discard_deps: Repair active plugins

Plugins that are unused currently can still contribute to
active plugins by the specs added to their "dependencies" field.

This commit adds functionality to the "parse" method,
repairing only the active plugins that could be affected by the
aforementioned specs.

This can be achieved by maintaining an extra administration table
containing a copy of each added plugin instance.

lua/lazy/core/plugin.lua

@@ -7,7 +7,9 @@ local M = {}
 M.loading = false
 
 ---@class LazySpecLoader
+---@field repair_info table<string,LazyPlugin[]>
 ---@field plugins table<string, LazyPlugin>
+---@field optional_only table<string, LazyPlugin>
 ---@field disabled table<string, LazyPlugin>
 ---@field modules string[]
 ---@field notifs {msg:string, level:number, file?:string}[]
@@ -20,7 +22,9 @@ M.Spec = Spec
 ---@param opts? {optional?:boolean}
 function Spec.new(spec, opts)
   local self = setmetatable({}, { __index = Spec })
+  self.repair_info = {}
   self.plugins = {}
+  self.optional_only = {}
   self.disabled = {}
   self.modules = {}
   self.notifs = {}
@@ -32,18 +36,75 @@ function Spec.new(spec, opts)
 end
 
 function Spec:parse(spec)
+  -- spec -> self.plugins
   self:normalize(spec)
+  -- self.plugins -> self.optional_only, self.disabled
+  self:fix_disabled()
 
-  -- calculate handlers
-  for _, plugin in pairs(self.plugins) do
+  if self:has_unused_plugins() then
+    self:fix_active()
+  end
+
+  self:calculate_handlers(self.plugins)
+  self:calculate_handlers(self.disabled)
+end
+
+function Spec:has_unused_plugins()
+  return not (vim.tbl_isempty(self.optional_only) and vim.tbl_isempty(self.disabled))
+end
+
+function Spec:fix_active()
+  ---@type table<string,LazyPlugin>
+  local unused_plugins = vim.tbl_extend("keep", self.optional_only, self.disabled)
+
+  ---@type string[]
+  local candidates = {}
+  for _, plugin in pairs(unused_plugins) do
+    if plugin.dependencies then
+      vim.list_extend(candidates, plugin.dependencies)
+    end
+  end
+
+  -- merge candidate again without instances supplied by unused plugins
+  for _, candidate in pairs(candidates) do
+    local fix = false
+    local instances = vim.tbl_filter(function(instance)
+      if instance._.parent_name and unused_plugins[instance._.parent_name] then
+        fix = true
+        return false
+      end
+      return instance
+    end, self.repair_info[candidate])
+    if fix then
+      self.plugins[candidate] = self:fix_merge(instances)
+    end
+  end
+end
+
+---@param instances_of_plugin LazyPlugin[]
+---@return LazyPlugin
+function Spec:fix_merge(instances_of_plugin)
+  local last
+  for index, inst in ipairs(instances_of_plugin) do
+    if index == 1 then
+      last = inst
+    else
+      self:merge(last, inst)
+      last = inst
+    end
+  end
+  return last
+end
+
+---@param plugins table<string, LazyPlugin>
+function Spec:calculate_handlers(plugins)
+  for _, plugin in pairs(plugins) do
     for _, handler in pairs(Handler.types) do
       if plugin[handler] then
         plugin[handler] = M.values(plugin, handler, true)
       end
     end
   end
-
-  self:fix_disabled()
 end
 
 -- PERF: optimized code to get package name without using lua patterns
@@ -56,8 +117,8 @@ end
 
 ---@param plugin LazyPlugin
 ---@param results? string[]
----@param is_dep? boolean
-function Spec:add(plugin, results, is_dep)
+---@param parent_name? string
+function Spec:add(plugin, results, parent_name)
   -- check if we already processed this spec. Can happen when a user uses the same instance of a spec in multiple specs
   -- see https://github.com/folke/lazy.nvim/issues/45
   if rawget(plugin, "_") then
@@ -125,9 +186,10 @@ function Spec:add(plugin, results, is_dep)
   end
 
   plugin._ = {}
-  plugin._.dep = is_dep
+  plugin._.dep = (parent_name ~= nil) or nil
 
-  plugin.dependencies = plugin.dependencies and self:normalize(plugin.dependencies, {}, true) or nil
+  plugin.dependencies = plugin.dependencies and self:normalize(plugin.dependencies, {}, plugin.name) or nil
+  self:add_to_repair_info(plugin, parent_name)
   if self.plugins[plugin.name] then
     plugin = self:merge(self.plugins[plugin.name], plugin)
   end
@@ -138,6 +200,15 @@ function Spec:add(plugin, results, is_dep)
   return plugin
 end
 
+---@param plugin LazyPlugin
+---@param parent_name? string
+function Spec:add_to_repair_info(plugin, parent_name)
+  local copy = vim.deepcopy(plugin) -- copy the instance of the plugin
+  copy._.parent_name = parent_name
+  self.repair_info[copy.name] = self.repair_info[copy.name] or {}
+  table.insert(self.repair_info[copy.name], copy)
+end
+
 function Spec:error(msg)
   self:log(msg, vim.log.levels.ERROR)
 end
@@ -197,6 +268,7 @@ function Spec:fix_optional()
     for _, plugin in pairs(self.plugins) do
       if plugin.optional and all_optional(plugin) then
         self.plugins[plugin.name] = nil
+        self.optional_only[plugin.name] = plugin
         if plugin.dependencies then
           vim.list_extend(all_optional_deps, plugin.dependencies)
         end
@@ -243,7 +315,9 @@ function Spec:fix_disabled()
 
   -- fix deps of plugins that are completely optional
   self:fix_dependencies(all_optional_deps, dep_of, function(dep_name)
+    local plugin = self.plugins[dep_name]
     self.plugins[dep_name] = nil
+    self.optional_only[dep_name] = plugin
   end)
   -- fix deps of disabled plugins
   self:fix_dependencies(disabled_deps, dep_of, function(dep_name)
@@ -271,8 +345,9 @@ end
 
 ---@param spec LazySpec|LazySpecImport
 ---@param results? string[]
----@param is_dep? boolean
-function Spec:normalize(spec, results, is_dep)
+---@param parent_name? string
+function Spec:normalize(spec, results, parent_name)
+  local is_dep = parent_name ~= nil
   if type(spec) == "string" then
     if is_dep and not spec:find("/", 1, true) then
       -- spec is a plugin name
@@ -280,16 +355,16 @@ function Spec:normalize(spec, results, is_dep)
         table.insert(results, spec)
       end
     else
-      self:add({ spec }, results, is_dep)
+      self:add({ spec }, results, parent_name)
     end
   elseif #spec > 1 or Util.is_list(spec) then
     ---@cast spec LazySpec[]
     for _, s in ipairs(spec) do
-      self:normalize(s, results, is_dep)
+      self:normalize(s, results, parent_name)
     end
   elseif spec[1] or spec.dir or spec.url then
     ---@cast spec LazyPlugin
-    local plugin = self:add(spec, results, is_dep)
+    local plugin = self:add(spec, results, parent_name)
     ---@diagnostic disable-next-line: cast-type-mismatch
     ---@cast plugin LazySpecImport
     if plugin and plugin.import then