[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[Erich Keane via cfe-commits]

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@@ -1419,6 +1419,44 @@ is not specified.
   }];
 }
 
+def ExplicitInitDocs : Documentation {
+  let Category = DocCatField;
+  let Content = [{
+The ``clang::requires_explicit_initialization`` attribute indicates that the
+field of an aggregate must be initialized explicitly by users when the class
+is constructed. Its usage is invalid on non-aggregates.
+
+Note that this attribute is *not* a memory safety feature, and is *not* intended
+to guard against use of uninitialized memory.
+
+Rather, it is intended for use in "parameter-objects", used to simulate the
+passing of named parameters:
+
+.. code-block:: c++
+
+  struct ArrayIOParams {
+    size_t count [[clang::requires_explicit_initialization]];
+    size_t element_size [[clang::requires_explicit_initialization]];
+    int flags = 0;
+  };
+
+  size_t ReadArray(FILE *file, void *buffer, ArrayIOParams params);
+
+  int main() {
+    unsigned int buf[512];
+    ReadArray(stdin, buf, {
+      .count = sizeof(buf) / sizeof(*buf),
+      .element_size = sizeof(*buf)
+    });
+  }
+
+This attribute marks such "named parameters" as mandatory, thus informing
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erichkeane wrote:

mandatory implies an error diagnostic here without further elaboration.  Something that shows the warning behavior/example + output if you were to forget this would be appreciated.

https://github.com/llvm/llvm-project/pull/102040