test: Rename meth_pcall to pcall_err

- Rename `meth_pcall`.
- Make `pcall_err` raise an error if the function does not fail.
- Add `vim.pesc()` to treat a string as literal where a Lua pattern is
  expected.

runtime/lua/vim/shared.lua

@@ -178,9 +178,20 @@ local function trim(s)
   return s:match('^%s*(.*%S)') or ''
 end
 
+--- Escapes magic chars in a Lua pattern string.
+---
+--@see https://github.com/rxi/lume
+--@param s  String to escape
+--@returns  %-escaped pattern string
+local function pesc(s)
+  assert(type(s) == 'string')
+  return s:gsub('[%(%)%.%%%+%-%*%?%[%]%^%$]', '%%%1')
+end
+
 local module = {
   deepcopy = deepcopy,
   gsplit = gsplit,
+  pesc = pesc,
   split = split,
   tbl_contains = tbl_contains,
   tbl_extend = tbl_extend,

test/functional/api/buffer_spec.lua

@@ -10,11 +10,11 @@ local exc_exec = helpers.exc_exec
 local feed_command = helpers.feed_command
 local insert = helpers.insert
 local NIL = helpers.NIL
-local meth_pcall = helpers.meth_pcall
 local command = helpers.command
 local bufmeths = helpers.bufmeths
 local feed = helpers.feed
 local expect_err = helpers.expect_err
+local pcall_err = helpers.pcall_err
 
 describe('api/buf', function()
   before_each(clear)
@@ -191,11 +191,8 @@ describe('api/buf', function()
 
     it('fails correctly when input is not valid', function()
       eq(1, curbufmeths.get_number())
-      local err, emsg = pcall(bufmeths.set_lines, 1, 1, 2, false, {'b\na'})
-      eq(false, err)
-      local exp_emsg = 'String cannot contain newlines'
-      -- Expected {filename}:{lnum}: {exp_emsg}
-      eq(': ' .. exp_emsg, emsg:sub(-#exp_emsg - 2))
+      expect_err([[String cannot contain newlines]],
+        bufmeths.set_lines, 1, 1, 2, false, {'b\na'})
     end)
 
     it("fails if 'nomodifiable'", function()
@@ -407,8 +404,8 @@ describe('api/buf', function()
       eq(16, get_offset(3))
       eq(24, get_offset(4))
       eq(29, get_offset(5))
-      eq({false,'Index out of bounds'}, meth_pcall(get_offset, 6))
-      eq({false,'Index out of bounds'}, meth_pcall(get_offset, -1))
+      eq('Index out of bounds', pcall_err(get_offset, 6))
+      eq('Index out of bounds', pcall_err(get_offset, -1))
 
       curbufmeths.set_option('eol', false)
       curbufmeths.set_option('fixeol', false)
@@ -441,15 +438,15 @@ describe('api/buf', function()
       eq(1, funcs.exists('b:lua'))
       curbufmeths.del_var('lua')
       eq(0, funcs.exists('b:lua'))
-      eq({false, 'Key not found: lua'}, meth_pcall(curbufmeths.del_var, 'lua'))
+      eq( 'Key not found: lua', pcall_err(curbufmeths.del_var, 'lua'))
       curbufmeths.set_var('lua', 1)
       command('lockvar b:lua')
-      eq({false, 'Key is locked: lua'}, meth_pcall(curbufmeths.del_var, 'lua'))
-      eq({false, 'Key is locked: lua'}, meth_pcall(curbufmeths.set_var, 'lua', 1))
-      eq({false, 'Key is read-only: changedtick'},
-         meth_pcall(curbufmeths.del_var, 'changedtick'))
-      eq({false, 'Key is read-only: changedtick'},
-         meth_pcall(curbufmeths.set_var, 'changedtick', 1))
+      eq('Key is locked: lua', pcall_err(curbufmeths.del_var, 'lua'))
+      eq('Key is locked: lua', pcall_err(curbufmeths.set_var, 'lua', 1))
+      eq('Key is read-only: changedtick',
+         pcall_err(curbufmeths.del_var, 'changedtick'))
+      eq('Key is read-only: changedtick',
+         pcall_err(curbufmeths.set_var, 'changedtick', 1))
     end)
   end)
 

