Improve mojom syntax highlighting in Vim

Add missing keywords, literals, and attributes. Clean up the vimscript a
bit.

The file still does not conform to the Google style guide, but I wanted
to keep these changes minimal.

Before/after screenshots: see bug.
Text for testing: https://pastebin.com/raw/xAxJ5LvH

Bug: 727553

Change-Id: I2e3519514c30bb627b825861d722edba91c824de
Reviewed-on: https://chromium-review.googlesource.com/722122
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509249}

tools/vim/mojom/syntax/mojom.vim

@@ -2,47 +2,60 @@
 " Use of this source code is governed by a BSD-style license that can be
 " found in the LICENSE file.
 
-" Vim syntax file " Language: Mojom
+" Vim syntax file
+" Language: Mojom
 " To get syntax highlighting for .mojom files, add the following to your .vimrc
 " file:
 "     set runtimepath^=/path/to/src/tools/vim/mojom
 
+if exists("b:current_syntax")
+  finish
+endif
+
 syn case match
 
 syntax region mojomFold start="{" end="}" transparent fold
 
-" keyword definitions
-syntax keyword mojomType        bool int8 int16 int32 int64 uint8 uint16 uint32 uint64 float double array
+" Keywords
+syntax keyword mojomType        bool string int8 int16 int32 int64 uint8 uint16
+syntax keyword mojomType        uint32 uint64 float double array
 syntax match mojomImport        "^\(import\)\s"
-syntax keyword mojomKeyword     const module interface enum struct union
+syntax keyword mojomKeyword     const module interface enum struct union associated
 syntax match mojomOperator      /=>/
 syntax match mojomOperator      /?/
 
 " Comments
 syntax keyword mojomTodo           contained TODO FIXME XXX
+syntax region  mojomDocLink        contained start="\[" end="\]"
 syntax region  mojomComment        start="/\*"  end="\*/" contains=mojomTodo,mojomDocLink,@Spell
-syntax match   mojomLineComment    "//.*" contains=mojomTodo,@Spell
-syntax match   mojomLineDocComment "///.*" contains=mojomTodo,mojomDocLink,@Spell
-syntax region  mojomDocLink        contained start=+\[+ end=+\]+
+syntax match   mojomLineComment    "//.*" contains=mojomTodo,mojomDocLink,@Spell
+
+" Literals
+syntax match mojomBoolean       /true\|false/
+" Negative lookahead for "." so floats are not partly highlighted as integers.
+syntax match mojomInteger       /-\=[0-9]\(\.\)\@!/
+syntax match mojomFloat         /[0-9]\+\.[0-9]*\|[0-9]*\.[0-9]\+/
+syntax region mojomString       start=+"+ skip=+\\\\\|\\"+ end=+"+ contains=@Spell
 
-" Strings
-syn region mojomString start=+L\="+ skip=+\\\\\|\\"+ end=+"+ contains=@Spell
-hi def link mojomString            String
+" Attributes
+syntax match mojomAttribute  /\[[^\]]*\]/
 
 " The default highlighting.
 highlight default link mojomTodo            Todo
 highlight default link mojomComment         Comment
 highlight default link mojomLineComment     Comment
-highlight default link mojomLineDocComment  Comment
 highlight default link mojomDocLink         SpecialComment
 highlight default link mojomType            Type
 highlight default link mojomImport          Include
 highlight default link mojomKeyword         Keyword
 highlight default link mojomOperator        Operator
+highlight default link mojomString          String
+highlight default link mojomInteger         Number
+highlight default link mojomBoolean         Boolean
+highlight default link mojomFloat           Float
+highlight default link mojomAttribute       Label
 
 let b:current_syntax = "mojom"
 let b:spell_options = "contained"
 
 syn sync minlines=500
-
-let b:current_syntax = "mojom"