4 files changed, 128 insertions, 5 deletions
diff --git a/lua/mason-registry/init.lua b/lua/mason-registry/init.lua
index 535e6b57..5806c30a 100644
--- a/lua/mason-registry/init.lua
+++ b/lua/mason-registry/init.lua
@@ -49,7 +49,6 @@ function Registry.get_installed_package_names()
             directories[#directories + 1] = entry.name
         end
     end
-    -- TODO: validate that entry is a mason package
     return directories
 end
 
@@ -68,7 +67,10 @@ function Registry.get_all_package_names()
 end
 
 function Registry.get_all_packages()
-    return vim.tbl_map(Registry.get_package, Registry.get_all_package_names())
+    local _ = require "mason-core.functional"
+    local packages =
+        _.uniq_by(_.identity, _.concat(Registry.get_all_package_names(), Registry.get_installed_package_names()))
+    return vim.tbl_map(Registry.get_package, packages)
 end
 
 function Registry.get_all_package_specs()
diff --git a/lua/mason-registry/sources/github.lua b/lua/mason-registry/sources/github.lua
index 597e7d84..2b177bdd 100644
--- a/lua/mason-registry/sources/github.lua
+++ b/lua/mason-registry/sources/github.lua
@@ -1,5 +1,4 @@
 local InstallLocation = require "mason-core.installer.InstallLocation"
-local Optional = require "mason-core.optional"
 local Result = require "mason-core.result"
 local _ = require "mason-core.functional"
 local fetch = require "mason-core.fetch"
diff --git a/lua/mason-registry/sources/init.lua b/lua/mason-registry/sources/init.lua
index 765d4904..f468a8ef 100644
--- a/lua/mason-registry/sources/init.lua
+++ b/lua/mason-registry/sources/init.lua
@@ -11,7 +11,7 @@ local log = require "mason-core.log"
 ---@field serialize fun(self: RegistrySource): InstallReceiptRegistry
 ---@field is_same_location fun(self: RegistrySource, other: RegistrySource): boolean
 
----@alias RegistrySourceType '"github"' | '"lua"' | '"file"'
+---@alias RegistrySourceType '"github"' | '"lua"' | '"file"' | '"synthesized"'
 
 ---@class LazySource
 ---@field type RegistrySourceType
@@ -54,6 +54,11 @@ function LazySource.File(id)
     }
 end
 
+function LazySource.Synthesized()
+    local SynthesizedSource = require "mason-registry.sources.synthesized"
+    return SynthesizedSource:new()
+end
+
 ---@param type RegistrySourceType
 ---@param id string
 ---@param init fun(id: string): RegistrySource
@@ -115,6 +120,7 @@ end
 
 ---@class LazySourceCollection
 ---@field list LazySource[]
+---@field synthesized LazySource
 local LazySourceCollection = {}
 LazySourceCollection.__index = LazySourceCollection
 
@@ -123,6 +129,7 @@ function LazySourceCollection:new()
     local instance = {}
     setmetatable(instance, self)
     instance.list = {}
+    instance.synthesized = LazySource:new("synthesized", "synthesized", LazySource.Synthesized)
     return instance
 end
 
@@ -184,7 +191,7 @@ function LazySourceCollection:checksum()
     return vim.fn.sha256(table.concat(registry_ids, ""))
 end
 
----@param opts? { include_uninstalled?: boolean }
+---@param opts? { include_uninstalled?: boolean, include_synthesized?: boolean }
 function LazySourceCollection:iterate(opts)
     opts = opts or {}
 
@@ -197,6 +204,12 @@ function LazySourceCollection:iterate(opts)
                 return source
             end
         end
+
+        -- We've exhausted the true registry sources, fall back to the synthesized registry source.
+        if idx == #self.list + 1 and opts.include_synthesized ~= false then
+            idx = idx + 1
+            return self.synthesized:get()
+        end
     end
 end
 
diff --git a/lua/mason-registry/sources/synthesized.lua b/lua/mason-registry/sources/synthesized.lua
new file mode 100644
index 00000000..75638cd6
--- /dev/null
+++ b/lua/mason-registry/sources/synthesized.lua
@@ -0,0 +1,109 @@
+local Package = require "mason-core.package"
+local Result = require "mason-core.result"
+local _ = require "mason-core.functional"
+local InstallReceipt = require("mason-core.receipt").InstallReceipt
+local InstallLocation = require "mason-core.installer.InstallLocation"
+local fs = require "mason-core.fs"
+local log = require "mason-core.log"
+
+---@class SynthesizedRegistrySource : RegistrySource
+---@field buffer table<string, Package>
+local SynthesizedRegistrySource = {}
+SynthesizedRegistrySource.__index = SynthesizedRegistrySource
+
+function SynthesizedRegistrySource:new()
+    ---@type SynthesizedRegistrySource
+    local instance = {}
+    setmetatable(instance, self)
+    instance.buffer = {}
+    return instance
+end
+
+function SynthesizedRegistrySource:is_installed()
+    return true
+end
+
+---@return RegistryPackageSpec[]
+function SynthesizedRegistrySource:get_all_package_specs()
+    return {}
+end
+
+---@param pkg_name string
+---@param receipt InstallReceipt
+---@return Package
+function SynthesizedRegistrySource:load_package(pkg_name, receipt)
+    local installed_version = receipt:get_installed_package_version()
+    local source = {
+        id = ("pkg:mason/%s@%s"):format(pkg_name, installed_version or "N%2FA"), -- N%2FA = N/A
+        install = function()
+            error("This package can no longer be installed because it has been removed from the registry.", 0)
+        end,
+    }
+    ---@type RegistryPackageSpec
+    local spec = {
+        schema = "registry+v1",
+        name = pkg_name,
+        description = "",
+        categories = {},
+        languages = {},
+        homepage = "",
+        licenses = {},
+        deprecation = {
+            since = receipt:get_installed_package_version() or "N/A",
+            message = "This package has been removed from the registry.",
+        },
+        source = source,
+    }
+    local existing_pkg = self.buffer[pkg_name]
+    if existing_pkg then
+        existing_pkg:update(spec, self)
+        return existing_pkg
+    else
+        local pkg = Package:new(spec, self)
+        self.buffer[pkg_name] = pkg
+        return pkg
+    end
+end
+
+---@param pkg_name string
+---@return Package?
+function SynthesizedRegistrySource:get_package(pkg_name)
+    local receipt_path = InstallLocation.global():receipt(pkg_name)
+    if fs.sync.file_exists(receipt_path) then
+        local ok, receipt_json = pcall(vim.json.decode, fs.sync.read_file(receipt_path))
+        if ok then
+            local receipt = InstallReceipt.from_json(receipt_json)
+            return self:load_package(pkg_name, receipt)
+        else
+            log.error("Failed to decode package receipt", pkg_name, receipt_json)
+        end
+    end
+end
+
+function SynthesizedRegistrySource:get_all_package_names()
+    return vim.tbl_keys(self.buffer)
+end
+
+---@async
+function SynthesizedRegistrySource:install()
+    return Result.success()
+end
+
+function SynthesizedRegistrySource:get_display_name()
+    return "SynthesizedRegistrySource"
+end
+
+function SynthesizedRegistrySource:serialize()
+    return {}
+end
+
+---@param other SynthesizedRegistrySource
+function SynthesizedRegistrySource:is_same_location(other)
+    return true
+end
+
+function SynthesizedRegistrySource:__tostring()
+    return "SynthesizedRegistrySource"
+end
+
+return SynthesizedRegistrySource