test/functional/api/server_requests_spec.lua

@@ -10,7 +10,7 @@ local ok = helpers.ok
 local meths = helpers.meths
 local spawn, merge_args = helpers.spawn, helpers.merge_args
 local set_session = helpers.set_session
-local meth_pcall = helpers.meth_pcall
+local pcall_err = helpers.pcall_err
 
 describe('server -> client', function()
   local cid
@@ -220,8 +220,8 @@ describe('server -> client', function()
     end)
 
     it('returns an error if the request failed', function()
-      eq({false, "Vim:Error invoking 'does-not-exist' on channel 3:\nInvalid method: does-not-exist" },
-         meth_pcall(eval, "rpcrequest(vim, 'does-not-exist')"))
+      eq("Vim:Error invoking 'does-not-exist' on channel 3:\nInvalid method: does-not-exist",
+         pcall_err(eval, "rpcrequest(vim, 'does-not-exist')"))
     end)
   end)
 

test/functional/api/tabpage_spec.lua

@@ -6,7 +6,7 @@ local curtabmeths = helpers.curtabmeths
 local funcs = helpers.funcs
 local request = helpers.request
 local NIL = helpers.NIL
-local meth_pcall = helpers.meth_pcall
+local pcall_err = helpers.pcall_err
 local command = helpers.command
 
 describe('api/tabpage', function()
@@ -34,11 +34,11 @@ describe('api/tabpage', function()
       eq(1, funcs.exists('t:lua'))
       curtabmeths.del_var('lua')
       eq(0, funcs.exists('t:lua'))
-      eq({false, 'Key not found: lua'}, meth_pcall(curtabmeths.del_var, 'lua'))
+      eq('Key not found: lua', pcall_err(curtabmeths.del_var, 'lua'))
       curtabmeths.set_var('lua', 1)
       command('lockvar t:lua')
-      eq({false, 'Key is locked: lua'}, meth_pcall(curtabmeths.del_var, 'lua'))
-      eq({false, 'Key is locked: lua'}, meth_pcall(curtabmeths.set_var, 'lua', 1))
+      eq('Key is locked: lua', pcall_err(curtabmeths.del_var, 'lua'))
+      eq('Key is locked: lua', pcall_err(curtabmeths.set_var, 'lua', 1))
     end)
 
     it('tabpage_set_var returns the old value', function()

test/functional/api/vim_spec.lua

@@ -8,7 +8,6 @@ local eval = helpers.eval
 local expect = helpers.expect
 local funcs = helpers.funcs
 local iswin = helpers.iswin
-local meth_pcall = helpers.meth_pcall
 local meths = helpers.meths
 local ok, nvim_async, feed = helpers.ok, helpers.nvim_async, helpers.feed
 local is_os = helpers.is_os
@@ -17,6 +16,7 @@ local request = helpers.request
 local source = helpers.source
 local next_msg = helpers.next_msg
 
+local pcall_err = helpers.pcall_err
 local expect_err = helpers.expect_err
 local format_string = helpers.format_string
 local intchar2lua = helpers.intchar2lua
@@ -326,25 +326,20 @@ describe('API', function()
     end)
 
     it('reports errors', function()
-      eq({false, 'Error loading lua: [string "<nvim>"]:1: '..
-                 "'=' expected near '+'"},
-         meth_pcall(meths.execute_lua, 'a+*b', {}))
+      eq([[Error loading lua: [string "<nvim>"]:1: '=' expected near '+']],
+        pcall_err(meths.execute_lua, 'a+*b', {}))
 
