Speed up (format "%s" STRING) and the like

Although the Lisp manual said that ‘format’ returns a
newly-allocated string, this was not true for a few cases like
(format "%s" ""), and fixing the documentation to allow reuse of
arguments lets us improve performance in common cases like
(format "foo") and (format "%s" "foo") (Bug#28625).
* doc/lispref/strings.texi (Formatting Strings):
* etc/NEWS:
Say that the result of ‘format’ might not be newly allocated.
* src/callint.c (Fcall_interactively):
* src/dbusbind.c (XD_OBJECT_TO_STRING):
* src/editfns.c (Fmessage, Fmessage_box):
* src/xdisp.c (vadd_to_log, Ftrace_to_stderr):
Just use Fformat or Fformat_message, as that’s simpler and no
longer makes unnecessary copies.
* src/editfns.c (styled_format): Remove last argument, as it
is no longer needed: all callers now want it to behave as if it
were true.  All remaining callers changed.  Make this function
static again.  Simplify the function now that we no longer
need to worry about whether the optimization is allowed.

1 files changed, 7 insertions, 3 deletions

diff --git a/doc/lispref/strings.texi b/doc/lispref/strings.texi
index dd004927caf..09c3bdf71f6 100644
--- a/doc/lispref/strings.texi
+++ b/doc/lispref/strings.texi
@@ -812,15 +812,19 @@ formatting feature described here; they differ from @code{format-message} only
 in how they use the result of formatting.
 
 @defun format string &rest objects
-This function returns a new string that is made by copying
-@var{string} and then replacing any format specification
-in the copy with encodings of the corresponding @var{objects}.  The
+This function returns a string equal to @var{string}, replacing any format
+specifications with encodings of the corresponding @var{objects}.  The
 arguments @var{objects} are the computed values to be formatted.
 
 The characters in @var{string}, other than the format specifications,
 are copied directly into the output, including their text properties,
 if any.  Any text properties of the format specifications are copied
 to the produced string representations of the argument @var{objects}.
+
+The output string need not be newly-allocated.  For example, if
+@code{x} is the string @code{"foo"}, the expressions @code{(eq x
+(format x))} and @code{(eq x (format "%s" x))} might both yield
+@code{t}.
 @end defun
 
 @defun format-message string &rest objects