fix(completion): use proper checks for possible functions in config

Resolve #1648

Author: echasnovski

lua/mini/completion.lua

@@ -595,7 +595,8 @@ H.setup_config = function(config)
     H.error('`lsp_completion.source_func` should be one of "completefunc" or "omnifunc"')
   end
   H.check_type('lsp_completion.auto_setup', config.lsp_completion.auto_setup, 'boolean')
-  H.check_type('lsp_completion.process_items', config.lsp_completion.process_items, 'nil')
+  H.check_type('lsp_completion.process_items', config.lsp_completion.process_items, 'callable', true)
+  H.check_type('lsp_completion.snippet_insert', config.lsp_completion.snippet_insert, 'callable', true)
 
   H.check_type('mappings.force_twostep', config.mappings.force_twostep, 'string')
   H.check_type('mappings.force_fallback', config.mappings.force_fallback, 'string')

tests/test_completion.lua

@@ -156,6 +156,7 @@ T['setup()']['creates `config` field'] = function()
   expect_config('lsp_completion.source_func', 'completefunc')
   expect_config('lsp_completion.auto_setup', true)
   expect_config('lsp_completion.process_items', vim.NIL)
+  expect_config('lsp_completion.snippet_insert', vim.NIL)
   expect_config('fallback_action', '<C-n>')
   expect_config('mappings.force_twostep', '<C-Space>')
   expect_config('mappings.force_fallback', '<A-Space>')
@@ -198,7 +199,8 @@ T['setup()']['validates `config` argument'] = function()
     '"completefunc" or "omnifunc"'
   )
   expect_config_error({ lsp_completion = { auto_setup = 'a' } }, 'lsp_completion.auto_setup', 'boolean')
-  expect_config_error({ lsp_completion = { process_items = 'a' } }, 'lsp_completion.process_items', 'function')
+  expect_config_error({ lsp_completion = { process_items = 'a' } }, 'lsp_completion.process_items', 'callable')
+  expect_config_error({ lsp_completion = { snippet_insert = 'a' } }, 'lsp_completion.snippet_insert', 'callable')
   expect_config_error({ fallback_action = 1 }, 'fallback_action', 'function or string')
   expect_config_error({ mappings = 'a' }, 'mappings', 'table')
   expect_config_error({ mappings = { force_twostep = 1 } }, 'mappings.force_twostep', 'string')