diff --git a/src/buffer.cc b/src/buffer.cc
index 60dd5ba9..6088c8a8 100644
--- a/src/buffer.cc
+++ b/src/buffer.cc
@@ -195,12 +195,12 @@ BufferCoord Buffer::clamp(BufferCoord coord) const
     return coord;
 }
 
-BufferCoord Buffer::offset_coord(BufferCoord coord, CharCount offset, ColumnCount, bool)
+BufferCoord Buffer::offset_coord(BufferCoord coord, CharCount offset, ColumnCount, bool) const
 {
     return utf8::advance(iterator_at(coord), offset < 0 ? begin() : end()-1, offset).coord();
 }
 
-BufferCoordAndTarget Buffer::offset_coord(BufferCoordAndTarget coord, LineCount offset, ColumnCount tabstop, bool avoid_eol)
+BufferCoordAndTarget Buffer::offset_coord(BufferCoordAndTarget coord, LineCount offset, ColumnCount tabstop, bool avoid_eol) const
 {
     const auto column = coord.target == -1 ? get_column(*this, tabstop, coord) : coord.target;
     const auto line = Kakoune::clamp(coord.line + offset, 0_line, line_count()-1);
diff --git a/src/buffer.hh b/src/buffer.hh
index ab9d3248..a586e828 100644
--- a/src/buffer.hh
+++ b/src/buffer.hh
@@ -188,8 +188,8 @@ public:
     // returns nearest valid coordinates from given ones
     BufferCoord clamp(BufferCoord coord) const;
 
-    BufferCoord offset_coord(BufferCoord coord, CharCount offset, ColumnCount, bool);
-    BufferCoordAndTarget offset_coord(BufferCoordAndTarget coord, LineCount offset, ColumnCount tabstop, bool avoid_eol);
+    BufferCoord offset_coord(BufferCoord coord, CharCount offset, ColumnCount, bool) const;
+    BufferCoordAndTarget offset_coord(BufferCoordAndTarget coord, LineCount offset, ColumnCount tabstop, bool avoid_eol) const;
 
     const String& name() const { return m_name; }
     const String& display_name() const { return m_display_name; }
diff --git a/src/client.cc b/src/client.cc
index c2fa6f69..dff40bb7 100644
--- a/src/client.cc
+++ b/src/client.cc
@@ -212,7 +212,7 @@ void Client::redraw_ifn()
     if (m_ui_pending == 0)
         return;
 
-    auto& faces = context().faces();
+    const auto& faces = context().faces();
 
     if (m_ui_pending & Draw)
         m_ui->draw(window.update_display_buffer(context()),
diff --git a/src/client.hh b/src/client.hh
index b7351626..e9da7588 100644
--- a/src/client.hh
+++ b/src/client.hh
@@ -49,7 +49,7 @@ public:
     void info_hide(bool even_modal = false);
 
     void print_status(DisplayLine status_line);
-    const DisplayLine& current_status() { return m_status_line; }
+    const DisplayLine& current_status() const { return m_status_line; }
 
     DisplayCoord dimensions() const;
 
diff --git a/src/command_manager.cc b/src/command_manager.cc
index 294d3eab..099ee244 100644
--- a/src/command_manager.cc
+++ b/src/command_manager.cc
@@ -284,12 +284,12 @@ Token parse_percent_token(Reader& reader, bool throw_on_unterminated)
     }
 }
 
-auto expand_option(Option& opt, std::true_type)
+auto expand_option(const Option& opt, std::true_type)
 {
     return opt.get_as_string(Quoting::Raw);
 }
 
-auto expand_option(Option& opt, std::false_type)
+auto expand_option(const Option& opt, std::false_type)
 {
     return opt.get_as_strings();
 }
diff --git a/src/commands.cc b/src/commands.cc
index cb4b203b..e8694a92 100644
--- a/src/commands.cc
+++ b/src/commands.cc
@@ -278,7 +278,7 @@ struct ShellCandidatesCompleter
         CandidateList res;
         // Gather best max_count matches
         for_n_best(matches, max_count, [](auto& lhs, auto& rhs) { return rhs < lhs; },
-                   [&] (RankedMatch& m) {
+                   [&] (const RankedMatch& m) {
             if (not res.empty() and res.back() == m.candidate())
                 return false;
             res.push_back(m.candidate().str());
@@ -519,7 +519,7 @@ const CommandDesc force_write_cmd = {
     write_buffer<true>,
 };
 
