impl FromDatum for CString (pgcentralfoundation#1896)

As much as I would like FromDatum to unexist, it should probably be
usable for all std types we even half-support, especially since it's
less dubious, in several ways, than the `&'a CStr` impl.

Author: workingjubilee

pgrx/src/datum/from.rs

@@ -12,6 +12,7 @@
 use crate::{
     pg_sys, varlena, varlena_to_byte_slice, AllocatedByPostgres, IntoDatum, PgBox, PgMemoryContexts,
 };
+use alloc::ffi::CString;
 use core::{ffi::CStr, mem::size_of};
 use std::num::NonZeroUsize;
 
@@ -448,7 +449,7 @@ impl FromDatum for char {
 }
 
 /// for cstring
-impl<'a> FromDatum for &'a core::ffi::CStr {
+impl<'a> FromDatum for &'a CStr {
     #[inline]
     unsafe fn from_polymorphic_datum(
         datum: pg_sys::Datum,
@@ -458,7 +459,7 @@ impl<'a> FromDatum for &'a core::ffi::CStr {
         if is_null || datum.is_null() {
             None
         } else {
-            Some(core::ffi::CStr::from_ptr(datum.cast_mut_ptr()))
+            Some(CStr::from_ptr(datum.cast_mut_ptr()))
         }
     }
 
@@ -474,14 +475,42 @@ impl<'a> FromDatum for &'a core::ffi::CStr {
         if is_null || datum.is_null() {
             None
         } else {
-            let copy = memory_context
-                .switch_to(|_| core::ffi::CStr::from_ptr(pg_sys::pstrdup(datum.cast_mut_ptr())));
+            let copy =
+                memory_context.switch_to(|_| CStr::from_ptr(pg_sys::pstrdup(datum.cast_mut_ptr())));
 
             Some(copy)
         }
     }
 }
 
+impl FromDatum for CString {
+    #[inline]
+    unsafe fn from_polymorphic_datum(
+        datum: pg_sys::Datum,
+        is_null: bool,
+        _: pg_sys::Oid,
+    ) -> Option<CString> {
+        if is_null || datum.is_null() {
+            None
+        } else {
+            Some(CStr::from_ptr(datum.cast_mut_ptr()).to_owned())
+        }
+    }
+
+    /// Nonsensical because CString is always allocated within Rust memory.
+    unsafe fn from_datum_in_memory_context(
+        _memory_context: PgMemoryContexts,
+        datum: pg_sys::Datum,
+        is_null: bool,
+        _typoid: pg_sys::Oid,
+    ) -> Option<Self>
+    where
+        Self: Sized,
+    {
+        Self::from_polymorphic_datum(datum, is_null, _typoid)
+    }
+}
+
 /// for bytea
 impl<'a> FromDatum for &'a [u8] {
     #[inline]