grayout.vim

grayout.vim is a simple plugin that grays out inactive preprocessor blocks.

Instead of adding static syntax regions for each (manually) defined macro, like ifdef highlighting does, this plugin invokes the compiler on the source file and parses its output. This approach is way more flexible, because the actual parsing is done by the compiler. See "How it works" for further information.

Although it's intended to be used for C/C++/Obj-C, it should also work for every language with C-style preprocessor commands.

The plugin was only tested on Linux but should work on other platforms, too (with custom command lines).

Installation

1. Make sure your vim version is compiled with python support
2. Install the plugin
   • Using a plugin manager like vundle or pathogen
   • Alternatively copy the plugin directory into your .vim folder
3. Configure it as you like (see "Configuration")
4. ...
5. Profit

Usage

• Run :GrayoutUpdate to update the grayout.
• Run :GrayoutClear to clear all grayouts.
• Run :GrayoutReloadConfig to reload the config file.

All these commands only affect the current buffer.

How it works

When calling :GrayoutUpdate the source file is parsed for #if-#elif-#else-#endif blocks and their line start and end positions are stored in a list.

Then a special "tag", containing a random UUID followed by the ID and line information for the current block, is inserted at the start of every block. The modified source is then sent over stdin to the compiler (only to the preprocessor to be precise) using the specified command line (clang -x c++ -w -P -E - by default).

The compiler output is parsed again for tag-lines. If a tag was found it means that the block it was injected in is active. The script can then grayout certain lines depending on which tags were found and which tags weren't, since tags inside a inactive block don't show up in the compiler output.

Configuration

Key mappings

The plugin comes with no default keymappings, it only provides the commands listed above.

To map :GrayoutUpdate to e.g. <F5> you can add this to your .vimrc:

nnoremap <F5> :GrayoutUpdate<CR>

Options

• Set the command to invoke the compiler

  let g:grayout_cmd_line = 'clang -x c++ -w -P -E -'

  If you set a custom command, remember to pass the flags for only invoking the preprocessor and reading the source from stdin (-E and - for gcc and clang), otherwise it won't work. The language needs to be specified using the -x flag, too.

  Defining certain macros is done by adding the corresponding compiler flags to the command line. See "Config files" on how to make settings project specific.

  For example, if you want to use gcc instead of clang and need FOOBAR_MACRO to be defined, you can write:

  let g:grayout_cmd_line = 'gcc -x c++ -DFOOBAR_MACRO -w -P -E -'

• Ask for confirmation when loading a config file.

  let g:grayout_confirm = 1

  By default the user is asked for confirmation when a config file is found. This might be desirable because config files could potentially contain malicious code. See also "Config files".

  To disable confirmations:

  let g:grayout_confirm = 0

• Enable debug messages inside vim

  let g:grayout_debug = 0

• Enable writing debug messages to grayout-log.txt

  let g:grayout_debug_logfile = 0

• Log compiler in- and output

  let g:grayout_debug_compiler_inout = 0

  If g:grayout_debug or g:grayout_debug_logfile is enabled, this option will also log the compiler in- and output. Default is 0.

Config files

For project specific configuration you can create a .grayout.conf file containing the compiler command. This basically works like g:grayout_cmd_line.

The script searches the parent directories until it finds a config file. If no config file was found, the command in g:grayout_cmd_line is used.

To reload the config file run :GrayoutReloadConfig. Be aware that this only reloads the config for the current buffer. Other buffers won't get updated this way, even if they share the same file.

The syntax is pretty straight forward. The file only contains the command to invoke the compiler. Linebreaks are ignored. Remember to pass the flags for only invoking the preprocessor and reading the source from stdin (-E and - for gcc and clang).

For example, for a C project you might write:

gcc -x c -w -P -E
    -DENABLE_FEATURE_X
    -DANOTHER_FLAG
    -DSOME_CONSTANT=42
    -nostdinc
    -

Useful Compiler flags

Every flag that reduces compiler output speeds up the grayout process because less lines have to be processed (see "How it works").

Since you usually don't need to check macros from standard libraries, it's a good idea to add -nostdinc and/or -nostdinc++ to your cmd line to tell the compiler to ignore system includes. This could give a significant speed boost as it greatly reduces the compiler output.

-nostdinc

Do not search the standard system directories for header files. Only the directories you have specified with -I options (and the directory of the current file, if appropriate) are searched.

-nostdinc++

Do not search for header files in the standard directories specific to C++, but do still search the other standard directories.

-P

Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.

-w

Inhibit all warning messages.

The -P and -w flags are set by default.

Colors

If you don't like the default colors for grayouts you can change them by altering the PreprocessorGrayout style.

The default is:

highlight PreprocessorGrayout cterm=italic gui=italic ctermfg=DarkGray guifg=DarkGray

Todo

• Write vim doc
• Maybe rewrite the core functionality in vimscript to get rid of the python dependency
• Add support for Makefiles