Merge bitcoin/bitcoin#29906: Disable util::Result copying and assignment

6a8b2befeab25e4e92d8e947a23e78014695e06c refactor: Avoid copying util::Result values (Ryan Ofsky)
834f65e82405bbed336f98996bc8cef366bbed0f refactor: Drop util::Result operator= (Ryan Ofsky)

Pull request description:

  This PR just contains the first two commits of #25665.

  It disables copying of `util::Result` objects because unnecessary copies are inefficient and not possible after #25665, which makes `util::Result` object move-only.

  It disables the assignment operator and replaces it with an `Update()` method, because #25665 adds more information to `util::Result` objects (warning and error messages and failure values) and having an assignment operator that overwrites data instead of merging it would make it easy to accidentally erase existing information while trying to assign new information.

ACKs for top commit:
  stickies-v:
    re-ACK 6a8b2befeab25e4e92d8e947a23e78014695e06c
  achow101:
    ACK 6a8b2befeab25e4e92d8e947a23e78014695e06c
  furszy:
    re-ACK https://github.com/bitcoin/bitcoin/commit/6a8b2befeab25e4e92d8e947a23e78014695e06c

Tree-SHA512: 3f21af9031d50d6c68cca69133de03080f69b1ddcf8b140bdeb762069f14645209b2586037236d15b6ebd8973af0fbefd7e83144aeb7b84078a4cb4df812f984

1 files changed, 12 insertions, 0 deletions

diff --git a/src/util/result.h b/src/util/result.h
index b99995c7e5..122a7638fa 100644
--- a/src/util/result.h
+++ b/src/util/result.h
@@ -39,6 +39,16 @@ private:
 
     std::variant<bilingual_str, T> m_variant;
 
+    //! Disallow copy constructor, require Result to be moved for efficiency.
+    Result(const Result&) = delete;
+
+    //! Disallow operator= to avoid confusion in the future when the Result
+    //! class gains support for richer error reporting, and callers should have
+    //! ability to set a new result value without clearing existing error
+    //! messages.
+    Result& operator=(const Result&) = delete;
+    Result& operator=(Result&&) = delete;
+
     template <typename FT>
     friend bilingual_str ErrorString(const Result<FT>& result);
 
@@ -46,6 +56,8 @@ public:
     Result() : m_variant{std::in_place_index_t<1>{}, std::monostate{}} {}  // constructor for void
     Result(T obj) : m_variant{std::in_place_index_t<1>{}, std::move(obj)} {}
     Result(Error error) : m_variant{std::in_place_index_t<0>{}, std::move(error.message)} {}
+    Result(Result&&) = default;
+    ~Result() = default;
 
     //! std::optional methods, so functions returning optional<T> can change to
     //! return Result<T> with minimal changes to existing code, and vice versa.