pymethods: support buffer protocol

CHANGELOG.md

@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - All PyO3 proc-macros except the deprecated `#[pyproto]` now accept a supplemental attribute `#[pyo3(crate = "some::path")]` that specifies
   where to find the `pyo3` crate, in case it has been renamed or is re-exported and not found at the crate root. [#2022](https://github.com/PyO3/pyo3/pull/2022)
 - Expose `pyo3-build-config` APIs for cross-compiling and Python configuration discovery for use in other projects. [#1996](https://github.com/PyO3/pyo3/pull/1996)
+- Add buffer magic methods `__getbuffer__` and `__releasebuffer__` to `#[pymethods]`. [#2067](https://github.com/PyO3/pyo3/pull/2067)
 - Accept paths in `wrap_pyfunction` and `wrap_pymodule`. [#2081](https://github.com/PyO3/pyo3/pull/2081)
 
 ### Changed

guide/src/class.md

@@ -899,12 +899,6 @@ impl pyo3::class::impl_::PyClassImpl for MyClass {
         visitor(collector.buffer_protocol_slots());
         visitor(collector.methods_protocol_slots());
     }
-
-    fn get_buffer() -> Option<&'static pyo3::class::impl_::PyBufferProcs> {
-        use pyo3::class::impl_::*;
-        let collector = PyClassImplCollector::<Self>::new();
-        collector.buffer_procs()
-    }
 }
 # Python::with_gil(|py| {
 #     let cls = py.get_type::<MyClass>();

guide/src/class/protocols.md

@@ -175,7 +175,8 @@ TODO; see [#1884](https://github.com/PyO3/pyo3/issues/1884)
 
 #### Buffer objects
 
-TODO; see [#1884](https://github.com/PyO3/pyo3/issues/1884)
+  - `__getbuffer__(<self>, *mut ffi::Py_buffer, flags) -> ()`
+  - `__releasebuffer__(<self>, *mut ffi::Py_buffer)` (no return value, not even `PyResult`)
 
 #### Garbage Collector Integration
 

pyo3-macros-backend/src/method.rs

@@ -229,6 +229,7 @@ pub struct FnSpec<'a> {
     pub convention: CallingConvention,
     pub text_signature: Option<TextSignatureAttribute>,
     pub krate: syn::Path,
+    pub unsafety: Option<syn::Token![unsafe]>,
 }
 
 pub fn get_return_info(output: &syn::ReturnType) -> syn::Type {
@@ -316,6 +317,7 @@ impl<'a> FnSpec<'a> {
             deprecations,
             text_signature,
             krate,
+            unsafety: sig.unsafety,
         })
     }
 

pyo3-macros-backend/src/pyclass.rs

@@ -847,12 +847,8 @@ impl<'a> PyClassImplsBuilder<'a> {
                     #methods_protos
                 }
 
-                fn get_buffer() -> ::std::option::Option<&'static _pyo3::class::impl_::PyBufferProcs> {
-                    use _pyo3::class::impl_::*;
-                    let collector = PyClassImplCollector::<Self>::new();
-                    collector.buffer_procs()
-                }
                 #dict_offset
+
                 #weaklist_offset
             }
 

pyo3-macros-backend/src/pyfunction.rs

@@ -431,6 +431,7 @@ pub fn impl_wrap_pyfunction(
         deprecations: options.deprecations,
         text_signature: options.text_signature,
         krate: krate.clone(),
+        unsafety: func.sig.unsafety,
     };
 
     let wrapper_ident = format_ident!("__pyo3_raw_{}", spec.name);

pyo3-macros-backend/src/pymethod.rs

@@ -103,7 +103,7 @@ pub fn gen_py_method(
             ensure_no_forbidden_protocol_attributes(spec, &method.method_name)?;
             match proto_kind {
                 PyMethodProtoKind::Slot(slot_def) => {
-                    let slot = slot_def.generate_type_slot(cls, spec)?;
+                    let slot = slot_def.generate_type_slot(cls, spec, &method.method_name)?;
                     GeneratedPyMethod::Proto(slot)
                 }
                 PyMethodProtoKind::Call => {
@@ -556,6 +556,14 @@ const __IOR__: SlotDef = SlotDef::new("Py_nb_inplace_or", "binaryfunc")
     .arguments(&[Ty::Object])
     .extract_error_mode(ExtractErrorMode::NotImplemented)
     .return_self();
+const __GETBUFFER__: SlotDef = SlotDef::new("Py_bf_getbuffer", "getbufferproc")
+    .arguments(&[Ty::PyBuffer, Ty::Int])
+    .ret_ty(Ty::Int)
+    .require_unsafe();
+const __RELEASEBUFFER__: SlotDef = SlotDef::new("Py_bf_releasebuffer", "releasebufferproc")
+    .arguments(&[Ty::PyBuffer])
+    .ret_ty(Ty::Void)
+    .require_unsafe();
 
