rusticl/util: add CStrExt::from_ptr_or_empty

Reviewed-by: @LingMan
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/42021>

src/gallium/frontends/rusticl/util/string.rs

@@ -30,10 +30,23 @@ pub unsafe fn char_arr_to_cstr(c_str: &[c_char]) -> &CStr {
 }
 
 pub trait CStrExt: ToOwned {
+    /// # Safety
+    ///
+    /// The same as CStr::from_ptr except that ptr can be a NULL pointer.
+    unsafe fn from_ptr_or_empty<'a>(ptr: &'a *const c_char) -> &'a Self;
     fn concat(&self, other: impl AsRef<Self>) -> Self::Owned;
 }
 
 impl CStrExt for CStr {
+    #[inline]
+    unsafe fn from_ptr_or_empty<'a>(ptr: &'a *const c_char) -> &'a Self {
+        if ptr.is_null() {
+            return c"";
+        }
+        // SAFETY: Callers responsibility
+        unsafe { CStr::from_ptr(*ptr) }
+    }
+
     fn concat(&self, other: impl AsRef<Self>) -> Self::Owned {
         let other = other.as_ref();
 
@@ -50,6 +63,16 @@ impl CStrExt for CStr {
     }
 }
 
+#[test]
+fn test_from_ptr_or_empty() {
+    assert_eq!(unsafe { CStr::from_ptr_or_empty(&std::ptr::null()) }, c"");
+    let some_str = c"SomeStr";
+    assert_eq!(
+        unsafe { CStr::from_ptr_or_empty(&some_str.as_ptr()) },
+        some_str
+    );
+}
+
 #[test]
 fn test_concat_cstr() {
     assert_eq!(c"Test".concat(c"Other"), c"TestOther".to_owned());