-void write_all_buffers(Context& context, bool sync = false)
+void write_all_buffers(const Context& context, bool sync = false)
 {
     // Copy buffer list because hooks might be creating/deleting buffers
     Vector<SafePtr<Buffer>> buffers;
@@ -1105,7 +1105,7 @@ void define_command(const ParametersParser& parser, Context& context, const Shel
                        size_t token_to_complete, ByteCount pos_in_token)
         {
              const String& prefix = params[token_to_complete];
-             auto& ignored_files = context.options()["ignored_files"].get<Regex>();
+             const auto& ignored_files = context.options()["ignored_files"].get<Regex>();
              return Completions{0_byte, pos_in_token,
                                 complete_filename(prefix, ignored_files,
                                                   pos_in_token, FilenameFlags::Expand),
@@ -1739,7 +1739,7 @@ void context_wrap(const ParametersParser& parser, Context& context, StringView d
         (int)(bool)parser.get_switch("try-client") > 1)
         throw runtime_error{"only one of -buffer, -client or -try-client can be specified"};
 
-    auto& register_manager = RegisterManager::instance();
+    const auto& register_manager = RegisterManager::instance();
     auto make_register_restorer = [&](char c) {
         return on_scope_end([&, c, save=register_manager[c].save(context)] {
             try
diff --git a/src/highlighters.cc b/src/highlighters.cc
index d8a95832..000befef 100644
--- a/src/highlighters.cc
+++ b/src/highlighters.cc
@@ -899,9 +899,9 @@ struct WrapHighlighter : Highlighter
         }
 
         return pos;
-    };
+    }
 
-    ColumnCount line_indent(const Buffer& buffer, int tabstop, LineCount line) const
+    static ColumnCount line_indent(const Buffer& buffer, int tabstop, LineCount line)
     {
         StringView l = buffer[line];
         auto col = 0_byte;
@@ -947,7 +947,7 @@ struct TabulationHighlighter : Highlighter
     void do_highlight(HighlightContext context, DisplayBuffer& display_buffer, BufferRange) override
     {
         const ColumnCount tabstop = context.context.options()["tabstop"].get<int>();
-        auto& buffer = context.context.buffer();
+        const auto& buffer = context.context.buffer();
         auto win_column = context.setup.window_pos.column;
         for (auto& line : display_buffer.lines())
         {
@@ -1029,11 +1029,11 @@ struct ShowWhitespacesHighlighter : Highlighter
     }
 
 private:
-    void do_highlight(HighlightContext context, DisplayBuffer& display_buffer, BufferRange)
+    void do_highlight(HighlightContext context, DisplayBuffer& display_buffer, BufferRange) override
     {
         const int tabstop = context.context.options()["tabstop"].get<int>();
         auto whitespaceface = context.context.faces()["Whitespace"];
-        auto& buffer = context.context.buffer();
+        const auto& buffer = context.context.buffer();
         auto win_column = context.setup.window_pos.column;
         for (auto& line : display_buffer.lines())
         {
@@ -1112,7 +1112,7 @@ private:
         if (contains(context.disabled_ids, ms_id))
             return;
 
-        auto& faces = context.context.faces();
+        const auto& faces = context.context.faces();
         const Face face = faces["LineNumbers"];
         const Face face_wrapped = faces["LineNumbersWrapped"];
         const Face face_absolute = faces["LineNumberCursor"];
@@ -1153,7 +1153,7 @@ private:
         unique_ids.push_back(ms_id);
     }
 
-    int compute_digit_count(const Context& context) const
+    static int compute_digit_count(const Context& context)
     {
         int digit_count = 0;
         LineCount last_line = context.buffer().line_count();
@@ -1268,7 +1268,7 @@ void highlight_selections(HighlightContext context, DisplayBuffer& display_buffe
 
 void expand_unprintable(HighlightContext context, DisplayBuffer& display_buffer, BufferRange)
 {
-    auto& buffer = context.context.buffer();
+    const auto& buffer = context.context.buffer();
     auto error = context.context.faces()["Error"];
     for (auto& line : display_buffer.lines())
     {
@@ -1367,7 +1367,7 @@ private:
     void do_highlight(HighlightContext context, DisplayBuffer& display_buffer, BufferRange) override
     {
         auto& line_flags = context.context.options()[m_option_name].get_mutable<LineAndSpecList>();
-        auto& buffer = context.context.buffer();
+        const auto& buffer = context.context.buffer();
         update_line_specs_ifn(buffer, line_flags);
 
