eval.c: New functions `defvar-1` and `defconst-1` (bug#55156)

The bytecode interpreter can't directly call special forms, so
the byte-compiler usually converts special forms into some sequence of
byte codes (basically, providing a duplicate definition of the special
form).  There are still two exceptions to this: `defconst` and `defvar`,
where the compiler instead generates a convoluted chunk of code like:

    (funcall '(lambda (x) (defvar <sym> x <doc>)) <value>)

where the quote makes sure we keep the function non-compiled, so as
to end up running the special form at run time.

Get rid of this workaround by introducing `defvar-1` and `defconst-1`
which provide a *functional* interface to the functionality of the
corresponding special form.

* src/eval.c (defvar, Fdefvar_1, Fdefconst_1): New functions, extracted from
`Fdefvar` and `Fdefconst`.
(Fdefvar, Fdefconst): Use them.
(syms_of_eval): `defsubr` the new functions.

* lisp/emacs-lisp/bytecomp.el (byte-compile-tmp-var): Delete const.
(byte-compile-defvar): Simplify using the new functions.

* doc/lispref/variables.texi (Defining Variables): Adjust the doc of
`defvar` to reflect the actual semantics implemented.

1 files changed, 5 insertions, 4 deletions

diff --git a/doc/lispref/variables.texi b/doc/lispref/variables.texi
index f0e3f337a69..c29547d00db 100644
--- a/doc/lispref/variables.texi
+++ b/doc/lispref/variables.texi
@@ -527,10 +527,11 @@ If @var{symbol} has a buffer-local binding in the current buffer,
 rather than the buffer-local binding.  It sets the default value if
 the default value is void.  @xref{Buffer-Local Variables}.
 
-If @var{symbol} is already lexically bound (e.g., if the @code{defvar}
-form occurs in a @code{let} form with lexical binding enabled), then
-@code{defvar} sets the dynamic value.  The lexical binding remains in
-effect until its binding construct exits.  @xref{Variable Scoping}.
+If @var{symbol} is already let bound (e.g., if the @code{defvar}
+form occurs in a @code{let} form), then @code{defvar} sets the toplevel
+default value, like @code{set-default-toplevel-value}.
+The let binding remains in effect until its binding construct exits.
+@xref{Variable Scoping}.
 
 @cindex @code{eval-defun}, and @code{defvar} forms
 @cindex @code{eval-last-sexp}, and @code{defvar} forms