Move elements when converting rvalue containers to JSON

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/basic_json.md

@@ -68,6 +68,14 @@ basic_json(basic_json&& other) noexcept;
      `std::unordered_map`, `std::multimap`, and `std::unordered_multimap` with a `key_type` compatible to `string_t`
      and a `value_type` from which a `basic_json` value can be constructed.
     - **strings**: `string_t`, string literals, and all compatible string containers can be used.
+
+    !!! note "Rvalue containers are moved from"
+
+        If `val` is an rvalue of a compatible array-like or object-like container other than `array_t`/`object_t`
+        (e.g., `json j = std::move(vec);` or `json j(std::vector<std::string>{...});`), the elements are moved into
+        the JSON value instead of copied. Afterwards, the elements of the source container are in a valid but
+        unspecified state (**moved-from**). Keys of associative containers are `const` and therefore still copied.
+        Lvalues and `const` rvalues are copied, and the source is left unchanged.
     - **numbers**: [`number_integer_t`](number_integer_t.md), [`number_unsigned_t`](number_unsigned_t.md),
      [`number_float_t`](number_float_t.md), and all convertible number types such as `int`, `size_t`, `int64_t`, `float`
      or `double` can be used.
@@ -236,7 +244,9 @@ basic_json(basic_json&& other) noexcept;
 2. No-throw guarantee: this constructor never throws exceptions.
 3. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no
    `to_json()` function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any
-   JSON value.
+   JSON value. **Exception:** when `val` is an rvalue array-like or object-like container, its elements are moved one
+   by one; if an exception is thrown in the middle of the conversion (e.g., an allocation failure), the JSON value is
+   unchanged, but the source container may be left partially moved-from.
 4. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no
    `to_json()` function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any
    JSON value.

docs/mkdocs/docs/home/releases.md

@@ -9,6 +9,15 @@ published on the [GitHub releases page](https://github.com/nlohmann/json/release
     This documentation is built from the `develop` branch and may describe changes that are not part of a release
     yet. Their version numbers are followed by an <span class="unreleased-version">unreleased</span> badge.
 
+## Unreleased
+
+- Converting an rvalue container to JSON (`json j = std::move(vec);`, `json j = std::map<std::string, std::string>{...};`)
+  now **moves** the elements instead of copying them (e.g., `std::vector<std::string>` with 100,000 strings: 4.7 ms to
+  2.3 ms). Two consequences: the source container is left in a valid but unspecified (moved-from) state, and if an
+  exception is thrown during the conversion, the source may be partially moved-from, so the strong exception guarantee
+  no longer holds for the source of an rvalue conversion (the JSON value itself is unaffected). Lvalues and `const`
+  objects are still copied. See [`basic_json`](../api/basic_json/basic_json.md).
+
 ## v3.12.0 (2025-04-11)
 
 Fixes bugs found in 3.11.3 and adds several features. All changes are backward-compatible.