build-time: hide boost stacktrace in a .cc file

Saves about 16s of CPU time. Not a lot but not nothing. Feels more like
the principle of the thing.

Change-Id: I0992d4024317c20d6985a7977d5649edfb9f46bb

2 files changed, 15 insertions, 7 deletions

diff --git a/src/libutil/fmt.cc b/src/libutil/fmt.cc
index 400fb7ea0..bff5af020 100644
--- a/src/libutil/fmt.cc
+++ b/src/libutil/fmt.cc
@@ -1,4 +1,9 @@
 #include "fmt.hh" // IWYU pragma: keep
+// Darwin and FreeBSD stdenv do not define _GNU_SOURCE but do have _Unwind_Backtrace.
+#if __APPLE__ || __FreeBSD__
+#define BOOST_STACKTRACE_GNU_SOURCE_NOT_REQUIRED
+#endif
+#include <boost/stacktrace/stacktrace.hpp>
 
 template class boost::basic_format<char>;
 
@@ -11,4 +16,9 @@ template HintFmt::HintFmt(const std::string &, const uint64_t &, const char * co
 
 HintFmt::HintFmt(const std::string & literal) : HintFmt("%s", Uncolored(literal)) {}
 
+void printStackTrace()
+{
+    std::cerr << boost::stacktrace::stacktrace() << std::endl;
+}
+
 }
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index 7589e51e2..ee3e1e2e7 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -4,11 +4,6 @@
 #include <iostream>
 #include <string>
 #include <boost/format.hpp>
-// Darwin and FreeBSD stdenv do not define _GNU_SOURCE but do have _Unwind_Backtrace.
-#if __APPLE__ || __FreeBSD__
-#define BOOST_STACKTRACE_GNU_SOURCE_NOT_REQUIRED
-#endif
-#include <boost/stacktrace.hpp>
 #include "ansicolor.hh"
 
 // Explicit instantiation in fmt.cc
@@ -16,6 +11,9 @@ extern template class boost::basic_format<char>;
 
 namespace nix {
 
+/** Prints a C++ stack trace to stderr using boost stacktrace */
+void printStackTrace();
+
 /**
  * Values wrapped in this struct are printed in magenta.
  *
@@ -176,14 +174,14 @@ public:
             std::cerr << "HintFmt received incorrect number of format args. Original format string: '";
             std::cerr << format << "'; number of arguments: " << sizeof...(args) << "\n";
             // And regardless of the coredump give me a damn stacktrace.
-            std::cerr << boost::stacktrace::stacktrace() << std::endl;
+            printStackTrace();
             abort();
         }
     } catch (boost::io::format_error & ex) {
         // Same thing, but for anything that happens in the member initializers.
         std::cerr << "HintFmt received incorrect format string. Original format string: '";
         std::cerr << format << "'; number of arguments: " << sizeof...(args) << "\n";
-        std::cerr << boost::stacktrace::stacktrace() << std::endl;
+        printStackTrace();
         abort();
     }