diff --git a/src/buffer_manager.cc b/src/buffer_manager.cc
index 88ed99a5..a7175d7e 100644
--- a/src/buffer_manager.cc
+++ b/src/buffer_manager.cc
@@ -95,10 +95,10 @@ void BufferManager::set_last_used_buffer(Buffer& buffer)
     m_buffers.emplace(m_buffers.begin(), &buffer);
 }
 
-CandidateList BufferManager::complete_buffer_name(const String& prefix,
+CandidateList BufferManager::complete_buffer_name(StringView prefix,
                                                   ByteCount cursor_pos)
 {
-    String real_prefix = prefix.substr(0, cursor_pos);
+    auto real_prefix = prefix.substr(0, cursor_pos);
     const bool include_dirs = contains(real_prefix, '/');
     CandidateList result;
     CandidateList subsequence_result;
diff --git a/src/buffer_manager.hh b/src/buffer_manager.hh
index 97124a2c..287ef672 100644
--- a/src/buffer_manager.hh
+++ b/src/buffer_manager.hh
@@ -33,7 +33,7 @@ public:
     Buffer& get_buffer(const String& name);
     void    set_last_used_buffer(Buffer& buffer);
 
-    CandidateList complete_buffer_name(const String& prefix,
+    CandidateList complete_buffer_name(StringView prefix,
                                        ByteCount cursor_pos = -1);
 
 private:
diff --git a/src/client_manager.cc b/src/client_manager.cc
index 12043456..6e3fd638 100644
--- a/src/client_manager.cc
+++ b/src/client_manager.cc
@@ -170,10 +170,10 @@ void ClientManager::redraw_clients() const
         client->redraw_ifn();
 }
 
-CandidateList ClientManager::complete_client_name(const String& prefix,
+CandidateList ClientManager::complete_client_name(StringView prefix,
                                                   ByteCount cursor_pos) const
 {
-    String real_prefix = prefix.substr(0, cursor_pos);
+    auto real_prefix = prefix.substr(0, cursor_pos);
     CandidateList result;
     CandidateList subsequence_result;
     for (auto& client : m_clients)
diff --git a/src/client_manager.hh b/src/client_manager.hh
index b25fe525..d3334486 100644
--- a/src/client_manager.hh
+++ b/src/client_manager.hh
@@ -39,7 +39,7 @@ public:
     bool validate_client_name(const String& name) const;
     void remove_client(Client& client);
 
-    CandidateList complete_client_name(const String& name,
+    CandidateList complete_client_name(StringView name,
                                        ByteCount cursor_pos = -1) const;
 
 private:
diff --git a/src/color_registry.cc b/src/color_registry.cc
index 0820409f..4867234d 100644
--- a/src/color_registry.cc
+++ b/src/color_registry.cc
@@ -36,11 +36,11 @@ void ColorRegistry::register_alias(const String& name, const String& colordesc,
                       it->second : parse_color_pair(colordesc);
 }
 
-CandidateList ColorRegistry::complete_alias_name(const String& prefix,
+CandidateList ColorRegistry::complete_alias_name(StringView prefix,
                                                  ByteCount cursor_pos) const
 {
     CandidateList res;
-    String real_prefix = prefix.substr(0, cursor_pos);
+    auto real_prefix = prefix.substr(0, cursor_pos);
     for (auto& alias : m_aliases)
     {
         if (prefix_match(alias.first, real_prefix))
diff --git a/src/color_registry.hh b/src/color_registry.hh
index 3eff6fab..24fcb5eb 100644
--- a/src/color_registry.hh
+++ b/src/color_registry.hh
@@ -19,7 +19,7 @@ public:
     void register_alias(const String& name, const String& colordesc,
                         bool override = false);
 