-      eq({false, 'Error loading lua: [string "<nvim>"]:1: '..
-                 "unexpected symbol near '1'"},
-         meth_pcall(meths.execute_lua, '1+2', {}))
+      eq([[Error loading lua: [string "<nvim>"]:1: unexpected symbol near '1']],
+        pcall_err(meths.execute_lua, '1+2', {}))
 
-      eq({false, 'Error loading lua: [string "<nvim>"]:1: '..
-                 "unexpected symbol"},
-         meth_pcall(meths.execute_lua, 'aa=bb\0', {}))
+      eq([[Error loading lua: [string "<nvim>"]:1: unexpected symbol]],
+        pcall_err(meths.execute_lua, 'aa=bb\0', {}))
 
-      eq({false, 'Error executing lua: [string "<nvim>"]:1: '..
-                 "attempt to call global 'bork' (a nil value)"},
-         meth_pcall(meths.execute_lua, 'bork()', {}))
+      eq([[Error executing lua: [string "<nvim>"]:1: attempt to call global 'bork' (a nil value)]],
+        pcall_err(meths.execute_lua, 'bork()', {}))
 
-      eq({false, 'Error executing lua: [string "<nvim>"]:1: '..
-                 "did\nthe\nfail"},
-         meth_pcall(meths.execute_lua, 'error("did\\nthe\\nfail")', {}))
+      eq('Error executing lua: [string "<nvim>"]:1: did\nthe\nfail',
+        pcall_err(meths.execute_lua, 'error("did\\nthe\\nfail")', {}))
     end)
 
     it('uses native float values', function()
@@ -571,10 +566,10 @@ describe('API', function()
         yyybc line 2
         line 3
         ]])
-      eq({false, "Invalid type: 'bx'"},
-         meth_pcall(meths.put, {'xxx', 'yyy'}, 'bx', false, true))
-      eq({false, "Invalid type: 'b3x'"},
-         meth_pcall(meths.put, {'xxx', 'yyy'}, 'b3x', false, true))
+      eq("Invalid type: 'bx'",
+         pcall_err(meths.put, {'xxx', 'yyy'}, 'bx', false, true))
+      eq("Invalid type: 'b3x'",
+         pcall_err(meths.put, {'xxx', 'yyy'}, 'b3x', false, true))
     end)
   end)
 
@@ -607,13 +602,13 @@ describe('API', function()
       eq(1, funcs.exists('g:lua'))
       meths.del_var('lua')
       eq(0, funcs.exists('g:lua'))
-      eq({false, "Key not found: lua"}, meth_pcall(meths.del_var, 'lua'))
+      eq("Key not found: lua", pcall_err(meths.del_var, 'lua'))
       meths.set_var('lua', 1)
 
       -- Set locked g: var.
       command('lockvar lua')
-      eq({false, 'Key is locked: lua'}, meth_pcall(meths.del_var, 'lua'))
-      eq({false, 'Key is locked: lua'}, meth_pcall(meths.set_var, 'lua', 1))
+      eq('Key is locked: lua', pcall_err(meths.del_var, 'lua'))
+      eq('Key is locked: lua', pcall_err(meths.set_var, 'lua', 1))
     end)
 
     it('nvim_get_vvar, nvim_set_vvar', function()
@@ -1195,8 +1190,8 @@ describe('API', function()
       eq({info=info}, meths.get_var("info_event"))
       eq({[1]=testinfo,[2]=stderr,[3]=info}, meths.list_chans())
 
