clang-tidy check infrastructure

This brings in infrastructure for developing new custom clang-tidy lints
and refactors for Lix.

Change-Id: I3df5f5855712ab4f97d4e84d771e5e818f81f881

6 files changed, 187 insertions, 0 deletions

diff --git a/clang-tidy/.clang-format b/clang-tidy/.clang-format
new file mode 100644
index 000000000..cd8995543
--- /dev/null
+++ b/clang-tidy/.clang-format
@@ -0,0 +1 @@
+BasedOnStyle: llvm
diff --git a/clang-tidy/HasPrefixSuffix.cc b/clang-tidy/HasPrefixSuffix.cc
new file mode 100644
index 000000000..e29b67e7c
--- /dev/null
+++ b/clang-tidy/HasPrefixSuffix.cc
@@ -0,0 +1,80 @@
+#include "HasPrefixSuffix.hh"
+#include <clang/AST/ASTTypeTraits.h>
+#include <clang/AST/Expr.h>
+#include <clang/AST/PrettyPrinter.h>
+#include <clang/AST/Type.h>
+#include <clang/ASTMatchers/ASTMatchers.h>
+#include <clang/Basic/Diagnostic.h>
+#include <clang/Frontend/FrontendAction.h>
+#include <clang/Frontend/FrontendPluginRegistry.h>
+#include <clang/Tooling/Transformer/SourceCode.h>
+#include <clang/Tooling/Transformer/SourceCodeBuilders.h>
+#include <iostream>
+
+namespace nix::clang_tidy {
+using namespace clang::ast_matchers;
+using namespace clang;
+
+void HasPrefixSuffixCheck::registerMatchers(ast_matchers::MatchFinder *Finder) {
+  Finder->addMatcher(
+      traverse(clang::TK_AsIs,
+               callExpr(callee(functionDecl(anyOf(hasName("hasPrefix"),
+                                                  hasName("hasSuffix")))
+                                   .bind("callee-decl")),
+                        optionally(hasArgument(
+                            0, cxxConstructExpr(
+                                   hasDeclaration(functionDecl(hasParameter(
+                                       0, parmVarDecl(hasType(
+                                              asString("const char *")))))))
+                                   .bind("implicit-cast"))))
+                   .bind("call")),
+      this);
+}
+
+void HasPrefixSuffixCheck::check(
+    const ast_matchers::MatchFinder::MatchResult &Result) {
+
+  const auto *CalleeDecl = Result.Nodes.getNodeAs<FunctionDecl>("callee-decl");
+  auto FuncName = std::string(CalleeDecl->getName());
+  std::string NewName;
+  if (FuncName == "hasPrefix") {
+    NewName = "starts_with";
+  } else if (FuncName == "hasSuffix") {
+    NewName = "ends_with";
+  } else {
+    llvm_unreachable("nix-has-prefix: invalid callee");
+  }
+
+  const auto *MatchedDecl = Result.Nodes.getNodeAs<CallExpr>("call");
+  const auto *ImplicitConvertArg =
+      Result.Nodes.getNodeAs<CXXConstructExpr>("implicit-cast");
+
+  const auto *Lhs = MatchedDecl->getArg(0);
+  const auto *Rhs = MatchedDecl->getArg(1);
+  auto Diag = diag(MatchedDecl->getExprLoc(), FuncName + " is deprecated");
+
+  std::string Text = "";
+
+  // Form possible cast to string_view, or nothing.
+  if (ImplicitConvertArg) {
+    Text = "std::string_view(";
+    Text.append(tooling::getText(*Lhs, *Result.Context));
+    Text.append(").");
+  } else {
+    Text.append(*tooling::buildAccess(*Lhs, *Result.Context));
+  }
+
+  // Call .starts_with.
+  Text.append(NewName);
+  Text.push_back('(');
+  Text.append(tooling::getText(*Rhs, *Result.Context));
+  Text.push_back(')');
+
+  Diag << FixItHint::CreateReplacement(MatchedDecl->getSourceRange(), Text);
+
+  // for (const auto *arg : MatchedDecl->arguments()) {
+  //   arg->dumpColor();
+  //   arg->getType().dump();
+  // }
+}
+}; // namespace nix::clang_tidy
diff --git a/clang-tidy/HasPrefixSuffix.hh b/clang-tidy/HasPrefixSuffix.hh
new file mode 100644
index 000000000..8693b6767
--- /dev/null
+++ b/clang-tidy/HasPrefixSuffix.hh
@@ -0,0 +1,25 @@
+#pragma once
+///@file
+/// This is an example of a clang-tidy automated refactoring against the Nix
+/// codebase. The refactoring has been completed in
+/// https://gerrit.lix.systems/c/lix/+/565 so this code is around as