-    CandidateList complete_alias_name(const String& prefix,
+    CandidateList complete_alias_name(StringView prefix,
                                       ByteCount cursor_pos) const;
 private:
     std::unordered_map<String, ColorPair>    m_aliases;
diff --git a/src/commands.cc b/src/commands.cc
index f5ed63ff..53b4fa67 100644
--- a/src/commands.cc
+++ b/src/commands.cc
@@ -495,7 +495,7 @@ HookManager& get_hook_manager(const String& scope, Context& context)
     throw runtime_error("error: no such hook container " + scope);
 }
 
-CandidateList complete_scope(const String& prefix)
+CandidateList complete_scope(StringView prefix)
 {
     CandidateList res;
     for (auto scope : { "global", "buffer", "window" })
diff --git a/src/completion.cc b/src/completion.cc
index a3e99ce4..9fc3ed78 100644
--- a/src/completion.cc
+++ b/src/completion.cc
@@ -6,7 +6,7 @@ namespace Kakoune
 {
 
 Completions shell_complete(const Context& context, CompletionFlags flags,
-                           const String& prefix, ByteCount cursor_pos)
+                           StringView prefix, ByteCount cursor_pos)
 {
     ByteCount word_start = 0;
     ByteCount word_end = 0;
diff --git a/src/completion.hh b/src/completion.hh
index fc58d1c4..6cda2e6c 100644
--- a/src/completion.hh
+++ b/src/completion.hh
@@ -5,13 +5,13 @@
 #include <functional>
 
 #include "units.hh"
+#include "string.hh"
 
 namespace Kakoune
 {
 
 class Context;
 
-class String;
 using CandidateList = std::vector<String>;
 
 struct Completions
@@ -36,16 +36,16 @@ enum class CompletionFlags
     Fast
 };
 using Completer = std::function<Completions (const Context&, CompletionFlags,
-                                             const String&, ByteCount)>;
+                                             StringView, ByteCount)>;
 
 inline Completions complete_nothing(const Context& context, CompletionFlags,
-                                    const String&, ByteCount cursor_pos)
+                                    StringView, ByteCount cursor_pos)
 {
     return Completions(cursor_pos, cursor_pos);
 }
 
 Completions shell_complete(const Context& context, CompletionFlags,
-                           const String&, ByteCount cursor_pos);
+                           StringView, ByteCount cursor_pos);
 
 }
 #endif // completion_hh_INCLUDED
diff --git a/src/file.cc b/src/file.cc
index c295217f..c3df5f54 100644
--- a/src/file.cc
+++ b/src/file.cc
@@ -311,7 +311,7 @@ std::vector<String> list_files(const String& prefix,
     return result.empty() ? subseq_result : result;
 }
 
-std::vector<String> complete_filename(const String& prefix,
+std::vector<String> complete_filename(StringView prefix,
                                       const Regex& ignored_regex,
                                       ByteCount cursor_pos)
 {
@@ -346,7 +346,7 @@ std::vector<String> complete_filename(const String& prefix,
     return res;
 }
 
-std::vector<String> complete_command(const String& prefix, ByteCount cursor_pos)
+std::vector<String> complete_command(StringView prefix, ByteCount cursor_pos)
 {
     String real_prefix = parse_filename(prefix.substr(0, cursor_pos));
     String dirname;
diff --git a/src/file.hh b/src/file.hh
index 818d6de1..91e27d9d 100644
--- a/src/file.hh
+++ b/src/file.hh
@@ -35,12 +35,12 @@ String find_file(const String& filename, memoryview<String> paths);
 
 time_t get_fs_timestamp(const String& filename);
 
-std::vector<String> complete_filename(const String& prefix,
+std::vector<String> complete_filename(StringView prefix,
                                       const Regex& ignore_regex,
                                       ByteCount cursor_pos = -1);
 
-std::vector<String> complete_command(const String& prefix,
-                                      ByteCount cursor_pos = -1);
+std::vector<String> complete_command(StringView prefix,
+                                     ByteCount cursor_pos = -1);
 }
 