         auto def_face = context.context.faces()[m_default_face];
@@ -1417,7 +1417,7 @@ private:
     void do_compute_display_setup(HighlightContext context, DisplaySetup& setup) const override
     {
         auto& line_flags = context.context.options()[m_option_name].get_mutable<LineAndSpecList>();
-        auto& buffer = context.context.buffer();
+        const auto& buffer = context.context.buffer();
         update_line_specs_ifn(buffer, line_flags);
 
         ColumnCount width = 0;
diff --git a/src/insert_completer.cc b/src/insert_completer.cc
index abca58ab..0bb6186d 100644
--- a/src/insert_completer.cc
+++ b/src/insert_completer.cc
@@ -545,7 +545,7 @@ void InsertCompleter::on_option_changed(const Option& opt)
         m_current_candidate != m_completions.candidates.size() - 1)
         return;
 
-    auto& completers = m_options["completers"].get<InsertCompleterDescList>();
+    const auto& completers = m_options["completers"].get<InsertCompleterDescList>();
     for (auto& completer : completers)
     {
         if (completer.mode == InsertCompleterDesc::Option and
diff --git a/src/json_ui.cc b/src/json_ui.cc
index f4f39b84..51fde617 100644
--- a/src/json_ui.cc
+++ b/src/json_ui.cc
@@ -19,7 +19,7 @@ namespace Kakoune
 {
 
 struct invalid_rpc_request : runtime_error {
-    invalid_rpc_request(String message)
+    invalid_rpc_request(const String& message)
         : runtime_error(format("invalid json rpc request ({})", message)) {}
 };
 
diff --git a/src/ncurses_ui.cc b/src/ncurses_ui.cc
index 15417ff2..d5191eb8 100644
--- a/src/ncurses_ui.cc
+++ b/src/ncurses_ui.cc
@@ -638,7 +638,7 @@ Optional<Key> NCursesUI::get_next_key()
         }
         for (int count = 0; count < 16 and c >= 0x30 && c <= 0x3f; c = next_char())
         {
-            if ('0' <= 'c' and c <= '9')
+            if (isdigit(c))
                 params[count] = params[count] * 10 + c - '0';
             else if (c == ';')
                 ++count;
diff --git a/src/normal.cc b/src/normal.cc
index 3a7565e2..66957a2e 100644
--- a/src/normal.cc
+++ b/src/normal.cc
@@ -166,7 +166,7 @@ void repeat_last_select(Context& context, NormalParams)
     context.repeat_last_select();
 }
 
-String build_autoinfo_for_mapping(Context& context, KeymapMode mode,
+String build_autoinfo_for_mapping(const Context& context, KeymapMode mode,
                                   ConstArrayView<KeyInfo> built_ins)
 {
     auto& keymaps = context.keymaps();
@@ -445,7 +445,7 @@ void for_each_codepoint(Context& context, NormalParams)
     selections.insert(strings, InsertMode::Replace);
 }
 
-void command(Context& context, EnvVarMap env_vars)
+void command(const Context& context, EnvVarMap env_vars)
 {
     if (not CommandManager::has_instance())
         throw runtime_error{"commands are not supported"};
@@ -1190,7 +1190,7 @@ void deindent(Context& context, NormalParams params)
         indent_width = tabstop;
     indent_width = indent_width * count;
 
-    auto& buffer = context.buffer();
+    const auto& buffer = context.buffer();
     Vector<Selection> sels;
     LineCount last_line = 0;
     for (auto& sel : context.selections())
@@ -1378,7 +1378,7 @@ enum Direction { Backward = -1, Forward = 1 };
 template<Direction direction, bool half = false>
 void scroll(Context& context, NormalParams params)
 {
-    Window& window = context.window();
+    const Window& window = context.window();
     const int count = params.count ? params.count : 1;
     const LineCount offset = (window.dimensions().line - 2) / (half ? 2 : 1) * count;
 
diff --git a/src/normal.hh b/src/normal.hh
index e0eb8477..ad0f81be 100644
--- a/src/normal.hh
+++ b/src/normal.hh
@@ -37,7 +37,7 @@ struct KeyInfo
     StringView docstring;
 };
 
