bug#16136: 24.3.50; `minibuffer-prompt-properties' for `face'

bug#16136: 24.3.50; `minibuffer-prompt-properties' for `face'

[Drew Adams]

It is good to have variable `minibuffer-prompt-properties'.  And it is
good that by default its value includes `face' with a value of
`minibuffer-prompt'.  This allows the prompt, by default, to stand out
against the user input.

However, the design should be more supple.  It should use the `face'
property specified in `minibuffer-prompt-properties', if any, to apply
that face only to parts of the PROMPT string that do not already have a
`face' text property.  Any parts of PROMPT that are already propertized
with property `face' should be left alone, and not have their face
overridden by face `minibuffer-prompt'.

This would let users supply a PROMPT string that already has certain
parts highlighted with faces, so that they, in turn, stand out against
the rest of the prompt.

In order to get that effect now, users have to propertize the entire
PROMPT string, highlighting not only the particular part(s) they want to
emphasize, but also providing face `minibuffer-prompt' for the other
parts.  If they do not do the latter then the prompt as a whole does
not stand out against the user input: it has no special face except for
the particular part(s) highlighted.

This enhancement apparently needs to be done in the C code
(`read_minibuf'), which means also that users cannot implement this
improvement themselves, in Lisp.



In GNU Emacs 24.3.50.2 (i686-pc-mingw32)
 of 2013-11-28 on LEG570
Bzr revision: 115271 rgm@gnu.org-20131128203155-qjc1xsp19z2k64b2
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking 'CFLAGS=-O0 -g3' CPPFLAGS=-DGLYPH_DEBUG=1'

bug#16136: 24.3.50; `minibuffer-prompt-properties' for `face'

[Drew Adams]

Perhaps I should add, if it wasn't obvious, that anyone wanting
the current behavior can just do one of the following:

* Not propertize PROMPT using a `face' property, to get the
  default behavior.

* Propertize PROMPT using `face' zones, but bind
  `minibuffer-prompt-properties' to a value without `face', to
  get the same behavior as now when it has no `face' behavior.

IOW, this enhancement does not lose any current features.  It
just gives users more flexibility, letting them have prompts
that emphasize certain parts.