Fix some miscellaneous warnings

src/builtin_string.cpp

@@ -907,18 +907,17 @@ class literal_replacer_t : public string_replacer_t {
 
 static wcstring interpret_escapes(const wcstring &arg) {
     wcstring result;
-
-    const wchar_t *orig = arg.c_str();
-    const wchar_t *start = arg.c_str();
-    while (orig - start < arg.size()) {
-        if (*orig == L'\\') {
-            orig += read_unquoted_escape(orig, &result, true, false);
+    result.reserve(arg.size());
+    const wchar_t *cursor = arg.c_str();
+    const wchar_t *end = cursor + arg.size();
+    while (cursor < end) {
+        if (*cursor == L'\\') {
+            cursor += read_unquoted_escape(cursor, &result, true, false);
         } else {
-            result += *orig;
-            orig++;
+            result.push_back(*cursor);
+            cursor++;
         }
     }
-
     return result;
 }
 
@@ -948,15 +947,15 @@ bool literal_replacer_t::replace_matches(const wcstring &arg) {
     } else {
         auto &cmp_func = opts.ignore_case ? wcsncasecmp : wcsncmp;
         const wchar_t *cur = arg.c_str();
-        const wchar_t *start = arg.c_str();
-        while (cur - start < arg.size()) {
+        const wchar_t *end = cur + arg.size();
+        while (cur < end) {
             if ((opts.all || !replacement_occurred) && cmp_func(cur, pattern.c_str(), patlen) == 0) {
                 result += replacement;
                 cur += patlen;
                 replacement_occurred = true;
                 total_replaced++;
             } else {
-                result += *cur;
+                result.push_back(*cur);
                 cur++;
             }
         }

src/history.cpp

@@ -299,7 +299,7 @@ class history_file_contents_t {
     static std::unique_ptr<history_file_contents_t> create(int fd) {
         // Check that the file is seekable, and its size.
         off_t len = lseek(fd, 0, SEEK_END);
-        if (len <= 0 || len >= SIZE_MAX) return nullptr;
+        if (len <= 0 || static_cast<unsigned long>(len) >= SIZE_MAX) return nullptr;
         if (lseek(fd, 0, SEEK_SET) != 0) return nullptr;
 
         // Read the file, possibly ussing mmap.

src/reader.cpp

@@ -278,8 +278,7 @@ class reader_history_search_t {
     bool add_skip(const wcstring &str) { return skips_.insert(str).second; }
 
     /// Reset, beginning a new line or token mode search.
-    void reset_to_mode(const wcstring &text, history_t *hist, mode_t mode,
-                       wcstring_list_t skip_list = {}) {
+    void reset_to_mode(const wcstring &text, history_t *hist, mode_t mode) {
         assert(mode != inactive && "mode cannot be inactive in this setter");
         skips_ = {text};
         matches_ = {text};