 fn pyproto(method_name: &str) -> Option<&'static SlotDef> {
     match method_name {
@@ -594,6 +602,8 @@ fn pyproto(method_name: &str) -> Option<&'static SlotDef> {
         "__iand__" => Some(&__IAND__),
         "__ixor__" => Some(&__IXOR__),
         "__ior__" => Some(&__IOR__),
+        "__getbuffer__" => Some(&__GETBUFFER__),
+        "__releasebuffer__" => Some(&__RELEASEBUFFER__),
         _ => None,
     }
 }
@@ -608,6 +618,7 @@ enum Ty {
     PyHashT,
     PySsizeT,
     Void,
+    PyBuffer,
 }
 
 impl Ty {
@@ -619,6 +630,7 @@ impl Ty {
             Ty::PyHashT => quote! { _pyo3::ffi::Py_hash_t },
             Ty::PySsizeT => quote! { _pyo3::ffi::Py_ssize_t },
             Ty::Void => quote! { () },
+            Ty::PyBuffer => quote! { *mut _pyo3::ffi::Py_buffer },
         }
     }
 
@@ -680,7 +692,8 @@ impl Ty {
                     let #ident = #extract;
                 }
             }
-            Ty::Int | Ty::PyHashT | Ty::PySsizeT | Ty::Void => todo!(),
+            // Just pass other types through unmodified
+            Ty::PyBuffer | Ty::Int | Ty::PyHashT | Ty::PySsizeT | Ty::Void => quote! {},
         }
     }
 }
@@ -752,6 +765,7 @@ struct SlotDef {
     before_call_method: Option<TokenGenerator>,
     extract_error_mode: ExtractErrorMode,
     return_mode: Option<ReturnMode>,
+    require_unsafe: bool,
 }
 
 const NO_ARGUMENTS: &[Ty] = &[];
@@ -766,6 +780,7 @@ impl SlotDef {
             before_call_method: None,
             extract_error_mode: ExtractErrorMode::Raise,
             return_mode: None,
+            require_unsafe: false,
         }
     }
 
@@ -799,7 +814,17 @@ impl SlotDef {
         self
     }
 
-    fn generate_type_slot(&self, cls: &syn::Type, spec: &FnSpec) -> Result<TokenStream> {
+    const fn require_unsafe(mut self) -> Self {
+        self.require_unsafe = true;
+        self
+    }
+
+    fn generate_type_slot(
+        &self,
+        cls: &syn::Type,
+        spec: &FnSpec,
+        method_name: &str,
+    ) -> Result<TokenStream> {
         let SlotDef {
             slot,
             func_ty,
@@ -808,7 +833,14 @@ impl SlotDef {
             extract_error_mode,
             ret_ty,
             return_mode,
+            require_unsafe,
         } = self;
+        if *require_unsafe {
+            ensure_spanned!(
+                spec.unsafety.is_some(),
+                spec.name.span() => format!("`{}` must be `unsafe fn`", method_name)
+            );
+        }
         let py = syn::Ident::new("_py", Span::call_site());
         let method_arguments = generate_method_arguments(arguments);
         let ret_ty = ret_ty.ffi_type();

pyo3-macros-backend/src/pyproto.rs

@@ -134,31 +134,6 @@ fn impl_proto_methods(
     let slots_trait = proto.slots_trait();
     let slots_trait_slots = proto.slots_trait_slots();
 
