The former contain a codemap (which is per-crate), and the latter don't. This
will be useful in order to allow more than one crate to be compiled in one run
of the compiler.
This correctly fixes issue #1362.
chpos/byte_pos are now the offsets within a particular file, but
rather the offsets within a virtual file with is formed by combing all
of the modules within a crate. Thus, resetting them to 0 causes an
overlap and hence, bogus source locations.
Fix#1362 by moving chpos/byte_pos to parse_sess so that
new_parser_from_source_str has access to them and hence can chose an
initial value that is not already been used in the crate.
Note that the trigger for bug 1361 was that syntax/ext/expand.rs calls
parse_expr_from_source_str (which calls new_parser_from_source_str)
using the same codemap as the current crate (and hence causing overlap
with files in the crate as new_parser_from_source_str resets the
chpos/byte_pos to 0).
