implement custom cppcheck rules

I recently noticed there were several invocations of `wcwidth()` that should
have been `fish_wcwidth()`. This adds custom cppcheck rules to detect that
mistake.

.cppcheck.rule

@@ -0,0 +1,18 @@
+<?xml version="1.0"?>
+<rule version="1">
+    <pattern> wcwidth \(</pattern>
+    <message>
+        <id>wcwidthForbidden</id>
+        <severity>warning</severity>
+        <summary>Always use fish_wcwidth rather than wcwidth.</summary>
+    </message>
+</rule>
+
+<rule version="1">
+    <pattern> wcswidth \(</pattern>
+    <message>
+        <id>wcswidthForbidden</id>
+        <severity>warning</severity>
+        <summary>Always use fish_wcswidth rather than wcswidth.</summary>
+    </message>
+</rule>

CONTRIBUTING.md

@@ -30,6 +30,8 @@ Ultimately we want lint free code. However, at the moment a lot of cleanup is re
 
 To make linting the code easy there are two make targets: `lint` and `lint-all`. The latter does just what the name implies. The former will lint any modified but not committed `*.cpp` files. If there is no uncommitted work it will lint the files in the most recent commit.
 
+Fish has custom cppcheck rules in the file `.cppcheck.rule`. These help catch mistakes such as using `wcwidth()` rather than `fish_wcwidth()`. Please add a new rule if you find similar mistakes being made.
+
 ### Dealing With Lint Warnings
 
 You are strongly encouraged to address a lint warning by refactoring the code, changing variable names, or whatever action is implied by the warning.

build_tools/lint.fish

@@ -92,7 +92,7 @@ if set -q c_files[1]
         # The stderr to stdout redirection is because cppcheck, incorrectly IMHO, writes its
         # diagnostic messages to stderr. Anyone running this who wants to capture its output will
         # expect those messages to be written to stdout.
-        cppcheck -q --verbose --std=posix --std=c11 --language=c++ --template "[{file}:{line}]: {severity} ({id}): {message}" --suppress=missingIncludeSystem --inline-suppr --enable=$cppchecks $cppcheck_args $c_files 2>&1
+        cppcheck -q --verbose --std=posix --std=c11 --language=c++ --template "[{file}:{line}]: {severity} ({id}): {message}" --suppress=missingIncludeSystem --inline-suppr --enable=$cppchecks --rule-file=.cppcheck.rule $cppcheck_args $c_files 2>&1
     end
 
     if type -q oclint