diff --git a/src/assert.cc b/src/assert.cc
index 9d0635189..b193f595e 100644
--- a/src/assert.cc
+++ b/src/assert.cc
@@ -47,9 +47,7 @@ bool notify_fatal_error(const String& msg)
 
 void on_assert_failed(const char* message)
 {
-    char* callstack = Backtrace{}.desc();
-    String debug_info = format("pid: {}\ncallstack:\n{}", getpid(), callstack);
-    free(callstack);
+    String debug_info = format("pid: {}\ncallstack:\n{}", getpid(), Backtrace{}.desc());
     write_debug(format("assert failed: '{}'\n{}", message, debug_info));
 
     const auto msg = format("{}\n[Debug Infos]\n{}", message, debug_info);
diff --git a/src/backtrace.cc b/src/backtrace.cc
index 4f8aacf80..a4c65876e 100644
--- a/src/backtrace.cc
+++ b/src/backtrace.cc
@@ -1,12 +1,12 @@
 #include "backtrace.hh"
 
-#include <string.h>
-#include <stdlib.h>
+#include "string.hh"
 
 #if defined(__linux__) || defined(__APPLE__)
 # include <execinfo.h>
 #elif defined(__CYGWIN__)
 # include <windows.h>
+#include <stdio.h>
 #endif
 
 namespace Kakoune
@@ -21,31 +21,29 @@ Backtrace::Backtrace()
     #endif
 }
 
-char* Backtrace::desc() const
+String Backtrace::desc() const
 {
     #if defined(__linux__) || defined(__APPLE__)
     char** symbols = backtrace_symbols(stackframes, num_frames);
-    int size = 0;
+    ByteCount size = 0;
     for (int i = 0; i < num_frames; ++i)
-        size += strlen(symbols[i]) + 1;
+        size += StringView::strlen(symbols[i]) + 1;
 
-    char* res = (char*)malloc(size+1);
-    res[0] = 0;
+    String res; res.reserve(size);
     for (int i = 0; i < num_frames; ++i)
     {
-        strcat(res, symbols[i]);
-        strcat(res, "\n");
+        res += symbols[i];
+        res += "\n";
     }
     free(symbols);
     return res;
     #elif defined(__CYGWIN__)
-    char* res = (char*)malloc(num_frames * 20);
-    res[0] = 0;
+    String res; res.reserve(num_frames * 20);
     for (int i = 0; i < num_frames; ++i)
     {
         char addr[20];
         snprintf(addr, 20, "0x%p", stackframes[i]);
-        strcat(res, addr);
+        res += addr;
     }
     return res;
     #else
diff --git a/src/backtrace.hh b/src/backtrace.hh
index 8f1463724..a04c6f30a 100644
--- a/src/backtrace.hh
+++ b/src/backtrace.hh
@@ -4,6 +4,8 @@
 namespace Kakoune
 {
 
+class String;
+
 struct Backtrace
 {
     static constexpr int max_frames = 16;
@@ -11,7 +13,7 @@ struct Backtrace
     int num_frames = 0;
 
     Backtrace();
-    char* desc() const;
+    String desc() const;
 };
 
 }
diff --git a/src/main.cc b/src/main.cc
index 822d53fe5..7c7dc86d9 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -283,9 +283,7 @@ void signal_handler(int signal)
     }
     if (signal != SIGTERM)
     {
-        char* callstack = Backtrace{}.desc();
-        auto msg = format("Received {}, exiting.\nCallstack:\n{}", text, callstack);
-        free(callstack);
+        auto msg = format("Received {}, exiting.\nCallstack:\n{}", text, Backtrace{}.desc());
         write_stderr(msg);
         notify_fatal_error(msg);
     }
diff --git a/src/string.hh b/src/string.hh
index b5ad8e5a9..2108c64eb 100644
--- a/src/string.hh
+++ b/src/string.hh
@@ -164,13 +164,13 @@ public:
     };
     ZeroTerminatedString zstr() const { return {begin(), end()}; }
 
-private:
     [[gnu::optimize(3)]] // this is recursive for constexpr reason
     static constexpr ByteCount strlen(const char* s)
     {
         return *s == 0 ? 0 : strlen(s+1) + 1;
     }
 
+private:
     const char* m_data = nullptr;
     ByteCount m_length = 0;
 };