fix soundness issue with preallocated_gen_new

Stop this from being a generic function over all contexts, to only a
function generic over contexts where we can bound the lifetime
precisely.

Fixes #543

src/context.rs

@@ -318,9 +318,9 @@ unsafe impl<'buf> Context for AllPreallocated<'buf> {
     }
 }
 
-impl<'buf, C: Context + 'buf> Secp256k1<C> {
+impl<'buf> PreallocatedContext<'buf> for Secp256k1<SignOnlyPreallocated<'buf>> {
     /// 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> {
+    fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Self, Error> {
         #[cfg(target_arch = "wasm32")]
         ffi::types::sanity_checks_for_wasm();
 
@@ -331,13 +331,58 @@ impl<'buf, C: Context + 'buf> Secp256k1<C> {
             ctx: unsafe {
                 NonNull::new_unchecked(ffi::secp256k1_context_preallocated_create(
                     buf.as_mut_c_ptr() as *mut c_void,
-                    C::FLAGS,
+                    AllPreallocated::FLAGS,
                 ))
             },
             phantom: PhantomData,
         })
     }
 }
+impl<'buf> PreallocatedContext<'buf> for Secp256k1<VerifyOnlyPreallocated<'buf>> {
+    /// Lets you create a context with a preallocated buffer in a generic manner (sign/verify/all).
+    fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Self, Error> {
+        #[cfg(target_arch = "wasm32")]
+        ffi::types::sanity_checks_for_wasm();
+
+        if buf.len() < Self::preallocate_size_gen() {
+            return Err(Error::NotEnoughMemory);
+        }
+        Ok(Secp256k1 {
+            ctx: unsafe {
+                NonNull::new_unchecked(ffi::secp256k1_context_preallocated_create(
+                    buf.as_mut_c_ptr() as *mut c_void,
+                    VerifyOnlyPreallocated::FLAGS,
+                ))
+            },
+            phantom: PhantomData,
+        })
+    }
+}
+impl<'buf> PreallocatedContext<'buf> for Secp256k1<AllPreallocated<'buf>> {
+    /// Lets you create a context with a preallocated buffer in a generic manner (sign/verify/all).
+    fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Self, Error> {
+        #[cfg(target_arch = "wasm32")]
+        ffi::types::sanity_checks_for_wasm();
+
+        if buf.len() < Self::preallocate_size_gen() {
+            return Err(Error::NotEnoughMemory);
+        }
+        Ok(Secp256k1 {
+            ctx: unsafe {
+                NonNull::new_unchecked(ffi::secp256k1_context_preallocated_create(
+                    buf.as_mut_c_ptr() as *mut c_void,
+                    SignOnlyPreallocated::FLAGS,
+                ))
+            },
+            phantom: PhantomData,
+        })
+    }
+}
+
+trait PreallocatedContext<'buf>: Sized {
+    /// Lets you create a context with a preallocated buffer in a generic manner (sign/verify/all).
+    fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Self, Error>;
+}
 
 impl<'buf> Secp256k1<AllPreallocated<'buf>> {
     /// Creates a new Secp256k1 context with all capabilities.