
+use crate::alloc::{AllocError, Allocator, Global, Layout};
+#[cfg(not(no_global_oom_handling))]
 use crate::borrow::Cow;
+#[cfg(not(no_global_oom_handling))]
 use crate::raw_vec::RawVec;
+#[cfg(not(no_global_oom_handling))]
 use crate::str::from_boxed_utf8_unchecked;
+#[cfg(not(no_global_oom_handling))]
 use crate::vec::Vec;
 
 /// A pointer type for heap allocation.
@@ -177,6 +185,7 @@ impl<T> Box<T> {
     /// ```
     /// let five = Box::new(5);
     /// ```
+    #[cfg(not(no_global_oom_handling))]
     #[inline(always)]
     #[doc(alias = "alloc")]
     #[doc(alias = "malloc")]
@@ -203,6 +212,7 @@ impl<T> Box<T> {
     ///
     /// assert_eq!(*five, 5)
     /// ```
+    #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
     #[inline]
     pub fn new_uninit() -> Box<mem::MaybeUninit<T>> {
@@ -227,6 +237,7 @@ impl<T> Box<T> {
     /// ```
     ///
     /// [zeroed]: mem::MaybeUninit::zeroed
+    #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[doc(alias = "calloc")]
     #[unstable(feature = "new_uninit", issue = "63291")]
@@ -236,6 +247,7 @@ impl<T> Box<T> {
 
     /// Constructs a new `Pin<Box<T>>`. If `T` does not implement `Unpin`, then
     /// `x` will be pinned in memory and unable to be moved.
+    #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
     pub fn pin(x: T) -> Pin<Box<T>> {
@@ -329,6 +341,7 @@ impl<T, A: Allocator> Box<T, A> {
     ///
     /// let five = Box::new_in(5, System);
     /// ```
+    #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[inline]
     pub fn new_in(x: T, alloc: A) -> Self {
@@ -385,6 +398,7 @@ impl<T, A: Allocator> Box<T, A> {
     /// assert_eq!(*five, 5)
     /// ```
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[cfg(not(no_global_oom_handling))]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
@@ -447,6 +461,7 @@ impl<T, A: Allocator> Box<T, A> {
     ///
     /// [zeroed]: mem::MaybeUninit::zeroed
     #[unstable(feature = "allocator_api", issue = "32838")]
+    #[cfg(not(no_global_oom_handling))]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A> {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
@@ -490,6 +505,7 @@ impl<T, A: Allocator> Box<T, A> {
 
