Unix-style redirection from standard input for script mode (cmake -P)

Motivation

CMake's script mode is convenient, but it still requires that the script exist as a file on disk. That can be a hindrance, if all you really want to do is test the result of some one-off scripting command.

UNIX has had a solution to this for decades now: It's long been a convention that, where a command accepts a file path as an argument, using a single hyphen in place of the filename means "read the input from stdin instead of opening the named file".

Having the ability to pipe a script into cmake -P - would allow CMake commands to be quickly evaluated, for testing or experimentation purposes, without having to first create a file and write the commands into it.

Potential use patterns (examples)

A cmake -P - mode could permit all of the following uses:

Piping script commands to cmake

$ echo 'message(STATUS Test)' | cmake -P -
-- Test

Pasting/typing commands on stdin

$ cmake -P-
message(STATUS "This is my input")
^D
-- This is my input

Here-docs

cmake -P - << __SCRIPT__
message(STATUS "Here is the script")
message(STATUS "The script is here")
__SCRIPT__
-- Here is the script
-- The script is here

(Pointlessly) piping/redirecting from a file

# (Both Equivalent to `cmake -P some-script.cmake`)
$ cmake -P - < some-script.cmake
$ cat some-script.cmake | cmake -P-

With the virtual filename for script mode set to <stdin>, the output of cmake -P - --trace would resemble:

$ echo 'message(STATUS "Redirected")' | cmake -P - --trace
<stdin>(1): message(STATUS Redirected )
-- Redirected

Concerns

As its authors are probably well aware, the CMake command-line processing is very averse to accepting any arguments to a flag that themselves start with a -. There are checks throughout the code that reject any argument capture when nextArg[0] == '-'. Those checks are diametrically opposed to the needs of this feature, and implementing it would require relaxing them (at least for the -P argument).

To avoid any destabilization of existing checks for arguments other than -P, an entirely new values Type can be added to the cmCommandLineArgument::Values enum: Values::OneOrHyphen. This type would behave much like the previous Values::One type used for -P, but without the usual protection against nextArg[0] == '-'.

Special-casing the argument type for a single flag, and the redundancy it creates in the code, isn't ideal. But it feels like a safe approach to supporting commands like cmake -P - and cmake -P- without destabilizing the existing processing (and protections) for all of the other arguments.