Support exposing some env vars as part of the mode information

This should implement what #5131 proposed in a different way.

Closes #5131

src/client.cc

@@ -165,11 +165,12 @@ DisplayLine Client::generate_mode_line() const
     DisplayLine modeline;
     try
     {
+        ModeInfo mode_info = context().client().input_handler().mode_info();
         const String& modelinefmt = context().options()["modelinefmt"].get<String>();
-        HashMap<String, DisplayLine> atoms{{ "mode_info", context().client().input_handler().mode_line() },
+        HashMap<String, DisplayLine> atoms{{ "mode_info",  mode_info.display_line},
                                            { "context_info", {generate_context_info(context()),
                                                               context().faces()["Information"]}}};
-        auto expanded = expand(modelinefmt, context(), ShellContext{},
+        auto expanded = expand(modelinefmt, context(), {{}, mode_info.env_vars},
                                [](String s) { return escape(s, '{', '\\'); });
         modeline = parse_display_line(expanded, context().faces(), atoms);
     }

src/input_handler.cc

@@ -42,7 +42,7 @@ public:
     bool enabled() const { return &m_input_handler.current_mode() == this; }
     Context& context() const { return m_input_handler.context(); }
 
-    virtual DisplayLine mode_line() const = 0;
+    virtual ModeInfo mode_info() const = 0;
 
     virtual KeymapMode keymap_mode() const = 0;
 
@@ -370,7 +370,7 @@ public:
             m_idle_timer.set_next_date(Clock::now() + get_idle_timeout(context()));
     }
 
-    DisplayLine mode_line() const override
+    ModeInfo mode_info() const override
     {
         AtomList atoms;
         auto num_sel = context().selections().size();
@@ -390,7 +390,7 @@ public:
             atoms.emplace_back(" reg=", context().faces()["StatusLineInfo"]);
             atoms.emplace_back(StringView(m_params.reg).str(), context().faces()["StatusLineValue"]);
         }
-        return atoms;
+        return {atoms, {{"count", to_string(m_params.count)}, {"register", StringView(m_params.reg).str()}}};
     }
 
     KeymapMode keymap_mode() const override { return KeymapMode::Normal; }
@@ -951,9 +951,9 @@ public:
         }
     }
 
-    DisplayLine mode_line() const override
+    ModeInfo mode_info() const override
     {
-        return { "prompt", context().faces()["StatusLineMode"] };
+        return {{ "prompt", context().faces()["StatusLineMode"] }, {}};
     }
 
     KeymapMode keymap_mode() const override { return KeymapMode::Prompt; }
@@ -1126,9 +1126,9 @@ public:
         m_callback(key, context());
     }
 
-    DisplayLine mode_line() const override
+    ModeInfo mode_info() const override
     {
-        return { "enter key", context().faces()["StatusLineMode"] };
+        return {{ "enter key", context().faces()["StatusLineMode"] }, {}};
     }
 
     KeymapMode keymap_mode() const override { return m_keymap_mode; }
@@ -1376,15 +1376,16 @@ public:
         m_idle_timer.set_next_date(Clock::now() + get_idle_timeout(context()));
     }
 
-    DisplayLine mode_line() const override
+    ModeInfo mode_info() const override
     {
         auto num_sel = context().selections().size();
         auto main_index = context().selections().main_index();
-        return {AtomList{ { "insert", context().faces()["StatusLineMode"] },
-                          { " ", context().faces()["StatusLine"] },
-                          { num_sel == 1 ? format("{} sel", num_sel)
-                              : format("{} sels ({})", num_sel, main_index + 1),
-                            context().faces()["StatusLineInfo"] } }};
+        return {{AtomList{ { "insert", context().faces()["StatusLineMode"] },
+                           { " ", context().faces()["StatusLine"] },
+                           { num_sel == 1 ? format("{} sel", num_sel)
+                               : format("{} sels ({})", num_sel, main_index + 1),
+                             context().faces()["StatusLineInfo"] } }},
+                {}};
     }
 
     KeymapMode keymap_mode() const override { return KeymapMode::Insert; }
@@ -1707,9 +1708,9 @@ void InputHandler::stop_recording()
     m_recording_level = -1;
 }
 
-DisplayLine InputHandler::mode_line() const
+ModeInfo InputHandler::mode_info() const
 {
-    return current_mode().mode_line();
+    return current_mode().mode_info();
 }
 
 std::pair<CursorMode, DisplayCoord> InputHandler::get_cursor_info() const

src/input_handler.hh

@@ -4,6 +4,7 @@
 #include "completion.hh"
 #include "constexpr_utils.hh"
 #include "context.hh"
+#include "env_vars.hh"
 #include "face.hh"
 #include "normal.hh"
 #include "keys.hh"
@@ -52,6 +53,12 @@ enum class InsertMode : unsigned
     OpenLineAbove
 };
 
+struct ModeInfo
+{
+    DisplayLine display_line;
+    EnvVarMap env_vars;
+};
+
 class InputHandler : public SafeCountable
 {
 public:
@@ -97,7 +104,7 @@ public:
     Context& context() { return m_context; }
     const Context& context() const { return m_context; }
 
-    DisplayLine mode_line() const;
+    ModeInfo mode_info() const;
 
     std::pair<CursorMode, DisplayCoord> get_cursor_info() const;