cpp_build: disable LTO to preserve metadata in .data section (#116)

The cpp crate communicates type metadata (sizes, alignments, trait
flags) from the build script to the proc-macro by embedding a struct
with a magic byte pattern in the .data section of the compiled static
library. The proc-macro then scans the .a file for this pattern using
aho-corasick.

When LTO is enabled via environment CXXFLAGS (e.g. -flto=auto, as is
the default on Arch Linux's makepkg), the C++ compiler emits GIMPLE
intermediate representation instead of native machine code. This
leaves the .data and .text sections empty — all content lives in
.gnu.lto_* sections as serialized IR. The magic bytes are no longer
visible to the byte-level scan, causing the proc-macro to panic with:

  "Struct metadata not present in target library file."

Fix this by passing -fno-lto when compiling the generated C++ library.
This ensures the metadata struct is always present as raw bytes in
.data, regardless of the environment's LTO settings.

Fixes builds on distributions that enable LTO by default (Arch Linux,
Fedora, Gentoo with LTO profiles).

Author: okhsunrog

cpp_build/src/lib.rs

@@ -654,6 +654,15 @@ In order to provide a better error message, the build script will exit successfu
         if !self.std_flag_set {
             self.cc.flag_if_supported("-std=c++11");
         }
+        // Disable LTO for the generated C++ library. The cpp crate embeds
+        // metadata as raw bytes in the .data section of the compiled object,
+        // which the cpp_macros proc-macro later locates via byte pattern
+        // matching. When LTO is enabled (e.g. -flto=auto from environment
+        // CXXFLAGS), the compiler emits GIMPLE IR instead of native code,
+        // leaving the .data section empty and making the metadata invisible
+        // to the proc-macro.
+        self.cc.flag_if_supported("-fno-lto");
+
         // Build the C++ library
         if let Err(e) = self.cc.file(filename).try_compile(LIB_NAME) {
             let _ = writeln!(std::io::stderr(), "\n\nerror occurred: {}\n\n", e);