 #endif // file_hh_INCLUDED
diff --git a/src/function_group.hh b/src/function_group.hh
index a3371c53..3bac5159 100644
--- a/src/function_group.hh
+++ b/src/function_group.hh
@@ -54,12 +54,12 @@ public:
             return  *group;
     }
 
-    CandidateList complete_id(const String& prefix, ByteCount cursor_pos) const
+    CandidateList complete_id(StringView prefix, ByteCount cursor_pos) const
     {
         return m_functions.complete_id(prefix, cursor_pos);
     }
 
-    CandidateList complete_group_id(const String& prefix, ByteCount cursor_pos) const
+    CandidateList complete_group_id(StringView prefix, ByteCount cursor_pos) const
     {
         return m_functions.complete_id_if(
             prefix, cursor_pos, [](const FunctionAndId& func) {
diff --git a/src/function_registry.hh b/src/function_registry.hh
index d62f9ae8..2dbf1f02 100644
--- a/src/function_registry.hh
+++ b/src/function_registry.hh
@@ -32,7 +32,7 @@ public:
         return it->second;
     }
 
-    CandidateList complete_name(const String& prefix, ByteCount cursor_pos)
+    CandidateList complete_name(StringView prefix, ByteCount cursor_pos)
     {
         return m_functions.complete_id(prefix, cursor_pos);
     }
diff --git a/src/id_map.hh b/src/id_map.hh
index 32462fa5..4ba91fe3 100644
--- a/src/id_map.hh
+++ b/src/id_map.hh
@@ -67,11 +67,11 @@ public:
     }
 
     template<typename Condition>
-    CandidateList complete_id_if(const String& prefix,
+    CandidateList complete_id_if(StringView prefix,
                                  ByteCount cursor_pos,
                                  Condition condition) const
     {
-        String real_prefix = prefix.substr(0, cursor_pos);
+        auto real_prefix = prefix.substr(0, cursor_pos);
         CandidateList result;
         for (auto& value : m_content)
         {
@@ -84,7 +84,7 @@ public:
         return result;
     }
 
-    CandidateList complete_id(const String& prefix,
+    CandidateList complete_id(StringView prefix,
                               ByteCount cursor_pos) const
     {
         return complete_id_if(
diff --git a/src/option_manager.cc b/src/option_manager.cc
index ca9c145b..2bd9fd0c 100644
--- a/src/option_manager.cc
+++ b/src/option_manager.cc
@@ -85,14 +85,14 @@ CandidateList OptionManager::get_matching_names(MatchingFunc func)
     return result;
 }
 
-CandidateList OptionManager::complete_option_name(const String& prefix,
+CandidateList OptionManager::complete_option_name(StringView prefix,
                                                   ByteCount cursor_pos)
 {
     using namespace std::placeholders;
-    String real_prefix = prefix.substr(0, cursor_pos);
-    auto result = get_matching_names(std::bind(prefix_match, _1, std::ref(real_prefix)));
+    auto real_prefix = prefix.substr(0, cursor_pos);
+    auto result = get_matching_names(std::bind(prefix_match, _1, real_prefix));
     if (result.empty())
-        result = get_matching_names(std::bind(subsequence_match, _1, std::ref(real_prefix)));
+        result = get_matching_names(std::bind(subsequence_match, _1, real_prefix));
     return result;
 }
 
diff --git a/src/option_manager.hh b/src/option_manager.hh
index 6ceaac4c..32382b68 100644
--- a/src/option_manager.hh
+++ b/src/option_manager.hh
@@ -89,7 +89,7 @@ public:
     const Option& operator[] (const String& name) const;
     Option& get_local_option(const String& name);
 
-    CandidateList complete_option_name(const String& prefix,
+    CandidateList complete_option_name(StringView prefix,
                                        ByteCount cursor_pos);
 
     using OptionList = std::vector<const Option*>;