Reporting the position of the last valid token

metalua/grammar/generator.lua

@@ -149,17 +149,46 @@ end
 -- Generate a tracable parsing error (not implemented yet)
 -------------------------------------------------------------------------------
 function M.parse_error(lx, fmt, ...)
-   local li = lx:lineinfo_left()
+   local inf = lx:lineinfo_error()
    local file, line, column, offset, positions
-   if li then
+   local p_line, p_column
+   if inf then
+      local li = inf.last
       file, line, column, offset = li.source, li.line, li.column, li.offset
-      positions = { first = li, last = li }
+      local first = inf.first
+      positions = { first = first, last = li }
+
+      local facing = inf.first.facing
+      local no_invalid_comment = function(comments)
+         for _, c in ipairs(comments) do
+            if c[1]:match("%[%[") then
+               return false
+            end
+         end
+         return true
+      end
+      -- in case of a parse error, the consumed token usually facing the one
+      -- that caused the problem is the last valid one, allowing us to point
+      -- to the exact location where the invalid token starts
+      -- there are exceptions to this, identified so far:
+      -- * there's just one token yet
+      -- * there are comment(s) after the last valid token
+      if (facing.line >= first.line and facing.column >= first.column) or
+          (facing.comments and no_invalid_comment(facing.comments))
+      then
+         p_line, p_column = first.line, first.column
+      else
+         p_line, p_column = facing.line, facing.column
+      end
    else
-      line, column, offset = -1, -1, -1
+      line, column, offset, p_line, p_column = -1, -1, -1, -1, -1
    end
 
    local msg  = string.format("line %i, char %i: "..fmt, line, column, ...)
    if file and file~='?' then msg = "file "..file..", "..msg end
+   local prev_token = string.format("facing: line %i, char %i", p_line, p_column)
+   msg = string.format("%s\n%s", msg, prev_token)
+
 
    local src = lx.src
    if offset>0 and src then

metalua/grammar/lexer.lua

@@ -511,6 +511,7 @@ function lexer :next (n)
       -- TODO: is this used anywhere? I think not.  a.lineinfo.last may be nil.
       --self.lastline = a.lineinfo.last.line
    end
+   self.lineinfo_consumed = a.lineinfo
    self.lineinfo_last_consumed = a.lineinfo.last
    return a
 end
@@ -574,6 +575,10 @@ function lexer :lineinfo_left()
    return self.lineinfo_last_consumed
 end
 
+function lexer :lineinfo_error()
+   return self.lineinfo_consumed
+end
+
 ----------------------------------------------------------------------
 -- Create a new lexstream.
 ----------------------------------------------------------------------