diff --git a/src/main.cc b/src/main.cc
index 94a9ce8a..e8cab32a 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -64,7 +64,7 @@ void register_env_vars()
                                       { return b->display_name(); }), ':'); }
         }, {
             "timestamp",
-            [](StringView name, const Context& context)
+            [](StringView name, const Context& context) -> String
             { return to_string(context.buffer().timestamp()); }
         }, {
             "selection",
@@ -101,15 +101,15 @@ void register_env_vars()
             { return context.name(); }
         }, {
             "cursor_line",
-            [](StringView name, const Context& context)
+            [](StringView name, const Context& context) -> String
             { return to_string(context.selections().main().cursor().line + 1); }
         }, {
             "cursor_column",
-            [](StringView name, const Context& context)
+            [](StringView name, const Context& context) -> String
             { return to_string(context.selections().main().cursor().column + 1); }
         }, {
             "cursor_char_column",
-            [](StringView name, const Context& context)
+            [](StringView name, const Context& context) -> String
             { auto coord = context.selections().main().cursor();
               return to_string(context.buffer()[coord.line].char_count_to(coord.column) + 1); }
         }, {
@@ -129,11 +129,11 @@ void register_env_vars()
                 }), ':'); }
         }, {
             "window_width",
-            [](StringView name, const Context& context)
+            [](StringView name, const Context& context) -> String
             { return to_string(context.window().dimensions().column); }
         }, {
             "window_height",
-            [](StringView name, const Context& context)
+            [](StringView name, const Context& context) -> String
             { return to_string(context.window().dimensions().line); }
     } };
 
diff --git a/src/string.cc b/src/string.cc
index a9840d45..02e79f3f 100644
--- a/src/string.cc
+++ b/src/string.cc
@@ -115,25 +115,25 @@ int str_to_int(StringView str)
     return negative ? -(int)res : (int)res;
 }
 
-String to_string(int val)
+InplaceString<16> to_string(int val)
 {
-    char buf[16];
-    sprintf(buf, "%i", val);
-    return buf;
+    InplaceString<16> res;
+    res.m_length = sprintf(res.m_data, "%i", val);
+    return res;
 }
 
-String to_string(size_t val)
+InplaceString<24> to_string(size_t val)
 {
-    char buf[16];
-    sprintf(buf, "%zu", val);
-    return buf;
+    InplaceString<24> res;
+    res.m_length = sprintf(res.m_data, "%zu", val);
+    return res;
 }
 
-String to_string(float val)
+InplaceString<24> to_string(float val)
 {
-    char buf[32];
-    sprintf(buf, "%f", val);
-    return buf;
+    InplaceString<24> res;
+    res.m_length = sprintf(res.m_data, "%f", val);
+    return res;
 }
 
 bool subsequence_match(StringView str, StringView subseq)
diff --git a/src/string.hh b/src/string.hh
index 41dbccb8..6b66bce4 100644
--- a/src/string.hh
+++ b/src/string.hh
@@ -92,6 +92,7 @@ public:
 
     String() {}
     String(const char* content) : m_data(content) {}
+    String(const char* content, ByteCount len) : m_data(content, (size_t)(int)len) {}
     explicit String(char content, CharCount count = 1) : m_data((size_t)(int)count, content) {}
     explicit String(Codepoint cp, CharCount count = 1)
     {
@@ -256,12 +257,23 @@ inline String codepoint_to_str(Codepoint cp)
 
 int str_to_int(StringView str);
 
-String to_string(int val);
-String to_string(size_t val);
-String to_string(float val);
+template<size_t N>
+struct InplaceString
+{
+    constexpr operator StringView() const { return {m_data, m_length}; }
+    operator String() const { return {m_data, m_length}; }
+
+    ByteCount m_length;
+    char m_data[N];
+};
+
+InplaceString<16> to_string(int val);
+InplaceString<24> to_string(size_t val);
+InplaceString<24> to_string(float val);
 
 template<typename RealType, typename ValueType>
-String to_string(const StronglyTypedNumber<RealType, ValueType>& val)
+decltype(to_string(std::declval<ValueType>()))
+to_string(const StronglyTypedNumber<RealType, ValueType>& val)
 {
     return to_string((ValueType)val);
 }
@@ -283,7 +295,7 @@ namespace detail
 template<typename T> using IsString = std::is_convertible<T, StringView>;
 
 template<typename T, class = typename std::enable_if<!IsString<T>::value>::type>
-String format_param(const T& val) { return to_string(val); }
+auto format_param(const T& val) -> decltype(to_string(val)) { return to_string(val); }
 
 template<typename T, class = typename std::enable_if<IsString<T>::value>::type>
 StringView format_param(const T& val) { return val; }
@@ -295,7 +307,7 @@ String format(StringView fmt, ArrayView<const StringView> params);
 template<typename... Types>
 String format(StringView fmt, Types... params)
 {
-    return format(fmt, ArrayView<const StringView>{{detail::format_param(params)...}});
+    return format(fmt, ArrayView<const StringView>{detail::format_param(params)...});
 }
 
 }