-String build_autoinfo_for_mapping(Context& context, KeymapMode mode,
+String build_autoinfo_for_mapping(const Context& context, KeymapMode mode,
                                   ConstArrayView<KeyInfo> built_ins);
 
 }
diff --git a/src/optional.hh b/src/optional.hh
index e6ff2b83..e3e0c7e9 100644
--- a/src/optional.hh
+++ b/src/optional.hh
@@ -94,7 +94,7 @@ public:
     }
 
     template<typename U>
-    auto cast() -> Optional<U>
+    auto cast() const -> Optional<U>
     {
         if (not m_valid)
             return {};
diff --git a/src/ranges.hh b/src/ranges.hh
index 014dbd8b..0471dd41 100644
--- a/src/ranges.hh
+++ b/src/ranges.hh
@@ -356,7 +356,7 @@ struct ConcatView
         RangeIt2 m_it2;
     };
 
-    ConcatView(Range1& range1, Range2& range2)
+    ConcatView(const Range1& range1, const Range2& range2)
         : m_range1(range1), m_range2(range2) {}
 
     Iterator begin() const { return {m_range1.begin(), m_range1.end(), m_range2.begin()}; }
diff --git a/src/regex_impl.cc b/src/regex_impl.cc
index 4ad80c29..3d6d4455 100644
--- a/src/regex_impl.cc
+++ b/src/regex_impl.cc
@@ -161,7 +161,7 @@ struct RegexParser
 private:
     struct InvalidPolicy
     {
-        Codepoint operator()(Codepoint cp) { throw regex_error{"Invalid utf8 in regex"}; }
+        Codepoint operator()(Codepoint cp) const { throw regex_error{"Invalid utf8 in regex"}; }
     };
 
     enum class Flags
@@ -425,7 +425,7 @@ private:
         parse_error(format("unknown atom escape '{}'", cp));
     }
 
-    void normalize_ranges(Vector<CharacterClass::Range, MemoryDomain::Regex>& ranges)
+    static void normalize_ranges(Vector<CharacterClass::Range, MemoryDomain::Regex>& ranges)
     {
         if (ranges.empty())
             return;
@@ -1140,7 +1140,7 @@ String dump_regex(const CompiledRegex& program)
                 res += "match\n";
         }
     }
-    auto dump_start_desc = [&](CompiledRegex::StartDesc& desc, StringView name) {
+    auto dump_start_desc = [&](const CompiledRegex::StartDesc& desc, StringView name) {
         res += name + " start desc: [";
         for (size_t c = 0; c < CompiledRegex::StartDesc::count; ++c)
         {
diff --git a/src/regex_impl.hh b/src/regex_impl.hh
index ee374429..e49c5186 100644
--- a/src/regex_impl.hh
+++ b/src/regex_impl.hh
@@ -521,7 +521,7 @@ private:
         }
     }
 
-    void to_next_start(Iterator& start, const ExecConfig& config, const StartDesc& start_desc)
+    static void to_next_start(Iterator& start, const ExecConfig& config, const StartDesc& start_desc)
     {
         while (start != config.end)
         {
@@ -540,7 +540,7 @@ private:
     {
         using Lookaround = CompiledRegex::Lookaround;
 
-        if (not look_forward) 
+        if (not look_forward)
         {
             if (pos == config.subject_begin)
                 return m_program.lookarounds[index] == Lookaround::EndOfLookaround;
diff --git a/src/scope.hh b/src/scope.hh
index e29a3c94..870e1159 100644
--- a/src/scope.hh
+++ b/src/scope.hh
@@ -57,7 +57,7 @@ class GlobalScope : public Scope, public OptionManagerWatcher, public Singleton<
         OptionsRegistry& option_registry() { return m_option_registry; }
         const OptionsRegistry& option_registry() const { return m_option_registry; }
     private:
-        void on_option_changed(const Option& option);
+        void on_option_changed(const Option& option) override;
 
         OptionsRegistry m_option_registry;
 };
diff --git a/src/selection.cc b/src/selection.cc
index e0dbbecb..a5b2c7b1 100644
--- a/src/selection.cc
+++ b/src/selection.cc
@@ -45,7 +45,7 @@ void SelectionList::set(Vector<Selection> list, size_t main)
 
 bool compare_selections(const Selection& lhs, const Selection& rhs)
 {
-    const auto lmin = lhs.min(), rmin = rhs.min();
+    const auto& lmin = lhs.min(), rmin = rhs.min();
     return lmin == rmin ? lhs.max() < rhs.max() : lmin < rmin;
 }
 