-      eq({false, "Vim:Error invoking 'nvim_set_current_buf' on channel 3 (amazing-cat):\nWrong type for argument 1, expecting Buffer"},
-         meth_pcall(eval, 'rpcrequest(3, "nvim_set_current_buf", -1)'))
+      eq("Vim:Error invoking 'nvim_set_current_buf' on channel 3 (amazing-cat):\nWrong type for argument 1, expecting Buffer",
+         pcall_err(eval, 'rpcrequest(3, "nvim_set_current_buf", -1)'))
     end)
 
     it('works for :terminal channel', function()

test/functional/api/window_spec.lua

@@ -8,10 +8,10 @@ local curwinmeths = helpers.curwinmeths
 local funcs = helpers.funcs
 local request = helpers.request
 local NIL = helpers.NIL
-local meth_pcall = helpers.meth_pcall
 local meths = helpers.meths
 local command = helpers.command
 local expect_err = helpers.expect_err
+local pcall_err = helpers.pcall_err
 
 -- check if str is visible at the beginning of some line
 local function is_visible(str)
@@ -74,8 +74,7 @@ describe('API/win', function()
 
     it('does not leak memory when using invalid window ID with invalid pos',
     function()
-      eq({false, 'Invalid window id'},
-         meth_pcall(meths.win_set_cursor, 1, {"b\na"}))
+      eq('Invalid window id', pcall_err(meths.win_set_cursor, 1, {"b\na"}))
     end)
 
     it('updates the screen, and also when the window is unfocused', function()
@@ -185,11 +184,11 @@ describe('API/win', function()
       eq(1, funcs.exists('w:lua'))
       curwinmeths.del_var('lua')
       eq(0, funcs.exists('w:lua'))
-      eq({false, 'Key not found: lua'}, meth_pcall(curwinmeths.del_var, 'lua'))
+      eq('Key not found: lua', pcall_err(curwinmeths.del_var, 'lua'))
       curwinmeths.set_var('lua', 1)
       command('lockvar w:lua')
-      eq({false, 'Key is locked: lua'}, meth_pcall(curwinmeths.del_var, 'lua'))
-      eq({false, 'Key is locked: lua'}, meth_pcall(curwinmeths.set_var, 'lua', 1))
+      eq('Key is locked: lua', pcall_err(curwinmeths.del_var, 'lua'))
+      eq('Key is locked: lua', pcall_err(curwinmeths.set_var, 'lua', 1))
     end)
 
     it('window_set_var returns the old value', function()
@@ -222,7 +221,8 @@ describe('API/win', function()
       eq('', nvim('get_option', 'statusline'))
       command("set modified")
       command("enew") -- global-local: not preserved in new buffer
-      eq({false, "Failed to get value for option 'statusline'"}, meth_pcall(curwin, 'get_option', 'statusline'))
+      eq("Failed to get value for option 'statusline'",
+        pcall_err(curwin, 'get_option', 'statusline'))
       eq('', eval('&l:statusline')) -- confirm local value was not copied
     end)
   end)
@@ -317,8 +317,8 @@ describe('API/win', function()
       insert('text')
       command('new')
       local newwin = meths.get_current_win()
-      eq({false,"Vim:E37: No write since last change (add ! to override)"},
-         meth_pcall(meths.win_close, oldwin,false))
+      eq("Vim:E37: No write since last change (add ! to override)",
+         pcall_err(meths.win_close, oldwin,false))
       eq({newwin,oldwin}, meths.list_wins())
     end)
 

test/functional/autocmd/autocmd_spec.lua

@@ -7,7 +7,7 @@ local eval = helpers.eval
 local feed = helpers.feed
 local clear = helpers.clear
 local meths = helpers.meths
-local meth_pcall = helpers.meth_pcall
+local pcall_err = helpers.pcall_err
 local funcs = helpers.funcs
 local expect = helpers.expect
 local command = helpers.command
@@ -219,8 +219,8 @@ describe('autocmd', function()
     eq(7, eval('g:test'))
 
     -- API calls are blocked when aucmd_win is not in scope
-    eq({false, 'Vim(call):E5555: API call: Invalid window id'},
-       meth_pcall(command, "call nvim_set_current_win(g:winid)"))
+    eq('Vim(call):E5555: API call: Invalid window id',
+      pcall_err(command, "call nvim_set_current_win(g:winid)"))
 