-    let mut maybe_buffer_methods = None;
-
-    let build_config = pyo3_build_config::get();
-    const PY39: pyo3_build_config::PythonVersion =
-        pyo3_build_config::PythonVersion { major: 3, minor: 9 };
-
-    if build_config.version <= PY39 && proto.name == "Buffer" {
-        maybe_buffer_methods = Some(quote! {
-            impl _pyo3::class::impl_::PyBufferProtocolProcs<#ty>
-                for _pyo3::class::impl_::PyClassImplCollector<#ty>
-            {
-                fn buffer_procs(
-                    self
-                ) -> ::std::option::Option<&'static _pyo3::class::impl_::PyBufferProcs> {
-                    static PROCS: _pyo3::class::impl_::PyBufferProcs
-                        = _pyo3::class::impl_::PyBufferProcs {
-                            bf_getbuffer: ::std::option::Option::Some(_pyo3::class::buffer::getbuffer::<#ty>),
-                            bf_releasebuffer: ::std::option::Option::Some(_pyo3::class::buffer::releasebuffer::<#ty>),
-                        };
-                    ::std::option::Option::Some(&PROCS)
-                }
-            }
-        });
-    }
-
     let mut tokens = proto
         .slot_defs(method_names)
         .map(|def| {
@@ -178,8 +153,6 @@ fn impl_proto_methods(
     }
 
     quote! {
-        #maybe_buffer_methods
-
         impl _pyo3::class::impl_::#slots_trait<#ty>
             for _pyo3::class::impl_::PyClassImplCollector<#ty>
         {

src/class/impl_.rs

@@ -85,9 +85,6 @@ pub trait PyClassImpl: Sized {
     fn get_free() -> Option<ffi::freefunc> {
         None
     }
-    fn get_buffer() -> Option<&'static PyBufferProcs> {
-        None
-    }
     #[inline]
     fn dict_offset() -> Option<ffi::Py_ssize_t> {
         None
@@ -685,25 +682,6 @@ methods_trait!(PyDescrProtocolMethods, descr_protocol_methods);
 methods_trait!(PyMappingProtocolMethods, mapping_protocol_methods);
 methods_trait!(PyNumberProtocolMethods, number_protocol_methods);
 
-// On Python < 3.9 setting the buffer protocol using slots doesn't work, so these procs are used
-// on those versions to set the slots manually (on the limited API).
-
-#[cfg(not(Py_LIMITED_API))]
-pub use ffi::PyBufferProcs;
-
-#[cfg(Py_LIMITED_API)]
-pub struct PyBufferProcs;
-
-pub trait PyBufferProtocolProcs<T> {
-    fn buffer_procs(self) -> Option<&'static PyBufferProcs>;
-}
-
-impl<T> PyBufferProtocolProcs<T> for &'_ PyClassImplCollector<T> {
-    fn buffer_procs(self) -> Option<&'static PyBufferProcs> {
-        None
-    }
-}
-
 // Thread checkers
 
 #[doc(hidden)]

src/pyclass.rs

@@ -1,6 +1,6 @@
 //! `PyClass` and related traits.
 use crate::{
-    class::impl_::{fallback_new, tp_dealloc, PyBufferProcs, PyClassImpl},
+    class::impl_::{fallback_new, tp_dealloc, PyClassImpl},
     ffi,
     impl_::pyclass::{PyClassDict, PyClassWeakRef},
     PyCell, PyErr, PyMethodDefType, PyNativeType, PyResult, PyTypeInfo, Python,
@@ -55,7 +55,6 @@ where
             &T::for_each_proto_slot,
             T::IS_GC,
             T::IS_BASETYPE,
-            T::get_buffer(),
         )
     } {
         Ok(type_object) => type_object,
@@ -81,7 +80,6 @@ unsafe fn create_type_object_impl(
     for_each_proto_slot: &dyn Fn(&mut dyn FnMut(&[ffi::PyType_Slot])),
     is_gc: bool,
     is_basetype: bool,
-    buffer_procs: Option<&PyBufferProcs>,
 ) -> PyResult<*mut ffi::PyTypeObject> {
     let mut slots = Vec::new();
 
