Add lean.elan.toolchain.gc support.

(Without --delete at the minute.)

Author: Julian

lua/lean/elan.lua

@@ -22,6 +22,20 @@ local elan = { toolchain = {} }
 ---@field path string the path to the toolchain on this machine
 ---@field resolved_name string the identifier for this toolchain
 
+---Information about a toolchain which is in use by a project on the machine.
+---@class ElanUsedToolchain
+---@field toolchain string the name of the toolchain
+---@field user string a path (or in some cases reason) that causes the toolchain to be considered in-use
+
+---Determine which toolchains are in use.
+---@return string[] unused the unused toolchains currently installed
+---@return ElanUsedToolchain[] used any used toolchains
+function elan.toolchain.gc()
+  local stdout = subprocess_check_output { 'elan', 'toolchain', 'gc', '--json' }
+  local result = vim.json.decode(stdout)
+  return result.unused_toolchains, result.used_toolchains
+end
+
 ---List the installed toolchains.
 ---@return string[] toolchains the toolchains
 function elan.toolchain.list()