[cfe-dev] [clang-tdy] RFC: Support for plugin modules.

[Nathan James via cfe-dev]

Dear List,

I've heard a few small discussions about this topic and they seem to be
positive so I thought I'd query it here to see what the consensus is.

The general idea is to add a `add-plugin` command line option to clang-
tidy, akin to the plugins clang supports. This will be used to load one
or more plugins that link additional ClangTidyModules to better enable
users to utilise their own custom clang-tidy checks. 

Currently the only way to do that is to download the entire llvm
source, create the module and link it into clang-tidy the same way all
the built-in modules are linked.

Under this new proposal one could just get the required libraries. Then
build their module out of tree and tell clang-tidy to link at runtime
with `-add-plugin=<path-to-module>` command line option.

A typical modules source code could be as simple as this for just one
check

-----------------------------------------------------------------------

#include "ClangTidy.h"
#include "ClangTidyCheck.h"
#include "ClangTidyModule.h"
#include "ClangTidyModuleRegistry.h"

using namespace clang;
using namespace clang::tidy;
using namespace clang::ast_matchers;
using namespace llvm;

class AwesomeFunctionCheck : public ClangTidyCheck {
public:
  using ClangTidyCheck::ClangTidyCheck;

  void registerMatchers(MatchFinder *Finder) override {
    Finder->addMatcher(
        functionDecl(unless(matchesName("Awesome$"))).bind("Function"),
this);
  }
  void check(const MatchFinder::MatchResult &Result) override {
    const auto *Function =
Result.Nodes.getNodeAs<FunctionDecl>("Function");
    diag(Function->getLocation(), "Function '%0' is not sufficiently
awesome")
        << Function
        << FixItHint::CreateReplacement(
               Function->getLocation(),
               (Function->getName() + "Awesome").str());
  }
};

class MyPluginModule : public ClangTidyModule {
public:
  void addCheckFactories(ClangTidyCheckFactories &CheckFactories)
override {
    CheckFactories.registerCheck<AwesomeFunctionCheck>(
        "my-plugin-awesome-function");
  }
};

// Register the MyPluginModule using this statically initialized
variable.
static ClangTidyModuleRegistry::Add<MyPluginModule>
    X("my-plugin-module", "Adds my plugin checks.");

-----------------------------------------------------------------------

Obviously larger code bases may want multiple different checks in which
case it would be nicer to seperate the checks out into their own
implementation files (The way clang tidy is written now).

Once a plugin is linked its checks will still need to be turned on
using the `Checks` option from config files or the `checks` command
line option.

There will also need to be some documentation detailing how to create
plugins. https://clang.llvm.org/extra/clang-tidy/Contributing.html
already has some useful information but not everything needed for this purpose.

On the point of getting the required libraries I'm not too sure how
everything is packaged. From what I can see the clangTidy libraries are
packaged but the headers aren't. That would probably need addressing as
well.

Thanks for reading,
Nathan James