     /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement `Unpin`, then
     /// `x` will be pinned in memory and unable to be moved.
+    #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[inline(always)]
     pub fn pin_in(x: T, alloc: A) -> Pin<Self>
@@ -547,6 +563,7 @@ impl<T> Box<[T]> {
     ///
     /// assert_eq!(*values, [1, 2, 3])
     /// ```
+    #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
         unsafe { RawVec::with_capacity(len).into_box(len) }
@@ -570,6 +587,7 @@ impl<T> Box<[T]> {
     /// ```
     ///
     /// [zeroed]: mem::MaybeUninit::zeroed
+    #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_zeroed_slice(len: usize) -> Box<[mem::MaybeUninit<T>]> {
         unsafe { RawVec::with_capacity_zeroed(len).into_box(len) }
@@ -599,6 +617,7 @@ impl<T, A: Allocator> Box<[T], A> {
     ///
     /// assert_eq!(*values, [1, 2, 3])
     /// ```
+    #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_uninit_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
@@ -625,6 +644,7 @@ impl<T, A: Allocator> Box<[T], A> {
     /// ```
     ///
     /// [zeroed]: mem::MaybeUninit::zeroed
+    #[cfg(not(no_global_oom_handling))]
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Box<[mem::MaybeUninit<T>], A> {
@@ -1013,20 +1033,23 @@ impl<T: Default> Default for Box<T> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Default for Box<[T]> {
     fn default() -> Self {
         Box::<[T; 0]>::new([])
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "default_box_extra", since = "1.17.0")]
 impl Default for Box<str> {
     fn default() -> Self {
         unsafe { from_boxed_utf8_unchecked(Default::default()) }
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
     /// Returns a new box with a `clone()` of this box's contents.
@@ -1076,6 +1099,7 @@ impl<T: Clone, A: Allocator + Clone> Clone for Box<T, A> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl Clone for Box<str> {
     fn clone(&self) -> Self {
@@ -1182,6 +1206,7 @@ impl<T: ?Sized + Hasher, A: Allocator> Hasher for Box<T, A> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
 impl<T> From<T> for Box<T> {
     /// Converts a generic type `T` into a `Box<T>`
@@ -1214,6 +1239,7 @@ where
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_from_slice", since = "1.17.0")]
 impl<T: Copy> From<&[T]> for Box<[T]> {
     /// Converts a `&[T]` into a `Box<[T]>`
@@ -1239,6 +1265,7 @@ impl<T: Copy> From<&[T]> for Box<[T]> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_from_cow", since = "1.45.0")]
 impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
     #[inline]
@@ -1250,6 +1277,7 @@ impl<T: Copy> From<Cow<'_, [T]>> for Box<[T]> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_from_slice", since = "1.17.0")]
 impl From<&str> for Box<str> {
     /// Converts a `&str` into a `Box<str>`
@@ -1268,6 +1296,7 @@ impl From<&str> for Box<str> {
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_from_cow", since = "1.45.0")]
 impl From<Cow<'_, str>> for Box<str> {
     #[inline]
@@ -1567,13 +1596,15 @@ impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> fo
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "boxed_slice_from_iter", since = "1.32.0")]
 impl<I> FromIterator<I> for Box<[I]> {
     fn from_iter<T: IntoIterator<Item = I>>(iter: T) -> Self {
         iter.into_iter().collect::<Vec<_>>().into_boxed_slice()
     }
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone, A: Allocator + Clone> Clone for Box<[T], A> {
     fn clone(&self) -> Self {

library/alloc/src/collections/mod.rs

@@ -2,41 +2,52 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+#[cfg(not(no_global_oom_handling))]
 pub mod binary_heap;
+#[cfg(not(no_global_oom_handling))]
 mod btree;
+#[cfg(not(no_global_oom_handling))]
 pub mod linked_list;
+#[cfg(not(no_global_oom_handling))]
 pub mod vec_deque;
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub mod btree_map {
     //! A map based on a B-Tree.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub use super::btree::map::*;
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub mod btree_set {
     //! A set based on a B-Tree.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub use super::btree::set::*;
 }
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(no_inline)]
 pub use binary_heap::BinaryHeap;
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(no_inline)]
 pub use btree_map::BTreeMap;
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(no_inline)]
 pub use btree_set::BTreeSet;
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(no_inline)]
 pub use linked_list::LinkedList;
 
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(no_inline)]
 pub use vec_deque::VecDeque;

library/alloc/src/fmt.rs

@@ -546,6 +546,7 @@ pub use core::fmt::{LowerExp, UpperExp};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::fmt::{LowerHex, Pointer, UpperHex};
 
+#[cfg(not(no_global_oom_handling))]
 use crate::string;
 
 /// The `format` function takes an [`Arguments`] struct and returns the resulting
@@ -574,6 +575,7 @@ use crate::string;
 ///
 /// [`format_args!`]: core::format_args
 /// [`format!`]: crate::format
+#[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn format(args: Arguments<'_>) -> string::String {
     let capacity = args.estimated_capacity();

library/alloc/src/lib.rs

@@ -87,7 +87,7 @@
 #![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
-#![feature(const_btree_new)]
+#![cfg_attr(not(no_global_oom_handling), feature(const_btree_new))]
 #![cfg_attr(bootstrap, feature(const_fn))]
 #![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(cow_is_borrowed)]
@@ -183,7 +183,7 @@ pub mod str;
 pub mod string;
 #[cfg(target_has_atomic = "ptr")]
 pub mod sync;
-#[cfg(target_has_atomic = "ptr")]
+#[cfg(all(not(no_global_oom_handling), target_has_atomic = "ptr"))]
 pub mod task;
 #[cfg(test)]
 mod tests;