     -- second time aucmd_win is needed, a different code path is invoked
     -- to reuse the same window, so check again
@@ -257,8 +257,8 @@ describe('autocmd', function()
     eq(0, eval('g:had_value'))
     eq(7, eval('g:test'))
 
-    eq({false, 'Vim(call):E5555: API call: Invalid window id'},
-       meth_pcall(command, "call nvim_set_current_win(g:winid)"))
+    eq('Vim(call):E5555: API call: Invalid window id',
+      pcall_err(command, "call nvim_set_current_win(g:winid)"))
   end)
 
   it(':doautocmd does not warn "No matching autocommands" #10689', function()

test/functional/eval/api_functions_spec.lua

@@ -4,7 +4,7 @@ local lfs = require('lfs')
 local neq, eq, command = helpers.neq, helpers.eq, helpers.command
 local clear, curbufmeths = helpers.clear, helpers.curbufmeths
 local exc_exec, expect, eval = helpers.exc_exec, helpers.expect, helpers.eval
-local insert, meth_pcall = helpers.insert, helpers.meth_pcall
+local insert, pcall_err = helpers.insert, helpers.pcall_err
 local meths = helpers.meths
 
 describe('eval-API', function()
@@ -148,8 +148,8 @@ describe('eval-API', function()
   end)
 
   it('cannot be called from sandbox', function()
-    eq({false, 'Vim(call):E48: Not allowed in sandbox'},
-       meth_pcall(command, "sandbox call nvim_input('ievil')"))
+    eq('Vim(call):E48: Not allowed in sandbox',
+       pcall_err(command, "sandbox call nvim_input('ievil')"))
     eq({''}, meths.buf_get_lines(0, 0, -1, true))
   end)
 end)

test/functional/eval/changedtick_spec.lua

@@ -9,7 +9,7 @@ local meths = helpers.meths
 local command = helpers.command
 local exc_exec = helpers.exc_exec
 local redir_exec = helpers.redir_exec
-local meth_pcall = helpers.meth_pcall
+local pcall_err = helpers.pcall_err
 local curbufmeths = helpers.curbufmeths
 
 before_each(clear)
@@ -64,8 +64,8 @@ describe('b:changedtick', function()
        redir_exec('let b:.changedtick = ' .. ctn))
     eq('\nE46: Cannot change read-only variable "d.changedtick"',
        redir_exec('let d.changedtick = ' .. ctn))
-    eq({false, 'Key is read-only: changedtick'},
-       meth_pcall(curbufmeths.set_var, 'changedtick', ctn))
+    eq('Key is read-only: changedtick',
+      pcall_err(curbufmeths.set_var, 'changedtick', ctn))
 
     eq('\nE795: Cannot delete variable b:changedtick',
        redir_exec('unlet b:changedtick'))
@@ -75,8 +75,8 @@ describe('b:changedtick', function()
        redir_exec('unlet b:["changedtick"]'))
     eq('\nE46: Cannot change read-only variable "d.changedtick"',
        redir_exec('unlet d.changedtick'))
-    eq({false, 'Key is read-only: changedtick'},
-       meth_pcall(curbufmeths.del_var, 'changedtick'))
+    eq('Key is read-only: changedtick',
+      pcall_err(curbufmeths.del_var, 'changedtick'))
     eq(ct, changedtick())
 
     eq('\nE46: Cannot change read-only variable "b:["changedtick"]"',

test/functional/helpers.lua

@@ -687,14 +687,6 @@ function module.skip_fragile(pending_fn, cond)
   return false
 end
 
