diff --git a/src/file.cc b/src/file.cc
index a19d40bb..8331c596 100644
--- a/src/file.cc
+++ b/src/file.cc
@@ -383,7 +383,7 @@ std::vector<String> complete_command(StringView prefix, ByteCount cursor_pos)
             dirname += '/';
 
         struct stat st;
-        if (stat(dirname.substr(0_byte, dirname.length() - 1).c_str(), &st))
+        if (stat(dirname.substr(0_byte, dirname.length() - 1).zstr(), &st))
             continue;
 
         auto filter = [&](const dirent& entry) {
diff --git a/src/string.hh b/src/string.hh
index eb241e76..fa627b90 100644
--- a/src/string.hh
+++ b/src/string.hh
@@ -13,6 +13,8 @@ namespace Kakoune
 
 using Regex = boost::regex;
 
+class StringView;
+
 class String : public std::string
 {
 public:
@@ -47,16 +49,8 @@ public:
     String  operator+(Codepoint cp) const { String res = *this; utf8::dump(back_inserter(res), cp); return res; }
     String& operator+=(Codepoint cp) { utf8::dump(back_inserter(*this), cp); return *this; }
 
-    String substr(ByteCount pos, ByteCount length = -1) const
-    {
-        return String{std::string::substr((int)pos, (int)length)};
-    }
-    String substr(CharCount pos, CharCount length = INT_MAX) const
-    {
-        auto b = utf8::advance(begin(), end(), (int)pos);
-        auto e = utf8::advance(b, end(), (int)length);
-        return String(b,e);
-    }
+    StringView substr(ByteCount pos, ByteCount length = INT_MAX) const;
+    StringView substr(CharCount pos, CharCount length = INT_MAX) const;
 };
 
 class StringView
@@ -179,6 +173,16 @@ inline StringView StringView::substr(CharCount from, CharCount length) const
     return StringView{ beg, utf8::advance(beg, end(), length) };
 }
 
+inline StringView String::substr(ByteCount pos, ByteCount length) const
+{
+    return StringView{*this}.substr(pos, length);
+}
+
+inline StringView String::substr(CharCount pos, CharCount length) const
+{
+    return StringView{*this}.substr(pos, length);
+}
+
 inline const char* begin(StringView str)
 {
     return str.begin();
@@ -248,6 +252,13 @@ inline String operator+(Codepoint lhs, const String& rhs)
     return String(lhs) + rhs;
 }
 
+inline String operator+(StringView lhs, StringView rhs)
+{
+    String res{lhs.begin(), lhs.end()};
+    res += rhs;
+    return res;
+}
+
 std::vector<String> split(const String& str, char separator, char escape = 0);
 String escape(const String& str, char character, char escape);