Merge #3036

3036: fix const-ness of some FFI name & doc members r=adamreichold a=davidhewitt

I just noticed that these are [`*const` since Python 3.7](python/cpython@007d7ff#diff-830b405713d1c40982ffa918864e39c40c1b318d79b8dd2b523646dc3bd18b52).

This might break existing compiles, so will not include this in 0.18.2.

Co-authored-by: David Hewitt <1939362+davidhewitt@users.noreply.github.com>

newsfragments/3036.fixed.md

@@ -0,0 +1 @@
+Correct FFI definitions `PyGetSetDef`, `PyMemberDef`, `PyStructSequence_Field` and `PyStructSequence_Desc` to have `*const c_char` members for `name` and `doc` (not `*mut c_char`).

pyo3-ffi/src/descrobject.rs

@@ -11,7 +11,7 @@ pub type setter =
 #[repr(C)]
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub struct PyGetSetDef {
-    pub name: *mut c_char,
+    pub name: *const c_char,
     pub get: Option<getter>,
     pub set: Option<setter>,
     pub doc: *const c_char,
@@ -21,7 +21,7 @@ pub struct PyGetSetDef {
 impl Default for PyGetSetDef {
     fn default() -> PyGetSetDef {
         PyGetSetDef {
-            name: ptr::null_mut(),
+            name: ptr::null(),
             get: None,
             set: None,
             doc: ptr::null(),

pyo3-ffi/src/structmember.rs

@@ -6,11 +6,11 @@ use std::ptr;
 #[repr(C)]
 #[derive(Copy, Clone, Eq, PartialEq)]
 pub struct PyMemberDef {
-    pub name: *mut c_char,
+    pub name: *const c_char,
     pub type_code: c_int,
     pub offset: Py_ssize_t,
     pub flags: c_int,
-    pub doc: *mut c_char,
+    pub doc: *const c_char,
 }
 
 impl Default for PyMemberDef {

pyo3-ffi/src/structseq.rs

@@ -6,15 +6,15 @@ use std::os::raw::{c_char, c_int};
 #[repr(C)]
 #[derive(Copy, Clone)]
 pub struct PyStructSequence_Field {
-    pub name: *mut c_char,
-    pub doc: *mut c_char,
+    pub name: *const c_char,
+    pub doc: *const c_char,
 }
 
 #[repr(C)]
 #[derive(Copy, Clone)]
 pub struct PyStructSequence_Desc {
-    pub name: *mut c_char,
-    pub doc: *mut c_char,
+    pub name: *const c_char,
+    pub doc: *const c_char,
     pub fields: *mut PyStructSequence_Field,
     pub n_in_sequence: c_int,
 }