-function module.meth_pcall(...)
-  local ret = {pcall(...)}
-  if type(ret[2]) == 'string' then
-    ret[2] = ret[2]:gsub('^[^:]+:%d+: ', '')
-  end
-  return ret
-end
-
 module.funcs = module.create_callindex(module.call)
 module.meths = module.create_callindex(module.nvim)
 module.uimeths = module.create_callindex(ui)

test/functional/lua/utility_functions_spec.lua

@@ -7,12 +7,12 @@ local clear = helpers.clear
 local eq = helpers.eq
 local eval = helpers.eval
 local feed = helpers.feed
-local meth_pcall = helpers.meth_pcall
+local pcall_err = helpers.pcall_err
 local exec_lua = helpers.exec_lua
 
 before_each(clear)
 
-describe('lua function', function()
+describe('lua stdlib', function()
   -- İ: `tolower("İ")` is `i` which has length 1 while `İ` itself has
   --    length 2 (in bytes).
   -- Ⱥ: `tolower("Ⱥ")` is `ⱥ` which has length 2 while `Ⱥ` itself has
@@ -147,11 +147,11 @@ describe('lua function', function()
     eq({"yy","xx"}, exec_lua("return test_table"))
 
     -- type checked args
-    eq({false, 'Error executing lua: vim.schedule: expected function'},
-       meth_pcall(exec_lua, "vim.schedule('stringly')"))
+    eq('Error executing lua: vim.schedule: expected function',
+      pcall_err(exec_lua, "vim.schedule('stringly')"))
 
-    eq({false, 'Error executing lua: vim.schedule: expected function'},
-       meth_pcall(exec_lua, "vim.schedule()"))
+    eq('Error executing lua: vim.schedule: expected function',
+      pcall_err(exec_lua, "vim.schedule()"))
 
     exec_lua([[
       vim.schedule(function()
@@ -283,4 +283,9 @@ describe('lua function', function()
 
     assert(is_dc)
   end)
+
+  it('vim.pesc', function()
+    eq('foo%-bar', exec_lua([[return vim.pesc('foo-bar')]]))
+    eq('foo%%%-bar', exec_lua([[return vim.pesc(vim.pesc('foo-bar'))]]))
+  end)
 end)

test/functional/ui/float_spec.lua

@@ -10,7 +10,7 @@ local meths = helpers.meths
 local curbufmeths = helpers.curbufmeths
 local funcs = helpers.funcs
 local run = helpers.run
-local meth_pcall = helpers.meth_pcall
+local pcall_err = helpers.pcall_err
 
 describe('floating windows', function()
   before_each(function()
@@ -636,26 +636,26 @@ describe('floating windows', function()
 
