the
replacement list of a macro expansion carries the location of the token
within the '#define' directive, because cpplib expands a macro by
returning pointers to the tokens in its replacement list.  The current
implementation of cpplib assigns tokens created from built-in macros and
the '#' and '##' operators the location of the most recently lexed
token.  This is a because they are allocated from the lexer's token
runs, and because of the way the diagnostic routines infer the
appropriate location to report.

   The diagnostic routines in cpplib display the location of the most
recently _lexed_ token, unless they are passed a specific line and
column to report.  For diagnostics regarding tokens that arise from
macro expansions, it might also be helpful for the user to see the
original location in the macro definition that the token came from.
Since that is exactly the information each token carries, such an
enhancement could be made relatively easily in future.

   The stand-alone preprocessor faces a similar problem when determining
the correct line to output the token on: the position attached to a
token is fairly useless if the token came from a macro expansion.  All
tokens on a logical line should be output on its first physical line, so
the token's reported location is also wrong if it is part of a physical
line other than the first.

   To solve these issues, cpplib provides a callback that is generated
whenever it lexes a preprocessing token that starts a new logical line
other than a directive.  It passes this token (which may be a 'CPP_EOF'
token indicating the end of the translation unit) to the callback
routine, which can then use the line and column of this token to produce
correct output.

Representation of line numbers
==============================

As mentioned above, cpplib stores with each token the line number that
it was lexed on.  In fact, this number is not the number of the line in
the source file, but instead bears more resemblance to the number of the
line in the translation unit.

   The preprocessor maintains a monotonic increasing line count, which
is incremented at every new line character (and also at the end of any
buffer that does not end in a new line).  Since a line number of zero is
useful to indicate certain special states and conditions, this variable
starts counting from one.

   This variable therefore uniquely enumerates each line in the
translation unit.  With some simple infrastructure, it is straight
forward to map from this to the original source file and line number
pair, saving space whenever line number information needs to be saved.
The code the implements this mapping lies in the files 'line-map.c' and
'line-map.h'.

   Command-line macros and assertions are implemented by pushing a
buffer containing the right hand side of an equivalent '#define' or
'#assert' directive.  Some built-in macros are handled similarly.  Since
these are all processed before the first line of the main input file, it
will typically have an assigned line closer to twenty than to one.

