diff --git a/lib/message/mu-fields.cc b/lib/message/mu-fields.cc
index 7f53b6a7..d6f7bc0b 100644
--- a/lib/message/mu-fields.cc
+++ b/lib/message/mu-fields.cc
@@ -83,14 +83,16 @@ Field::xapian_term(const std::string& s) const
 	if (s.empty())
 		return res;
 
-	res.reserve(s.size() + 10);
-	/* slightly optimized common pure-ascii. */
-	if (G_LIKELY(g_str_is_ascii(s.c_str()))) {
-		res += s;
-		for (auto i = 1U; i != res.length(); ++i)
-			res[i] = g_ascii_tolower(res[i]);
-	} else
-		res += utf8_flatten(s);
+	res.reserve(s.size() + 1);
+	/* optimized common pure-ascii case */
+	for (auto&& c: s) {
+		if (G_UNLIKELY(!is_ascii(c))) { /* non-ascii after all */
+			res.erase(1U);
+			res += utf8_flatten(s);
+			break;
+		}
+		res.push_back(to_ascii_lower(c));
+	}
 
 	if (G_UNLIKELY(res.size() > MaxTermLength))
 		res.erase(MaxTermLength);
diff --git a/lib/message/mu-flags.hh b/lib/message/mu-flags.hh
index ee01702c..6c528b6c 100644
--- a/lib/message/mu-flags.hh
+++ b/lib/message/mu-flags.hh
@@ -105,8 +105,7 @@ struct MessageFlagInfo {
 	 * @return lower-case shortcut
 	 */
 	constexpr char shortcut_lower() const {
-		return shortcut >= 'A' && shortcut <= 'Z' ?
-			shortcut +  ('a' - 'A') : shortcut;
+		return to_ascii_lower(shortcut);
 	}
 };
 
diff --git a/lib/message/mu-labels.cc b/lib/message/mu-labels.cc
index 9b90abae..f10f1dbf 100644
--- a/lib/message/mu-labels.cc
+++ b/lib/message/mu-labels.cc
@@ -50,11 +50,11 @@ Mu::Labels::validate_label(const std::string &label)
 		if (g_unichar_isalnum(uc))
 			continue; // alphanum is okay
 
-		if (::iscntrl(uc))
+		if (is_ascii_cntrl(uc))
 			return Err(Error{Error::Code::InvalidArgument,
 					"control character {} not allowed in label",
 					static_cast<int>(uc)});
-		if (::isblank(uc))
+		if (is_ascii_blank(uc))
 			return Err(Error{Error::Code::InvalidArgument,
 					"blank character {} not allowed in label",
 					static_cast<int>(uc)});
diff --git a/lib/message/mu-message-part.cc b/lib/message/mu-message-part.cc
index 3ed0b9cb..4134fb5e 100644
--- a/lib/message/mu-message-part.cc
+++ b/lib/message/mu-message-part.cc
@@ -49,7 +49,8 @@ cook(const std::string& fname, const std::vector<char>& forbidden)
 	clean.reserve(fname.length());
 
 	for (auto& c: basename(fname))
-		if (seq_some(forbidden,[&](char fc){return ::iscntrl(c) || c == fc;}))
+		if (seq_some(forbidden,[&](char fc){
+			return is_ascii_cntrl(c) || c == fc;}))
 			clean += '-';
 		else
 			clean += c;
diff --git a/lib/mu-query-processor.cc b/lib/mu-query-processor.cc
index ef576ad3..33d71400 100644
--- a/lib/mu-query-processor.cc
+++ b/lib/mu-query-processor.cc
@@ -437,7 +437,7 @@ process(const std::string& expr)
 	/* all control chars become SPC */
 	std::string str{expr};
 	for (auto& c: str)
-		c = ::iscntrl(c) ? ' ' : c;
+		c = is_ascii_cntrl(c) ? ' ' : c;
 
 	while(!str.empty()) {
 		auto&& element = next_element(str, offset)
diff --git a/lib/mu-server.cc b/lib/mu-server.cc
index 35d38b31..9af1d48d 100644
--- a/lib/mu-server.cc
+++ b/lib/mu-server.cc
@@ -111,8 +111,9 @@ struct OutputStream {
 	void unlink () {
 		if (fname_.empty())
 			return;
-		if (auto&&res{::unlink(fname_.c_str())}; res != 0)
-			mu_warning("failed to unlink '{}'", ::strerror(res));
+		if (::unlink(fname_.c_str()) != 0)
+			mu_warning("failed to unlink '{}': {}", fname_,
+				   ::strerror(errno));
 		else
 			mu_debug("unlinked output-stream {}", fname_);
 	}
diff --git a/lib/utils/mu-html-to-text.cc b/lib/utils/mu-html-to-text.cc
index 69ff232a..ad773a4b 100644
--- a/lib/utils/mu-html-to-text.cc
+++ b/lib/utils/mu-html-to-text.cc
@@ -36,7 +36,7 @@ starts_with(std::string_view haystack, std::string_view needle)
 		return false;
 
 	for (auto&& c = 0U; c != needle.size(); ++c)
-		if (::tolower(haystack[c]) != ::tolower(needle[c]))
+		if (to_ascii_lower(haystack[c]) != to_ascii_lower(needle[c]))
 			return false;
 
 	return true;
@@ -143,7 +143,7 @@ public:
 	std::string_view eat_head_word() {
 		size_t start_pos{pos_};
 		while (!done()) {
-			if (!::isalpha(html_.at(pos_)))
+			if (!is_ascii_alpha(html_.at(pos_)))
 				break;
 			++pos_;
 		}
@@ -440,7 +440,7 @@ html_escape_char(Context& ctx)
 	auto unescape=[escs](std::string_view esc)->char {
 		if (esc.empty())
 			return ' ';
-		auto first{static_cast<char>(::tolower(esc.at(0)))};
+		auto first{to_ascii_lower(esc.at(0))};
 		auto rest=esc.substr(1);
 		if (seq_some(escs, [&](auto&& e){return starts_with(rest, e);}))
 			return first;
diff --git a/mu/mu-cmd-cfind.cc b/mu/mu-cmd-cfind.cc
index 00727909..25c75fc9 100644
--- a/mu/mu-cmd-cfind.cc
+++ b/mu/mu-cmd-cfind.cc
@@ -58,7 +58,7 @@ guess_nick(const Contact& contact)
 	auto cleanup = [](const std::string& str) {
 		std::string clean;
 		for (auto& c: str) // XXX: support non-ascii
-			if (!::ispunct(c) && !::isspace(c))
+			if (!is_ascii_punct(c) && !is_ascii_space(c))
 				clean += c;
 		return clean;
 	};