@@ -130,10 +128,26 @@ unsafe fn create_type_object_impl(
 
     // protocol methods
     let mut has_gc_methods = false;
+    // Before Python 3.9, need to patch in buffer methods manually (they don't work in slots)
+    #[cfg(all(not(Py_3_9), not(Py_LIMITED_API)))]
+    let mut buffer_procs: ffi::PyBufferProcs = Default::default();
+
     for_each_proto_slot(&mut |proto_slots| {
-        has_gc_methods |= proto_slots
+        for slot in proto_slots {
-            .iter()
+            has_gc_methods |= slot.slot == ffi::Py_tp_clear || slot.slot == ffi::Py_tp_traverse;
-            .any(|slot| slot.slot == ffi::Py_tp_clear || slot.slot == ffi::Py_tp_traverse);
+
+            #[cfg(all(not(Py_3_9), not(Py_LIMITED_API)))]
+            if slot.slot == ffi::Py_bf_getbuffer {
+                // Safety: slot.pfunc is a valid function pointer
+                buffer_procs.bf_getbuffer = Some(std::mem::transmute(slot.pfunc));
+            }
+
+            #[cfg(all(not(Py_3_9), not(Py_LIMITED_API)))]
+            if slot.slot == ffi::Py_bf_releasebuffer {
+                // Safety: slot.pfunc is a valid function pointer
+                buffer_procs.bf_releasebuffer = Some(std::mem::transmute(slot.pfunc));
+            }
+        }
         slots.extend_from_slice(proto_slots);
     });
 
@@ -153,8 +167,11 @@ unsafe fn create_type_object_impl(
         tp_init_additional(
             type_object as _,
             tp_doc,
-            buffer_procs,
+            #[cfg(all(not(Py_3_9), not(Py_LIMITED_API)))]
+            &buffer_procs,
+            #[cfg(not(Py_3_9))]
             dict_offset,
+            #[cfg(not(Py_3_9))]
             weaklist_offset,
         );
         Ok(type_object as _)
@@ -169,12 +186,12 @@ fn type_object_creation_failed(py: Python, e: PyErr, name: &'static str) -> ! {
 
 /// Additional type initializations necessary before Python 3.10
 #[cfg(all(not(Py_LIMITED_API), not(Py_3_10)))]
-fn tp_init_additional(
+unsafe fn tp_init_additional(
     type_object: *mut ffi::PyTypeObject,
     _tp_doc: &str,
-    _buffer_procs: Option<&PyBufferProcs>,
+    #[cfg(not(Py_3_9))] buffer_procs: &ffi::PyBufferProcs,
-    _dict_offset: Option<ffi::Py_ssize_t>,
+    #[cfg(not(Py_3_9))] dict_offset: Option<ffi::Py_ssize_t>,
-    _weaklist_offset: Option<ffi::Py_ssize_t>,
+    #[cfg(not(Py_3_9))] weaklist_offset: Option<ffi::Py_ssize_t>,
 ) {
     // Just patch the type objects for the things there's no
     // PyType_FromSpec API for... there's no reason this should work,
@@ -184,16 +201,14 @@ fn tp_init_additional(
     #[cfg(all(not(PyPy), not(Py_3_10)))]
     {
         if _tp_doc != "\0" {
-            unsafe {
+            // Until CPython 3.10, tp_doc was treated specially for
-                // Until CPython 3.10, tp_doc was treated specially for
+            // heap-types, and it removed the text_signature value from it.
-                // heap-types, and it removed the text_signature value from it.
+            // We go in after the fact and replace tp_doc with something
-                // We go in after the fact and replace tp_doc with something
+            // that _does_ include the text_signature value!
-                // that _does_ include the text_signature value!
+            ffi::PyObject_Free((*type_object).tp_doc as _);
-                ffi::PyObject_Free((*type_object).tp_doc as _);
+            let data = ffi::PyObject_Malloc(_tp_doc.len());
-                let data = ffi::PyObject_Malloc(_tp_doc.len());
+            data.copy_from(_tp_doc.as_ptr() as _, _tp_doc.len());
-                data.copy_from(_tp_doc.as_ptr() as _, _tp_doc.len());
+            (*type_object).tp_doc = data as _;
-                (*type_object).tp_doc = data as _;
-            }
         }
     }
 
@@ -201,23 +216,15 @@ fn tp_init_additional(
     // Python 3.9, so on older versions we must manually fixup the type object.
     #[cfg(not(Py_3_9))]
     {
-        if let Some(buffer) = _buffer_procs {
+        (*(*type_object).tp_as_buffer).bf_getbuffer = buffer_procs.bf_getbuffer;
-            unsafe {
+        (*(*type_object).tp_as_buffer).bf_releasebuffer = buffer_procs.bf_releasebuffer;
-                (*(*type_object).tp_as_buffer).bf_getbuffer = buffer.bf_getbuffer;
+
-                (*(*type_object).tp_as_buffer).bf_releasebuffer = buffer.bf_releasebuffer;
+        if let Some(dict_offset) = dict_offset {
-            }
+            (*type_object).tp_dictoffset = dict_offset;
         }
 
