Add .strdup() method to Option<AsRef<str>>

We already have a .strdup() method on AsRef<str>, this adds this
method also to an option of this.  In case the option is None a NULL
pointer is returned.

This is done by using a new trait, as the type system otherwise
considers such an implementation conflicting with the existing one.

deltachat-ffi/src/lib.rs

@@ -3227,7 +3227,7 @@ pub unsafe extern "C" fn dc_lot_get_text1(lot: *mut dc_lot_t) -> *mut libc::c_ch
     }
 
     let lot = &*lot;
-    strdup_opt(lot.get_text1())
+    lot.get_text1().strdup()
 }
 
 #[no_mangle]
@@ -3238,7 +3238,7 @@ pub unsafe extern "C" fn dc_lot_get_text2(lot: *mut dc_lot_t) -> *mut libc::c_ch
     }
 
     let lot = &*lot;
-    strdup_opt(lot.get_text2())
+    lot.get_text2().strdup()
 }
 
 #[no_mangle]
@@ -3323,13 +3323,6 @@ impl<T: Default, E: std::fmt::Display> ResultExt<T, E> for Result<T, E> {
     }
 }
 
-unsafe fn strdup_opt(s: Option<impl AsRef<str>>) -> *mut libc::c_char {
-    match s {
-        Some(s) => s.as_ref().strdup(),
-        None => ptr::null_mut(),
-    }
-}
-
 trait ResultNullableExt<T> {
     fn into_raw(self) -> *mut T;
 }

deltachat-ffi/src/string.rs

@@ -1,5 +1,6 @@
 use failure::Fail;
 use std::ffi::{CStr, CString};
+use std::ptr;
 
 /// Duplicates a string
 ///
@@ -144,7 +145,7 @@ pub trait CStringExt {
 
 impl CStringExt for CString {}
 
-/// Convenience methods to make transitioning from raw C strings easier.
+/// Convenience methods to turn strings into C strings.
 ///
 /// To interact with (legacy) C APIs we often need to convert from
 /// Rust strings to raw C strings.  This can be clumsy to do correctly
@@ -174,6 +175,35 @@ impl<T: AsRef<str>> StrExt for T {
     }
 }
 
+/// Convenience methods to turn optional strings into C strings.
+///
+/// This is the same as the [StrExt] trait but a different trait name
+/// to work around the type system not allowing to implement [StrExt]
+/// for `Option<impl StrExt>` When we already have an [StrExt] impl
+/// for `AsRef<&str>`.
+///
+/// When the [Option] is [Option::Some] this behaves just like
+/// [StrExt::strdup], when it is [Option::None] a null pointer is
+/// returned.
+pub trait OptStrExt {
+    /// Allocate a new raw C `*char` version of this string, or NULL.
+    ///
+    /// See [StrExt::strdup] for details.
+    unsafe fn strdup(&self) -> *mut libc::c_char;
+}
+
+impl<T: AsRef<str>> OptStrExt for Option<T> {
+    unsafe fn strdup(&self) -> *mut libc::c_char {
+        match self {
+            Some(s) => {
+                let tmp = CString::yolo(s.as_ref());
+                dc_strdup(tmp.as_ptr())
+            }
+            None => ptr::null_mut(),
+        }
+    }
+}
+
 pub fn to_string_lossy(s: *const libc::c_char) -> String {
     if s.is_null() {
         return "".into();
@@ -347,4 +377,19 @@ mod tests {
             assert_eq!(cmp, 0);
         }
     }
+
+    #[test]
+    fn test_strdup_opt_string() {
+        unsafe {
+            let s = Some("hello");
+            let c = s.strdup();
+            let cmp = strcmp(c, b"hello\x00" as *const u8 as *const libc::c_char);
+            free(c as *mut libc::c_void);
+            assert_eq!(cmp, 0);
+
+            let s: Option<&str> = None;
+            let c = s.strdup();
+            assert_eq!(c, ptr::null_mut());
+        }
+    }
 }