     it('API has proper error messages', function()
       local buf = meths.create_buf(false,false)
-      eq({false, "Invalid key 'bork'"},
-         meth_pcall(meths.open_win,buf, false, {width=20,height=2,bork=true}))
-      eq({false, "'win' key is only valid with relative='win'"},
-         meth_pcall(meths.open_win,buf, false, {width=20,height=2,relative='editor',row=0,col=0,win=0}))
-      eq({false, "Only one of 'relative' and 'external' must be used"},
-         meth_pcall(meths.open_win,buf, false, {width=20,height=2,relative='editor',row=0,col=0,external=true}))
-      eq({false, "Invalid value of 'relative' key"},
-         meth_pcall(meths.open_win,buf, false, {width=20,height=2,relative='shell',row=0,col=0}))
-      eq({false, "Invalid value of 'anchor' key"},
-         meth_pcall(meths.open_win,buf, false, {width=20,height=2,relative='editor',row=0,col=0,anchor='bottom'}))
-      eq({false, "All of 'relative', 'row', and 'col' has to be specified at once"},
-         meth_pcall(meths.open_win,buf, false, {width=20,height=2,relative='editor'}))
-      eq({false, "'width' key must be a positive Integer"},
-         meth_pcall(meths.open_win,buf, false, {width=-1,height=2,relative='editor'}))
-      eq({false, "'height' key must be a positive Integer"},
-         meth_pcall(meths.open_win,buf, false, {width=20,height=-1,relative='editor'}))
-      eq({false, "'height' key must be a positive Integer"},
-         meth_pcall(meths.open_win,buf, false, {width=20,height=0,relative='editor'}))
-      eq({false, "Must specify 'width' and 'height'"},
-         meth_pcall(meths.open_win,buf, false, {relative='editor'}))
+      eq("Invalid key 'bork'",
+         pcall_err(meths.open_win,buf, false, {width=20,height=2,bork=true}))
+      eq("'win' key is only valid with relative='win'",
+         pcall_err(meths.open_win,buf, false, {width=20,height=2,relative='editor',row=0,col=0,win=0}))
+      eq("Only one of 'relative' and 'external' must be used",
+         pcall_err(meths.open_win,buf, false, {width=20,height=2,relative='editor',row=0,col=0,external=true}))
+      eq("Invalid value of 'relative' key",
+         pcall_err(meths.open_win,buf, false, {width=20,height=2,relative='shell',row=0,col=0}))
+      eq("Invalid value of 'anchor' key",
+         pcall_err(meths.open_win,buf, false, {width=20,height=2,relative='editor',row=0,col=0,anchor='bottom'}))
+      eq("All of 'relative', 'row', and 'col' has to be specified at once",
+         pcall_err(meths.open_win,buf, false, {width=20,height=2,relative='editor'}))
+      eq("'width' key must be a positive Integer",
+         pcall_err(meths.open_win,buf, false, {width=-1,height=2,relative='editor'}))
+      eq("'height' key must be a positive Integer",
+         pcall_err(meths.open_win,buf, false, {width=20,height=-1,relative='editor'}))
+      eq("'height' key must be a positive Integer",
+         pcall_err(meths.open_win,buf, false, {width=20,height=0,relative='editor'}))
+      eq("Must specify 'width' and 'height'",
+         pcall_err(meths.open_win,buf, false, {relative='editor'}))
     end)
 
     it('can be placed relative window or cursor', function()
@@ -4528,8 +4528,8 @@ describe('floating windows', function()
             {0:~                             }|
         ]], float_pos=expected_pos}
         else
-          eq({false, "UI doesn't support external windows"},
-             meth_pcall(meths.win_set_config, 0, {external=true, width=30, height=2}))
+          eq("UI doesn't support external windows",
+             pcall_err(meths.win_set_config, 0, {external=true, width=30, height=2}))
           return
         end
 
@@ -4844,8 +4844,8 @@ describe('floating windows', function()
             {0:~                                       }|
         ]], float_pos=expected_pos}
         else
-          eq({false, "UI doesn't support external windows"},
-             meth_pcall(meths.win_set_config, 0, {external=true, width=65, height=4}))
+          eq("UI doesn't support external windows",
+             pcall_err(meths.win_set_config, 0, {external=true, width=65, height=4}))
         end
 
         feed(":tabnext<cr>")

test/helpers.lua

@@ -73,14 +73,26 @@ function module.matches(pat, actual)
   end
   error(string.format('Pattern does not match.\nPattern:\n%s\nActual:\n%s', pat, actual))
 end
--- Expect an error matching pattern `pat`.
-function module.expect_err(pat, ...)
-  local fn = select(1, ...)
+
+-- Expects an error matching Lua pattern `pat`.
+--
+function module.expect_err(pat, fn, ...)
+  assert(type(fn) == 'function')
   local fn_args = {...}
-  table.remove(fn_args, 1)
   assert.error_matches(function() return fn(unpack(fn_args)) end, pat)
 end
 
+-- Invokes `fn` and returns the error string, or raises an error if `fn` succeeds.
+function module.pcall_err(fn, ...)
+  assert(type(fn) == 'function')
+  local status, rv = pcall(fn, ...)
+  if status == true then
+    error('expected failure, but got success')
+  end
+  local errmsg = tostring(rv):gsub('^[^:]+:%d+: ', '')
+  return errmsg
+end
+
 -- initial_path:  directory to recurse into
 -- re:            include pattern (string)
 -- exc_re:        exclude pattern(s) (string or table)