Merge #555: Backport and bump version to 0.22.2

12e67f1 Bump version to 0.22.2 (Tobin C. Harding)
7604f32 Add saftey docs for PreallocatedContext trait (Tobin C. Harding)
357e800 context: introduce unsafe `PreallocatedContext` trait (Andrew Poelstra)

Pull request description:

  Backport #548 and bump version ready for release.

ACKs for top commit:
  Kixunil:
    ACK 12e67f1

Tree-SHA512: 6e2e700ecede75b00a6629e314f814946d4a929e5a5b1540b663cf5b338dae4fa7eda293dd6877888976892c834eb63a53ce29938d451e6062ed3d6db3ad6429

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 0.22.2 - 2022-12-05
+
+* Backport [fix soundness issue with `preallocated_gen_new`](https://github.com/rust-bitcoin/rust-secp256k1/pull/548)
+
 # 0.22.1 - 2022-03-10
 
 * [Reintroduce](https://github.com/rust-bitcoin/rust-secp256k1/pull/417) accidentally removed possibility to create `SharedSecret` from byte serialization

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "secp256k1"
-version = "0.22.1"
+version = "0.22.2"
 authors = [ "Dawid Ciężarkiewicz <dpc@ucore.info>",
             "Andrew Poelstra <apoelstra@wpsoftware.net>" ]
 license = "CC0-1.0"

src/context.rs

@@ -299,8 +299,22 @@ unsafe impl<'buf> Context for AllPreallocated<'buf> {
     }
 }
 
-impl<'buf, C: Context + 'buf> Secp256k1<C> {
-    /// Lets you create a context with preallocated buffer in a generic manner(sign/verify/all)
+/// Trait marking that a particular context object internally points to
+/// memory that must outlive `'a`
+///
+/// # Safety
+///
+/// This trait is used internally to gate which context markers can safely
+/// be used with the `preallocated_gen_new` function. Do not implement it
+/// on your own structures.
+pub unsafe trait PreallocatedContext<'a> {}
+
+unsafe impl<'buf> PreallocatedContext<'buf> for AllPreallocated<'buf> {}
+unsafe impl<'buf> PreallocatedContext<'buf> for SignOnlyPreallocated<'buf> {}
+unsafe impl<'buf> PreallocatedContext<'buf> for VerifyOnlyPreallocated<'buf> {}
+
+impl<'buf, C: Context + PreallocatedContext<'buf>> Secp256k1<C> {
+    /// Lets you create a context with a preallocated buffer in a generic manner (sign/verify/all).
     pub fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Secp256k1<C>, Error> {
         #[cfg(target_arch = "wasm32")]
         ffi::types::sanity_checks_for_wasm();