@@ -111,7 +111,7 @@ Iterator merge_overlapping(Iterator begin, Iterator end, size_t& main, OverlapsF
 BufferCoord& get_first(Selection& sel) { return sel.min(); }
 BufferCoord& get_last(Selection& sel) { return sel.max(); }
 
-Vector<Selection> compute_modified_ranges(Buffer& buffer, size_t timestamp)
+Vector<Selection> compute_modified_ranges(const Buffer& buffer, size_t timestamp)
 {
     Vector<Selection> ranges;
     auto changes = buffer.changes_since(timestamp);
@@ -211,7 +211,7 @@ void clamp_selections(Vector<Selection>& selections, const Buffer& buffer)
         clamp(sel, buffer);
 }
 
-void update_selections(Vector<Selection>& selections, size_t& main, Buffer& buffer, size_t timestamp, bool merge)
+void update_selections(Vector<Selection>& selections, size_t& main, const Buffer& buffer, size_t timestamp, bool merge)
 {
     if (timestamp == buffer.timestamp())
         return;
@@ -474,8 +474,8 @@ void SelectionList::erase()
 
 String selection_to_string(const Selection& selection)
 {
-    auto& cursor = selection.cursor();
-    auto& anchor = selection.anchor();
+    const auto& cursor = selection.cursor();
+    const auto& anchor = selection.anchor();
     return format("{}.{},{}.{}", anchor.line + 1, anchor.column + 1,
                   cursor.line + 1, cursor.column + 1);
 }
diff --git a/src/selection.hh b/src/selection.hh
index 34532f9b..5c47f878 100644
--- a/src/selection.hh
+++ b/src/selection.hh
@@ -65,7 +65,7 @@ inline bool overlaps(const Selection& lhs, const Selection& rhs)
 }
 
 void update_selections(Vector<Selection>& selections, size_t& main,
-                       Buffer& buffer, size_t timestamp, bool merge = true);
+                       const Buffer& buffer, size_t timestamp, bool merge = true);
 
 bool compare_selections(const Selection& lhs, const Selection& rhs);
 void sort_selections(Vector<Selection>& selections, size_t& main);
@@ -155,7 +155,7 @@ private:
     size_t m_timestamp;
 };
 
-Vector<Selection> compute_modified_ranges(Buffer& buffer, size_t timestamp);
+Vector<Selection> compute_modified_ranges(const Buffer& buffer, size_t timestamp);
 
 String selection_to_string(const Selection& selection);
 String selection_list_to_string(const SelectionList& selection);
diff --git a/src/string_utils.cc b/src/string_utils.cc
index a87d4cc3..43e7ef18 100644
--- a/src/string_utils.cc
+++ b/src/string_utils.cc
@@ -9,16 +9,6 @@
 namespace Kakoune
 {
 
-StringView trim_whitespaces(StringView str)
-{
-    auto beg = str.begin(), end = str.end();
-    while (beg != end and is_blank(*beg))
-        ++beg;
-    while (beg != end and is_blank(*(end-1)))
-        --end;
-    return {beg, end};
-}
-
 String trim_indent(StringView str)
 {
     if (str.empty())
diff --git a/src/string_utils.hh b/src/string_utils.hh
index 52dd719c..49f4baeb 100644
--- a/src/string_utils.hh
+++ b/src/string_utils.hh
@@ -9,7 +9,6 @@
 namespace Kakoune
 {
 
-StringView trim_whitespaces(StringView str);
 String trim_indent(StringView str);
 
 String escape(StringView str, StringView characters, char escape);
diff --git a/src/window.cc b/src/window.cc
index 29ac73de..0ef445ca 100644
--- a/src/window.cc
+++ b/src/window.cc
@@ -280,7 +280,7 @@ ColumnCount find_display_column(const DisplayLine& line, const Buffer& buffer,
 BufferCoord find_buffer_coord(const DisplayLine& line, const Buffer& buffer,
                               ColumnCount column)
 {
-    auto& range = line.range();
+    const auto& range = line.range();
     for (auto& atom : line)
     {
         ColumnCount len = atom.length();