-        if let Some(dict_offset) = _dict_offset {
+        if let Some(weaklist_offset) = weaklist_offset {
-            unsafe {
+            (*type_object).tp_weaklistoffset = weaklist_offset;
-                (*type_object).tp_dictoffset = dict_offset;
-            }
-        }
-
-        if let Some(weaklist_offset) = _weaklist_offset {
-            unsafe {
-                (*type_object).tp_weaklistoffset = weaklist_offset;
-            }
         }
     }
 }
@@ -226,9 +233,9 @@ fn tp_init_additional(
 fn tp_init_additional(
     _type_object: *mut ffi::PyTypeObject,
     _tp_doc: &str,
-    _buffer_procs: Option<&PyBufferProcs>,
+    #[cfg(all(not(Py_3_9), not(Py_LIMITED_API)))] _buffer_procs: &ffi::PyBufferProcs,
-    _dict_offset: Option<ffi::Py_ssize_t>,
+    #[cfg(not(Py_3_9))] _dict_offset: Option<ffi::Py_ssize_t>,
-    _weaklist_offset: Option<ffi::Py_ssize_t>,
+    #[cfg(not(Py_3_9))] _weaklist_offset: Option<ffi::Py_ssize_t>,
 ) {
 }
 
@@ -290,6 +297,7 @@ fn py_class_method_defs(
     });
 
     if !defs.is_empty() {
+        // Safety: Python expects a zeroed entry to mark the end of the defs
         defs.push(unsafe { std::mem::zeroed() });
     }
 
@@ -329,6 +337,7 @@ fn py_class_members(
     }
 
     if !members.is_empty() {
+        // Safety: Python expects a zeroed entry to mark the end of the defs
         members.push(unsafe { std::mem::zeroed() });
     }
 
@@ -370,6 +379,7 @@ fn py_class_properties(
     push_dict_getset(&mut props, is_dummy);
 
     if !props.is_empty() {
+        // Safety: Python expects a zeroed entry to mark the end of the defs
         props.push(unsafe { std::mem::zeroed() });
     }
     props

tests/test_buffer_protocol.rs

@@ -2,13 +2,12 @@
 #![cfg(not(Py_LIMITED_API))]
 
 use pyo3::buffer::PyBuffer;
-use pyo3::class::PyBufferProtocol;
 use pyo3::exceptions::PyBufferError;
 use pyo3::ffi;
 use pyo3::prelude::*;
 use pyo3::types::IntoPyDict;
 use pyo3::AsPyPointer;
-use std::ffi::CStr;
+use std::ffi::CString;
 use std::os::raw::{c_int, c_void};
 use std::ptr;
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -22,9 +21,13 @@ struct TestBufferClass {
     drop_called: Arc<AtomicBool>,
 }
 
-#[pyproto]
+#[pymethods]
-impl PyBufferProtocol for TestBufferClass {
+impl TestBufferClass {
-    fn bf_getbuffer(slf: PyRefMut<Self>, view: *mut ffi::Py_buffer, flags: c_int) -> PyResult<()> {
+    unsafe fn __getbuffer__(
+        mut slf: PyRefMut<Self>,
+        view: *mut ffi::Py_buffer,
+        flags: c_int,
+    ) -> PyResult<()> {
         if view.is_null() {
             return Err(PyBufferError::new_err("View is null"));
         }
@@ -33,43 +36,43 @@ impl PyBufferProtocol for TestBufferClass {
             return Err(PyBufferError::new_err("Object is not writable"));
         }
 
-        unsafe {
+        (*view).obj = ffi::_Py_NewRef(slf.as_ptr());
-            (*view).obj = ffi::_Py_NewRef(slf.as_ptr());
-        }
 
-        let bytes = &slf.vec;
+        (*view).buf = slf.vec.as_mut_ptr() as *mut c_void;
+        (*view).len = slf.vec.len() as isize;
+        (*view).readonly = 1;
+        (*view).itemsize = 1;
 
