LibIPC+Everywhere: Change IPC decoders to construct values in-place

Currently, the generated IPC decoders will default-construct the type to
be decoded, then pass that value by reference to the concrete decoder.
This, of course, requires that the type is default-constructible. This
was an issue for decoding Variants, which had to require the first type
in the Variant list is Empty, to ensure it is default constructible.

Further, this made it possible for values to become uninitialized in
user-defined decoders.

This patch makes the decoder interface such that the concrete decoders
themselves contruct the decoded type upon return from the decoder. To do
so, the default decoders in IPC::Decoder had to be moved to the IPC
namespace scope, as these decoders are now specializations instead of
overloaded methods (C++ requires specializations to be in a namespace
scope).

Userland/DevTools/HackStudio/AutoCompleteResponse.h

@@ -27,14 +27,15 @@ inline bool encode(Encoder& encoder, CodeComprehension::AutocompleteResultEntry
 }
 
 template<>
-inline ErrorOr<void> decode(Decoder& decoder, CodeComprehension::AutocompleteResultEntry& response)
+inline ErrorOr<CodeComprehension::AutocompleteResultEntry> decode(Decoder& decoder)
 {
-    TRY(decoder.decode(response.completion));
-    TRY(decoder.decode(response.partial_input_length));
-    TRY(decoder.decode(response.language));
-    TRY(decoder.decode(response.display_text));
-    TRY(decoder.decode(response.hide_autocomplete_after_applying));
-    return {};
+    auto completion = TRY(decoder.decode<DeprecatedString>());
+    auto partial_input_length = TRY(decoder.decode<size_t>());
+    auto language = TRY(decoder.decode<CodeComprehension::Language>());
+    auto display_text = TRY(decoder.decode<DeprecatedString>());
+    auto hide_autocomplete_after_applying = TRY(decoder.decode<CodeComprehension::AutocompleteResultEntry::HideAutocompleteAfterApplying>());
+
+    return CodeComprehension::AutocompleteResultEntry { move(completion), partial_input_length, language, move(display_text), hide_autocomplete_after_applying };
 }
 
 template<>
@@ -47,12 +48,13 @@ inline bool encode(Encoder& encoder, CodeComprehension::ProjectLocation const& l
 }
 
 template<>
-inline ErrorOr<void> decode(Decoder& decoder, CodeComprehension::ProjectLocation& location)
+inline ErrorOr<CodeComprehension::ProjectLocation> decode(Decoder& decoder)
 {
-    TRY(decoder.decode(location.file));
-    TRY(decoder.decode(location.line));
-    TRY(decoder.decode(location.column));
-    return {};
+    auto file = TRY(decoder.decode<DeprecatedString>());
+    auto line = TRY(decoder.decode<size_t>());
+    auto column = TRY(decoder.decode<size_t>());
+
+    return CodeComprehension::ProjectLocation { move(file), line, column };
 }
 
 template<>
@@ -67,13 +69,14 @@ inline bool encode(Encoder& encoder, CodeComprehension::Declaration const& decla
 }
 
 template<>
-inline ErrorOr<void> decode(Decoder& decoder, CodeComprehension::Declaration& declaration)
+inline ErrorOr<CodeComprehension::Declaration> decode(Decoder& decoder)
 {
-    TRY(decoder.decode(declaration.name));
-    TRY(decoder.decode(declaration.position));
-    TRY(decoder.decode(declaration.type));
-    TRY(decoder.decode(declaration.scope));
-    return {};
+    auto name = TRY(decoder.decode<DeprecatedString>());
+    auto position = TRY(decoder.decode<CodeComprehension::ProjectLocation>());
+    auto type = TRY(decoder.decode<CodeComprehension::DeclarationType>());
+    auto scope = TRY(decoder.decode<DeprecatedString>());
+
+    return CodeComprehension::Declaration { move(name), position, type, move(scope) };
 }
 
 template<>
@@ -87,13 +90,14 @@ inline bool encode(Encoder& encoder, CodeComprehension::TodoEntry const& entry)
 }
 
 template<>
-inline ErrorOr<void> decode(Decoder& decoder, CodeComprehension::TodoEntry& entry)
+inline ErrorOr<CodeComprehension::TodoEntry> decode(Decoder& decoder)
 {
-    TRY(decoder.decode(entry.content));
-    TRY(decoder.decode(entry.filename));
-    TRY(decoder.decode(entry.line));
-    TRY(decoder.decode(entry.column));
-    return {};
+    auto content = TRY(decoder.decode<DeprecatedString>());
+    auto filename = TRY(decoder.decode<DeprecatedString>());
+    auto line = TRY(decoder.decode<size_t>());
+    auto column = TRY(decoder.decode<size_t>());
+
+    return CodeComprehension::TodoEntry { move(content), move(filename), line, column };
 }
 
 template<>
@@ -109,18 +113,15 @@ inline bool encode(Encoder& encoder, CodeComprehension::TokenInfo const& locatio
 }
 
 template<>
-inline ErrorOr<void> decode(Decoder& decoder, CodeComprehension::TokenInfo& entry)
+inline ErrorOr<CodeComprehension::TokenInfo> decode(Decoder& decoder)
 {
-    u32 semantic_type { 0 };
-    static_assert(sizeof(semantic_type) == sizeof(entry.type));
+    auto type = TRY(decoder.decode<CodeComprehension::TokenInfo::SemanticType>());
+    auto start_line = TRY(decoder.decode<size_t>());
+    auto start_column = TRY(decoder.decode<size_t>());
+    auto end_line = TRY(decoder.decode<size_t>());
+    auto end_column = TRY(decoder.decode<size_t>());
 
-    TRY(decoder.decode(semantic_type));
-    entry.type = static_cast<CodeComprehension::TokenInfo::SemanticType>(semantic_type);
-    TRY(decoder.decode(entry.start_line));
-    TRY(decoder.decode(entry.start_column));
-    TRY(decoder.decode(entry.end_line));
-    TRY(decoder.decode(entry.end_column));
-    return {};
+    return CodeComprehension::TokenInfo { type, start_line, start_column, end_line, end_column };
 }
 
 }