cxx-qt-lib: use +whole_archive for std_types so that cargo-only works

When building with cargo we need to ensure that the statics aren't
optimised out for registering types with Qt 5.

Otherwise we cannot use numbers in QML.

Closes #592

Author: ahayzen-kdab

crates/cxx-qt-build/src/lib.rs

@@ -429,6 +429,7 @@ impl CxxQtBuilder {
         // https://github.com/rust-lang/rust/issues/110912
         cc_builder_whole_archive.link_lib_modifier("-bundle");
         for builder in [&mut self.cc_builder, &mut cc_builder_whole_archive] {
+            // Note, ensure our settings stay in sync across cxx-qt-build and cxx-qt-lib
             builder.cpp(true);
             // MSVC
             builder.flag_if_supported("/std:c++17");

crates/cxx-qt-lib/Cargo.toml

@@ -26,6 +26,7 @@ time = { version = "0.3.20", optional = true }
 url = { version = "2.3", optional = true }
 
 [build-dependencies]
+cc.workspace = true
 cxx-build.workspace = true
 cxx-qt-lib-headers.workspace = true
 qt-build-utils.workspace = true

crates/cxx-qt-lib/build.rs

@@ -232,10 +232,27 @@ fn main() {
     }
     builder.file("src/qt_types.cpp");
     println!("cargo:rerun-if-changed=src/qt_types.cpp");
-    builder.file("src/std_types.cpp");
-    println!("cargo:rerun-if-changed=src/std_types.cpp");
     println!("cargo:rerun-if-changed=src/assertion_utils.h");
 
+    // Setup compiler
+    // qRegisterMetaType needs to be linked with +whole-archive
+    // because they use static variables which need to be initialized before main
+    // (regardless of whether main is in Rust or C++). Normally linkers only copy symbols referenced
+    // from within main when static linking, which would result in discarding those static variables.
+    // Use a separate cc::Build for the little amount of code that needs to be linked with +whole-archive
+    // to avoid bloating the binary.
+    let mut cc_builder_whole_archive = cc::Build::new();
+    cc_builder_whole_archive.link_lib_modifier("+whole-archive");
+    // Workaround for spurious compiler error in Rust 1.69. The bug has been fixed, but keeping this
+    // workaround seems to be harmless.
+    // https://github.com/rust-lang/rust/issues/110912
+    cc_builder_whole_archive.link_lib_modifier("-bundle");
+    for include_path in qtbuild.include_paths() {
+        cc_builder_whole_archive.include(include_path);
+    }
+    cc_builder_whole_archive.file("src/std_types.cpp");
+    println!("cargo:rerun-if-changed=src/std_types.cpp");
+
     // Write this library's manually written C++ headers to files and add them to include paths
     let out_dir = std::env::var("OUT_DIR").unwrap();
     cxx_qt_lib_headers::write_headers(format!("{out_dir}/cxx-qt-lib"));
@@ -251,11 +268,17 @@ fn main() {
         builder.define("CXX_QT_QML_FEATURE", None);
     }
 
-    // MSVC
-    builder.flag_if_supported("/std:c++17");
-    builder.flag_if_supported("/Zc:__cplusplus");
-    builder.flag_if_supported("/permissive-");
-    // GCC + Clang
-    builder.flag_if_supported("-std=c++17");
+    for builder in [&mut builder, &mut cc_builder_whole_archive] {
+        // Note, ensure our settings stay in sync across cxx-qt-build and cxx-qt-lib
+        builder.cpp(true);
+        // MSVC
+        builder.flag_if_supported("/std:c++17");
+        builder.flag_if_supported("/Zc:__cplusplus");
+        builder.flag_if_supported("/permissive-");
+        // GCC + Clang
+        builder.flag_if_supported("-std=c++17");
+    }
+
+    cc_builder_whole_archive.compile("cxx-qt-lib-static-initializers");
     builder.compile("cxx-qt-lib");
 }