-        unsafe {
+        (*view).format = if (flags & ffi::PyBUF_FORMAT) == ffi::PyBUF_FORMAT {
-            (*view).buf = bytes.as_ptr() as *mut c_void;
+            let msg = CString::new("B").unwrap();
-            (*view).len = bytes.len() as isize;
+            msg.into_raw()
-            (*view).readonly = 1;
+        } else {
-            (*view).itemsize = 1;
+            ptr::null_mut()
+        };
 
-            (*view).format = ptr::null_mut();
+        (*view).ndim = 1;
-            if (flags & ffi::PyBUF_FORMAT) == ffi::PyBUF_FORMAT {
+        (*view).shape = if (flags & ffi::PyBUF_ND) == ffi::PyBUF_ND {
-                let msg = CStr::from_bytes_with_nul(b"B\0").unwrap();
+            &mut (*view).len
-                (*view).format = msg.as_ptr() as *mut _;
+        } else {
-            }
+            ptr::null_mut()
+        };
 
-            (*view).ndim = 1;
+        (*view).strides = if (flags & ffi::PyBUF_STRIDES) == ffi::PyBUF_STRIDES {
-            (*view).shape = ptr::null_mut();
+            &mut (*view).itemsize
-            if (flags & ffi::PyBUF_ND) == ffi::PyBUF_ND {
+        } else {
-                (*view).shape = &mut (*view).len;
+            ptr::null_mut()
-            }
+        };
 
-            (*view).strides = ptr::null_mut();
+        (*view).suboffsets = ptr::null_mut();
-            if (flags & ffi::PyBUF_STRIDES) == ffi::PyBUF_STRIDES {
+        (*view).internal = ptr::null_mut();
-                (*view).strides = &mut (*view).itemsize;
-            }
-
-            (*view).suboffsets = ptr::null_mut();
-            (*view).internal = ptr::null_mut();
-        }
 
         Ok(())
     }
 
-    fn bf_releasebuffer(_slf: PyRefMut<Self>, _view: *mut ffi::Py_buffer) {}
+    unsafe fn __releasebuffer__(&self, view: *mut ffi::Py_buffer) {
+        // Release memory held by the format string
+        drop(CString::from_raw((*view).format));
+    }
 }
 
