Re: Why does byte-compile-file copy the input file to a different buffer?

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sun, Dec 17, 2023 at 18:48:32 +0200, Eli Zaretskii wrote:
> > Date: Sun, 17 Dec 2023 16:06:44 +0000
> > From: Alan Mackenzie <acm@muc.de>

> > On executing a byte-compile-file command, rather than working on the
> > file's buffer directly, the byte compiler first copies the buffer/file
> > into another buffer with a boring name like " *Compiler Input*" or "
> > *Compiler Input*-1".

> > Why does it do this?  It makes it difficult, in the reader, to determine
> > the identity of the actual source buffer.  (Yes, I have reasons for
> > wanting to do this.)

> > Would it not be simpler just to compile directly from the source buffer,
> > thus avoiding a needless copying and making it clear what the actual
> > source buffer is?

> The command uses insert-file-contents to insert the file into the
> input buffer, and works on that, so that looks very natural to me.
> I'm not sure what is bothering you in that, or why.

From inside the reader, the buffer " *Compiler Input*" is effectively
anonymous: it gives no clue as to what the actual file or buffer is.  If
the byte compiler actually loaded the source file into a buffer with the
normal name (e.g. bytecomp.el), that name would be available for use in
the reader.

Identifying the source presented to the reader is a difficult business.
Quite bluntly, the code in lread.c is showing its age.  Sometimes a file
is loaded (and thus also read) by fragmented direct file access
routines.  Other times the reader is invoked from a file's buffer, other
times from an anonymous buffer such as " *Compiler Input*".

All I want is the name of the real buffer, or failing that, the name of
the real file.  When the reader sees " *Compiler Input*" does it have to
assume that byte-compile-current-file is bound and use that?  Even
recognising " *Compiler Input*-1" in C code is difficult - there're no
string functions in Emacs which can test that a given string is a prefix
of another string.  There's string-match, but it only works with a
regular expression, not a plain string.  By the time I put that sort of
code into a C routine, it is so bulky, it drowns out the prime purpose
of the routine.

It's difficult.

-- 
Alan Mackenzie (Nuremberg, Germany).