(format "%d" F) now truncates floating F

Problem reported by Paul Pogonyshev (Bug#31938).
* src/editfns.c: Include math.h, for trunc.
(styled_format): For %d, truncate floating-point numbers and
convert -0 to 0, going back to how Emacs 26 did things.
* doc/lispref/strings.texi (Formatting Strings):
Document behavior of %o, %d, %x, %X on floating-point numbers.
* src/floatfns.c (trunc) [!HAVE_TRUNC]: Rename from emacs_trunc
and make it an extern function, so that editfns.c can use it.
All callers changed.
* test/src/editfns-tests.el (format-%d-float): New test.

doc/lispref/strings.texi

@@ -922,18 +922,23 @@ Functions}).  Thus, strings are enclosed in @samp{"} characters, and
 @item %o
 @cindex integer to octal
 Replace the specification with the base-eight representation of an
-unsigned integer.
+unsigned integer.  The object can also be a nonnegative floating-point
+number that is formatted as an integer, dropping any fraction, if the
+integer does not exceed machine limits.
 
 @item %d
 Replace the specification with the base-ten representation of a signed
-integer.
+integer.  The object can also be a floating-point number that is
+formatted as an integer, dropping any fraction.
 
 @item %x
 @itemx %X
 @cindex integer to hexadecimal
 Replace the specification with the base-sixteen representation of an
 unsigned integer.  @samp{%x} uses lower case and @samp{%X} uses upper
-case.
+case.  The object can also be a nonnegative floating-point number that
+is formatted as an integer, dropping any fraction, if the integer does
+not exceed machine limits.
 
 @item %c
 Replace the specification with the character which is the value given.

src/editfns.c

@@ -47,6 +47,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include <errno.h>
 #include <float.h>
 #include <limits.h>
+#include <math.h>
 
 #ifdef HAVE_TIMEZONE_T
 # include <sys/param.h>
@@ -4671,6 +4672,12 @@ styled_format (ptrdiff_t nargs, Lisp_Object *args, bool message)
 		    {
 		      strcpy (f - pMlen - 1, "f");
 		      double x = XFLOAT_DATA (arg);
+
+		      /* Truncate and then convert -0 to 0, to be more
+			 consistent with %x etc.; see Bug#31938.  */
+		      x = trunc (x);
+		      x = x ? x : 0;
+
 		      sprintf_bytes = sprintf (sprintf_buf, convspec, 0, x);
 		      char c0 = sprintf_buf[0];
 		      bool signedp = ! ('0' <= c0 && c0 <= '9');

src/floatfns.c

@@ -435,11 +435,9 @@ emacs_rint (double d)
 }
 #endif
 
-#ifdef HAVE_TRUNC
-#define emacs_trunc trunc
-#else
-static double
-emacs_trunc (double d)
+#ifndef HAVE_TRUNC
+double
+trunc (double d)
 {
   return (d < 0 ? ceil : floor) (d);
 }
@@ -482,8 +480,7 @@ Rounds ARG toward zero.
 With optional DIVISOR, truncate ARG/DIVISOR.  */)
   (Lisp_Object arg, Lisp_Object divisor)
 {
-  return rounding_driver (arg, divisor, emacs_trunc, truncate2,
-			  "truncate");
+  return rounding_driver (arg, divisor, trunc, truncate2, "truncate");
 }
 
 
@@ -543,7 +540,7 @@ DEFUN ("ftruncate", Fftruncate, Sftruncate, 1, 1, 0,
 {
   CHECK_FLOAT (arg);
   double d = XFLOAT_DATA (arg);
-  d = emacs_trunc (d);
+  d = trunc (d);
   return make_float (d);
 }
 

src/lisp.h

@@ -3425,8 +3425,11 @@ extern Lisp_Object string_make_unibyte (Lisp_Object);
 extern void syms_of_fns (void);
 
 /* Defined in floatfns.c.  */
-extern void syms_of_floatfns (void);
+#ifndef HAVE_TRUNC
+extern double trunc (double);
+#endif
 extern Lisp_Object fmod_float (Lisp_Object x, Lisp_Object y);
+extern void syms_of_floatfns (void);
 
 /* Defined in fringe.c.  */
 extern void syms_of_fringe (void);

test/src/editfns-tests.el

@@ -176,6 +176,14 @@
   (should-error (format "%o" -1e-37)
                 :type 'overflow-error))
 
+;; Bug#31938
+(ert-deftest format-%d-float ()
+  (should (string-equal (format "%d" -1.1) "-1"))
+  (should (string-equal (format "%d" -0.9) "0"))
+  (should (string-equal (format "%d" -0.0) "0"))
+  (should (string-equal (format "%d" 0.0) "0"))
+  (should (string-equal (format "%d" 0.9) "0"))
+  (should (string-equal (format "%d" 1.1) "1")))
 
 ;;; Check format-time-string with various TZ settings.
 ;;; Use only POSIX-compatible TZ values, since the tests should work