Add support for env var name completion

Fixes #659

src/command_manager.cc

@@ -582,6 +582,11 @@ Completions CommandManager::complete(const Context& context,
         return offset_pos(shell_complete(context, flags, tokens[tok_idx].content(),
                                          cursor_pos_in_token), start);
 
+    case Token::Type::ValExpand:
+        return {start , cursor_pos,
+                ShellManager::instance().complete_env_var(
+                    tokens[tok_idx].content(), cursor_pos_in_token) };
+
     case Token::Type::Raw:
     case Token::Type::RawQuoted:
     case Token::Type::RawEval:

src/shell_manager.cc

@@ -234,4 +234,12 @@ String ShellManager::get_val(StringView name, const Context& context) const
     return env_var->func(name, context);
 }
 
+CandidateList ShellManager::complete_env_var(StringView prefix,
+                                             ByteCount cursor_pos) const
+{
+    return complete(prefix, cursor_pos, m_env_vars |
+                    transform([](const EnvVarDesc& desc) -> const String&
+                              { return desc.str; }));
+}
+
 }

src/shell_manager.hh

@@ -6,6 +6,7 @@
 #include "flags.hh"
 #include "string.hh"
 #include "utils.hh"
+#include "completion.hh"
 
 namespace Kakoune
 {
@@ -39,6 +40,8 @@ public:
     void register_env_var(StringView str, bool prefix, EnvVarRetriever retriever);
     String get_val(StringView name, const Context& context) const;
 
+    CandidateList complete_env_var(StringView prefix, ByteCount cursor_pos) const;
+
 private:
     struct EnvVarDesc { String str; bool prefix; EnvVarRetriever func; };
     Vector<EnvVarDesc> m_env_vars;