lib: make "std" independent of allocator_api

When compiling without allocator_api, assume that allocations cannot fail.
This way, nightly Rust features are not absolutely needed for pinned_init,
and it can be used with stable Rust; right now the minimum supported Rust
version is 1.82.0, where the new_uninit version was stabilized.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Cargo.toml

@@ -18,8 +18,8 @@ paste = "1.0"
 pinned-init-macro = { path = "./pinned-init-macro", version = "=0.0.5" }
 
 [features]
-default = ["std"]
-std = ["alloc"]
+default = ["std", "alloc"]
+std = []
 alloc = []
 
 [dev-dependencies]

README.md

@@ -26,13 +26,17 @@ it into it's final memory location.
 
 This library allows you to do in-place initialization safely.
 
-### Nightly Needed for `alloc` and `std` features
+### Nightly Needed for `alloc` feature
 
-This library requires the `allocator_api` unstable feature when the `alloc` or `std` features
-are enabled and thus can only be used with a nightly compiler.
+This library requires the `allocator_api` unstable feature when the `alloc` feature
+is enabled and thus this feature can only be used with a nightly compiler.
+When enabling the `alloc` feature, the user will be required to activate
+`allocator_api` as well.
 
-When enabling the `alloc` or `std` feature, the user will be required to activate `allocator_api`
-as well.
+The feature is enabled by default, thus by default `pinned-init` will require a
+nightly compiler. However, using the crate on stable compilers is possible by
+disabling `alloc`. In practice this will require the `std` feature, because
+stable compilers have neither `Box` nor `Arc` in no-std mode.
 
 ## Overview
 

src/lib.rs

@@ -18,13 +18,17 @@
 //!
 //! This library allows you to do in-place initialization safely.
 //!
-//! ## Nightly Needed for `alloc` and `std` features
+//! ## Nightly Needed for `alloc` feature
 //!
-//! This library requires the `allocator_api` unstable feature when the `alloc` or `std` features
-//! are enabled and thus can only be used with a nightly compiler.
+//! This library requires the `allocator_api` unstable feature when the `alloc` feature
+//! is enabled and thus this feature can only be used with a nightly compiler.
+//! When enabling the `alloc` feature, the user will be required to activate
+//! `allocator_api` as well.
 //!
-//! When enabling the `alloc` or `std` feature, the user will be required to activate `allocator_api`
-//! as well.
+//! The feature is enabled by default, thus by default `pinned-init` will require a
+//! nightly compiler. However, using the crate on stable compilers is possible by
+//! disabling `alloc`. In practice this will require the `std` feature, because
+//! stable compilers have neither `Box` nor `Arc` in no-std mode.
 //!
 //! # Overview
 //!
@@ -239,9 +243,9 @@
 extern crate alloc;
 
 #[cfg(all(feature = "alloc", not(feature = "std")))]
-use alloc::boxed::Box;
-#[cfg(feature = "alloc")]
-use alloc::sync::Arc;
+use alloc::{boxed::Box, sync::Arc};
+#[cfg(feature = "std")]
+use std::sync::Arc;
 
 use core::{
     cell::UnsafeCell,
@@ -1201,32 +1205,45 @@ pub trait InPlaceInit<T>: Sized {
 }
 
 #[cfg(feature = "alloc")]
+macro_rules! try_new_uninit {
+    ($type:ident) => {
+        $type::try_new_uninit()?
+    };
+}
+#[cfg(all(feature = "std", not(feature = "alloc")))]
+macro_rules! try_new_uninit {
+    ($type:ident) => {
+        $type::new_uninit()
+    };
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
 impl<T> InPlaceInit<T> for Box<T> {
     #[inline]
     fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
     where
         E: From<AllocError>,
     {
-        Box::try_new_uninit()?.write_pin_init(init)
+        try_new_uninit!(Box).write_pin_init(init)
     }
 
     #[inline]
     fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
     where
         E: From<AllocError>,
     {
-        Box::try_new_uninit()?.write_init(init)
+        try_new_uninit!(Box).write_init(init)
     }
 }
 
-#[cfg(feature = "alloc")]
+#[cfg(any(feature = "std", feature = "alloc"))]
 impl<T> InPlaceInit<T> for Arc<T> {
     #[inline]
     fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
     where
         E: From<AllocError>,
     {
-        let mut this = Arc::try_new_uninit()?;
+        let mut this = try_new_uninit!(Arc);
         let Some(slot) = Arc::get_mut(&mut this) else {
             // SAFETY: the Arc has just been created and has no external referecnes
             unsafe { core::hint::unreachable_unchecked() }
@@ -1244,7 +1261,7 @@ impl<T> InPlaceInit<T> for Arc<T> {
     where
         E: From<AllocError>,
     {
-        let mut this = Arc::try_new_uninit()?;
+        let mut this = try_new_uninit!(Arc);
         let Some(slot) = Arc::get_mut(&mut this) else {
             // SAFETY: the Arc has just been created and has no external referecnes
             unsafe { core::hint::unreachable_unchecked() }