+/// an example.
+
+#include <clang-tidy/ClangTidyCheck.h>
+#include <clang/ASTMatchers/ASTMatchFinder.h>
+#include <llvm/ADT/StringRef.h>
+
+namespace nix::clang_tidy {
+
+using namespace clang;
+using namespace clang::tidy;
+using namespace llvm;
+
+class HasPrefixSuffixCheck : public ClangTidyCheck {
+public:
+  HasPrefixSuffixCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+}; // namespace nix::clang_tidy
diff --git a/clang-tidy/NixClangTidyChecks.cc b/clang-tidy/NixClangTidyChecks.cc
new file mode 100644
index 000000000..8f0309107
--- /dev/null
+++ b/clang-tidy/NixClangTidyChecks.cc
@@ -0,0 +1,17 @@
+#include <clang-tidy/ClangTidyModule.h>
+#include <clang-tidy/ClangTidyModuleRegistry.h>
+#include "HasPrefixSuffix.hh"
+
+namespace nix::clang_tidy {
+using namespace clang;
+using namespace clang::tidy;
+
+class NixClangTidyChecks : public ClangTidyModule {
+    public:
+        void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
+            CheckFactories.registerCheck<HasPrefixSuffixCheck>("nix-hasprefixsuffix");
+        }
+};
+
+static ClangTidyModuleRegistry::Add<NixClangTidyChecks> X("nix-module", "Adds nix specific checks");
+};
diff --git a/clang-tidy/README.md b/clang-tidy/README.md
new file mode 100644
index 000000000..cf46c71ec
--- /dev/null
+++ b/clang-tidy/README.md
@@ -0,0 +1,56 @@
+# Clang tidy lints for Nix
+
+This is a skeleton of a clang-tidy lints library for Nix.
+
+Currently there is one check (which is already obsolete as it has served its
+goal and is there as an example), `HasPrefixSuffixCheck`.
+
+## Running fixes/checks
+
+One file:
+
+```
+ninja -C build && clang-tidy --checks='-*,nix-*' --load=build/libnix-clang-tidy.so -p ../compile_commands.json --fix ../src/libcmd/installables.cc
+```
+
+Several files, in parallel:
+
+```
+ninja -C build && run-clang-tidy -checks='-*,nix-*' -load=build/libnix-clang-tidy.so -p .. -fix ../src | tee -a clang-tidy-result
+```
+
+## Resources
+
+* https://firefox-source-docs.mozilla.org/code-quality/static-analysis/writing-new/clang-query.html
+* https://clang.llvm.org/docs/LibASTMatchersReference.html
+* https://devblogs.microsoft.com/cppblog/exploring-clang-tooling-part-3-rewriting-code-with-clang-tidy/
+
+## Developing new checks
+
+Put something like so in `myquery.txt`:
+
+```
+set traversal     IgnoreUnlessSpelledInSource
+# ^ Ignore implicit AST nodes. May need to use AsIs depending on how you are
+# working.
+set bind-root     true
+# ^ true unless you use any .bind("foo") commands
+set print-matcher true
+enable output     dump
+match callExpr(callee(functionDecl(hasName("hasPrefix"))), optionally(hasArgument( 0, cxxConstructExpr(hasDeclaration(functionDecl(hasParameter(0, parmVarDecl(hasType(asString("const char *"))).bind("meow2"))))))))
+```
+
+Then run, e.g. `clang-query --preload hasprefix.query -p compile_commands.json src/libcmd/installables.cc`.
+
+With this you can iterate a query before writing it in C++ and suffering from
+C++.
+
+### Tips and tricks for the C++
+
+There is a function `dump()` on many things that will dump to stderr. Also
+`llvm::errs()` lets you print to stderr.
+
+When I wrote `HasPrefixSuffixCheck`, I was not really able to figure out how
+the structured replacement system was supposed to work. In principle you can
+describe the replacement with a nice DSL. Look up the Stencil system in Clang
+for details.
diff --git a/clang-tidy/meson.build b/clang-tidy/meson.build
new file mode 100644
index 000000000..48770e39f
--- /dev/null
+++ b/clang-tidy/meson.build
@@ -0,0 +1,8 @@
+project('nix-clang-tidy', ['cpp', 'c'],
+  version : '0.1',
+  default_options : ['warning_level=3', 'cpp_std=c++20'])
+
+llvm = dependency('Clang', version: '>= 14', modules: ['libclang'])
+sources = ['HasPrefixSuffix.cc', 'NixClangTidyChecks.cc']
+shared_module('nix-clang-tidy', sources,
+  dependencies: llvm)