vim: support ":const" command which sets constants

[lacygoill]

This PR tries to add support for the Vim command :const. The latter is used in Vimscript to set a constant.

---

I did not try to create a new C function to parse the command because:

1. that's above my level
2. I don't think it's necessary. In Vim, :const is equivalent to :let followed by :lockvar; it's just a variable whose value is locked so that it can't change.

---

:const can be shortened into :cons, which is why I wrote 4 and not 5 here:

else if (wordMatchLen (line, "let", 3) || wordMatchLen (line, "const", 4))
                                                                       ^

---

I added a simple test consisting of an input file and an expected output tags file.
I made sure all the Vim tests passed:

make units LANGUAGES=Vim

[coveralls]

Coverage increased (+0.0006%) to 86.778% when pulling b92362f on lacygoill:vim-const into 2a45704 on universal-ctags:master.

[codecov]

Codecov Report

Merging #2593 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2593   +/-   ##
=======================================
  Coverage   86.67%   86.67%           
=======================================
  Files         183      183           
  Lines       38608    38610    +2     
=======================================
+ Hits        33462    33464    +2     
  Misses       5146     5146           

Impacted Files	Coverage Δ
parsers/vim.c	99.59% <100.00%> (+<0.01%)	⬆️

---

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[masatake]

that's above my level

I will asisst you. So the level is not a matter for us.

We know ctags. You know vim script. I know C. Your knowledge about vim script is very important.
See also #2494.

You reuse the code for handling "let". However, as far as my short study, let is for defining variables.
I think tagging a language object defined with :const as a variable is not good idea.
Instead, I think it should be tagged as a constant.

const s:CONSTANT = 42

=>

s:CONSTANT	input.vim	/^const s:CONSTANT = 42$/;"	v

Here v means a variable. I don't think s:CONSTANT is not a vairable.
How do you think?

[masatake]

I think ideal behavior is:

[jet@living]~/var/ctags% cat input.vim
cat input.vim
const s:CONSTANT = 42
[jet@living]~/var/ctags% ./ctags --list-kinds=Vim
./ctags --list-kinds=Vim
a  autocommand groups
c  user-defined commands
f  function definitions
m  maps
v  variable definitions
n  vimball filename
C  constant definitions
[jet@living]~/var/ctags% ./ctags -o - input.vim
./ctags -o - input.vim
s:CONSTANT	input.vim	/^const s:CONSTANT = 42$/;"	C

The change we need is for above result:

diff --git a/parsers/vim.c b/parsers/vim.c
index 0bdd1802..7ee767b6 100644
--- a/parsers/vim.c
+++ b/parsers/vim.c
@@ -46,7 +46,8 @@ typedef enum {
 	K_FUNCTION,
 	K_MAP,
 	K_VARIABLE,
-	K_FILENAME
+	K_FILENAME,
+	K_CONST,
 } vimKind;
 
 static kindDefinition VimKinds [] = {
@@ -56,6 +57,7 @@ static kindDefinition VimKinds [] = {
 	{ true,  'm', "map",      "maps" },
 	{ true,  'v', "variable", "variable definitions" },
 	{ true,  'n', "filename", "vimball filename" },
+	{ true,  'C', "constant", "constant definitions" },
 };
 
 /*
@@ -432,7 +434,7 @@ cleanUp:
 	return cmdProcessed;
 }
 
-static void parseLet (const unsigned char *line, int infunction)
+static void parseVariableOrConstant (const unsigned char *line, int infunction, int kindIndex)
 {
 	vString *name = vStringNew ();
 
@@ -482,7 +484,7 @@ static void parseLet (const unsigned char *line, int infunction)
 			vStringPut (name, (int) *cp);
 			++cp;
 		} while (isalnum ((int) *cp) || *cp == '_' || *cp == '#' || *cp == ':' || *cp == '$');
-		makeSimpleTag (name, K_VARIABLE);
+		makeSimpleTag (name, kindIndex);
 		vStringClear (name);
 	}
 
@@ -596,9 +598,12 @@ static bool parseVimLine (const unsigned char *line, int infunction)
 
 	else if (wordMatchLen (line, "let", 3))
 	{
-		parseLet (skipWord (line), infunction);
+		parseVariableOrConstant (skipWord (line), infunction, K_VARIABLE);
+	}
+	else if (wordMatchLen (line, "const", 4))
+	{
+		parseVariableOrConstant (skipWord (line), infunction, K_CONST);
 	}
-
 	return readNextLine;
 }

However, I don't know well about vim script. So, I would like to hear your comments.

[lacygoill]

I agree with you; technically, a constant is not a variable since its value can't change.
It should probably have its own kind. c is already taken for commands, so C is the next best choice.

I've applied your patch, compiled and installed. All the Vim tests still pass, and the ctags(1) binary correctly generates tags for constants defined with the :const keyword. I haven't had the time to read all of #2494, but if I understand correctly, the purpose is to give advice on Vimscript. I'm no expert, but I think I can help. If you have any question regarding the Vimscript language, let me know.

[lacygoill]

I made a mistake in the expected.tags file; I need to fix it.

[masatake]

Mathcing "const" is not needed here.

[lacygoill]

You are right. I forgot to revert my old patch, sorry.

[masatake]

Removng this empty line is not needed to support "const".

[lacygoill]

I think this is fixed; I added back the empty line.

[masatake]

Thank you.