Implement loader for Fennel

Tests can be written in Fennel if `fennel` is among the loaders.
Currently the traceback is wrong for failures and pending tests, but it
is wrong for tests written in Moonscript as well, so this seems to be a
general problem with busted.

busted-scm-1.rockspec

@@ -82,6 +82,7 @@ build = {
     ['busted.modules.files.lua']              = 'busted/modules/files/lua.lua',
     ['busted.modules.files.moonscript']       = 'busted/modules/files/moonscript.lua',
     ['busted.modules.files.terra']            = 'busted/modules/files/terra.lua',
+    ['busted.modules.files.fennel']           = 'busted/modules/files/fennel.lua',
 
     ['busted.outputHandlers.base']            = 'busted/outputHandlers/base.lua',
     ['busted.outputHandlers.utfTerminal']     = 'busted/outputHandlers/utfTerminal.lua',

busted/modules/files/fennel.lua

@@ -0,0 +1,97 @@
+-- NOTE: the traceback of `pending` and `error` is broken, and there does not
+-- seem to be anything we can do about it.  The Moonscript loader has the same
+-- problem.
+
+
+local path = require 'pl.path'
+local ok, fennel = pcall(require, 'fennel')
+
+local MESSAGE_PATTERN = '^([^\n]-):(%d+): (.*)'
+local MESSAGE_TEMPLATE = '%s:%d: %s'
+local source_maps = {}
+
+local ret = {}
+
+-- A Fennel variant of the standard Lua loadstring function; registers a source
+-- map for the given chunk as a side effect.
+local function loadstring_(str, chunk_name)
+  local options = {filename = chunk_name:sub(2)}
+  local lua, src_map = fennel.compileString(str, options)
+  source_maps[src_map.key] = src_map
+  -- NOTE: `loadstring` was deprecated in favour of a more general `load` in
+  -- Lua 5.2
+  return (loadstring or load)(lua, chunk_name)
+end
+
+-- A Fennel variant of the standard Lua loadfile function
+local function loadfile_(fname)
+  local file, err = io.open(fname)
+  if not (file) then
+    return nil, err
+  end
+  local text = assert(file:read("*a"))
+  file:close()
+  return loadstring_(text, '@' .. tostring(fname))
+end
+
+-- Sometimes Fennel gives files like [string "./filename.fnl"], so we'll chop
+-- it up to only get the filename.
+local rewrite_filename = function(filename)
+  return filename:match('string "(.+)"') or filename
+end
+
+-- Maps a Lua line number to the corresponding Fennel line number using a
+-- previously stored source map.
+local rewrite_linenumber = function(fname, lineno)
+  local src_map = source_maps['@' .. fname]
+  if not fname or not src_map then return lineno end
+
+  local entry = src_map[lineno]
+  if not entry then return lineno end
+
+  -- Assume that all lines of a Lua file are stored in the same Fennel file;
+  -- the format of all entries is {file_name, line_number}
+  return entry[2]
+end
+
+local getTrace = function(filename, info)
+  -- NOTE: `info` is the result of `debug.getinfo(level, 'Sl')`, enriched with
+  -- `traceback` and `message`. The `traceback` is result of
+  -- `debug.traceback('', level)`
+
+  local index = info.traceback:find('\n%s*%[C]')
+  info.traceback = info.traceback:sub(1, index)
+
+  info.short_src = rewrite_filename(info.short_src)
+  info.linedefined = rewrite_linenumber(filename, info.linedefined)
+  info.currentline = rewrite_linenumber(filename, info.currentline)
+
+  return info
+end
+
+local rewriteMessage = function(filename, message)
+  local fname, lineno, msg = message:match(MESSAGE_PATTERN)
+  if not fname then
+    return message
+  end
+
+  fname = rewrite_filename(fname)
+  lineno = rewrite_linenumber(fname, tonumber(lineno))
+
+  return MESSAGE_TEMPLATE:format(fname, tostring(lineno), msg)
+end
+
+ret.match = function(busted, filename)
+  local result = ok and path.extension(filename) == '.fnl'
+  return result
+end
+
+ret.load = function(busted, filename)
+  local file, err = loadfile_(filename)
+  if not file then
+    busted.publish({'error', 'file'}, {descriptor = 'file', name = filename}, nil, err, {})
+  end
+  return file, getTrace, rewriteMessage
+end
+
+return ret