 impl Drop for TestBufferClass {

tests/test_buffer_protocol_pyproto.rs

@@ -0,0 +1,132 @@
+#![cfg(feature = "macros")]
+#![cfg(not(Py_LIMITED_API))]
+
+use pyo3::buffer::PyBuffer;
+use pyo3::class::PyBufferProtocol;
+use pyo3::exceptions::PyBufferError;
+use pyo3::ffi;
+use pyo3::prelude::*;
+use pyo3::types::IntoPyDict;
+use pyo3::AsPyPointer;
+use std::ffi::CStr;
+use std::os::raw::{c_int, c_void};
+use std::ptr;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+
+mod common;
+
+#[pyclass]
+struct TestBufferClass {
+    vec: Vec<u8>,
+    drop_called: Arc<AtomicBool>,
+}
+
+#[pyproto]
+impl PyBufferProtocol for TestBufferClass {
+    fn bf_getbuffer(slf: PyRefMut<Self>, view: *mut ffi::Py_buffer, flags: c_int) -> PyResult<()> {
+        if view.is_null() {
+            return Err(PyBufferError::new_err("View is null"));
+        }
+
+        if (flags & ffi::PyBUF_WRITABLE) == ffi::PyBUF_WRITABLE {
+            return Err(PyBufferError::new_err("Object is not writable"));
+        }
+
+        unsafe {
+            (*view).obj = ffi::_Py_NewRef(slf.as_ptr());
+        }
+
+        let bytes = &slf.vec;
+
+        unsafe {
+            (*view).buf = bytes.as_ptr() as *mut c_void;
+            (*view).len = bytes.len() as isize;
+            (*view).readonly = 1;
+            (*view).itemsize = 1;
+
+            (*view).format = ptr::null_mut();
+            if (flags & ffi::PyBUF_FORMAT) == ffi::PyBUF_FORMAT {
+                let msg = CStr::from_bytes_with_nul(b"B\0").unwrap();
+                (*view).format = msg.as_ptr() as *mut _;
+            }
+
+            (*view).ndim = 1;
+            (*view).shape = ptr::null_mut();
+            if (flags & ffi::PyBUF_ND) == ffi::PyBUF_ND {
+                (*view).shape = &mut (*view).len;
+            }
+
+            (*view).strides = ptr::null_mut();
+            if (flags & ffi::PyBUF_STRIDES) == ffi::PyBUF_STRIDES {
+                (*view).strides = &mut (*view).itemsize;
+            }
+
+            (*view).suboffsets = ptr::null_mut();
+            (*view).internal = ptr::null_mut();
+        }
+
+        Ok(())
+    }
+
+    fn bf_releasebuffer(_slf: PyRefMut<Self>, _view: *mut ffi::Py_buffer) {}
+}
+
+impl Drop for TestBufferClass {
+    fn drop(&mut self) {
+        print!("dropped");
+        self.drop_called.store(true, Ordering::Relaxed);
+    }
+}
+
+#[test]
+fn test_buffer() {
+    let drop_called = Arc::new(AtomicBool::new(false));
+
+    {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let instance = Py::new(
+            py,
+            TestBufferClass {
+                vec: vec![b' ', b'2', b'3'],
+                drop_called: drop_called.clone(),
+            },
+        )
+        .unwrap();
+        let env = [("ob", instance)].into_py_dict(py);
+        py_assert!(py, *env, "bytes(ob) == b' 23'");
+    }
+
+    assert!(drop_called.load(Ordering::Relaxed));
+}
+
+#[test]
+fn test_buffer_referenced() {
+    let drop_called = Arc::new(AtomicBool::new(false));
+
+    let buf = {
+        let input = vec![b' ', b'2', b'3'];
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let instance: PyObject = TestBufferClass {
+            vec: input.clone(),
+            drop_called: drop_called.clone(),
+        }
+        .into_py(py);
+
+        let buf = PyBuffer::<u8>::get(instance.as_ref(py)).unwrap();
+        assert_eq!(buf.to_vec(py).unwrap(), input);
+        drop(instance);
+        buf
+    };
+
+    assert!(!drop_called.load(Ordering::Relaxed));
+
+    {
+        let _py = Python::acquire_gil().python();
+        drop(buf);
+    }
+
+    assert!(drop_called.load(Ordering::Relaxed));
+}

tests/test_compile_error.rs

@@ -25,6 +25,8 @@ fn _test_compile_errors() {
     t.compile_fail("tests/ui/invalid_pyclass_item.rs");
     t.compile_fail("tests/ui/invalid_pyfunctions.rs");
     t.compile_fail("tests/ui/invalid_pymethods.rs");
+    #[cfg(not(Py_LIMITED_API))]
+    t.compile_fail("tests/ui/invalid_pymethods_buffer.rs");
     t.compile_fail("tests/ui/invalid_pymethod_names.rs");
     t.compile_fail("tests/ui/invalid_pymodule_args.rs");
     t.compile_fail("tests/ui/missing_clone.rs");

tests/ui/invalid_pymethods_buffer.rs

@@ -0,0 +1,18 @@
+use pyo3::prelude::*;
+
+#[pyclass]
+struct MyClass {}
+
+#[pymethods]
+impl MyClass {
+    #[pyo3(name = "__getbuffer__")]
+    fn getbuffer_must_be_unsafe(&self, _view: *mut pyo3::ffi::Py_buffer, _flags: std::os::raw::c_int) {}
+}
+
+#[pymethods]
+impl MyClass {
+    #[pyo3(name = "__releasebuffer__")]
+    fn releasebuffer_must_be_unsafe(&self, _view: *mut pyo3::ffi::Py_buffer) {}
+}
+
+fn main() {}

tests/ui/invalid_pymethods_buffer.stderr

@@ -0,0 +1,11 @@
+error: `__getbuffer__` must be `unsafe fn`
+ --> tests/ui/invalid_pymethods_buffer.rs:9:8
+  |
+9 |     fn getbuffer_must_be_unsafe(&self, _view: *mut pyo3::ffi::Py_buffer, _flags: std::os::raw::c_int) {}
+  |        ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `__releasebuffer__` must be `unsafe fn`
+  --> tests/ui/invalid_pymethods_buffer.rs:15:8
+   |
+15 |     fn releasebuffer_must_be_unsafe(&self, _view: *mut pyo3::ffi::Py_buffer) {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^