src: Complete register names in %reg{} expansions

Builtin registers have name aliases that can be completed upon when
using a `%reg{}` expansion from the prompt.

src/command_manager.cc

@@ -675,6 +675,11 @@ Completions CommandManager::complete(const Context& context,
 
     switch (token.type)
     {
+    case Token::Type::RegisterExpand:
+        return {start , cursor_pos,
+                RegisterManager::instance().complete_register_name(
+                    token.content, cursor_pos_in_token) };
+
     case Token::Type::OptionExpand:
         return {start , cursor_pos,
                 GlobalScope::instance().option_registry().complete_option_name(

src/register_manager.cc

@@ -7,11 +7,6 @@
 namespace Kakoune
 {
 
-Register& RegisterManager::operator[](StringView reg) const
-{
-    if (reg.length() == 1)
-        return (*this)[reg[0_byte]];
-
 static const HashMap<String, Codepoint> reg_names = {
     { "slash", '/' },
     { "dquote", '"' },
@@ -24,6 +19,12 @@ Register& RegisterManager::operator[](StringView reg) const
     { "underscore", '_' },
     { "colon", ':' }
 };
+
+Register& RegisterManager::operator[](StringView reg) const
+{
+    if (reg.length() == 1)
+        return (*this)[reg[0_byte]];
+
     auto it = reg_names.find(reg);
     if (it == reg_names.end())
         throw runtime_error(format("no such register: '{}'", reg));
@@ -47,4 +48,9 @@ void RegisterManager::add_register(Codepoint c, std::unique_ptr<Register> reg)
     reg_ptr = std::move(reg);
 }
 
+CandidateList RegisterManager::complete_register_name(StringView prefix, ByteCount cursor_pos) const
+{
+    return complete(prefix, cursor_pos, reg_names | transform([](auto& i) { return i.key; }) | gather<Vector<String>>());
+}
+
 }

src/register_manager.hh

@@ -2,6 +2,7 @@
 #define register_manager_hh_INCLUDED
 
 #include "array_view.hh"
+#include "completion.hh"
 #include "exception.hh"
 #include "utils.hh"
 #include "hash_map.hh"
@@ -146,6 +147,7 @@ public:
     Register& operator[](StringView reg) const;
     Register& operator[](Codepoint c) const;
     void add_register(Codepoint c, std::unique_ptr<Register> reg);
+    CandidateList complete_register_name(StringView prefix, ByteCount cursor_pos) const;
 
 protected:
     HashMap<Codepoint, std::unique_ptr<Register>, MemoryDomain::Registers> m_registers;