From a5f0954c290157875b4dfb79edcf651f55742dc2 Mon Sep 17 00:00:00 2001
From: Jade Lovelace <lix@jade.fyi>
Date: Sun, 4 Aug 2024 22:02:53 -0700
Subject: clang-tidy: write a lint for charptr_cast

This lets us ensure that nobody is putting in new reinterpret_cast
instances where they could safely use charptr_cast instead.

Change-Id: I6358a3934c8133c7150042635843bdbb6b9218d4
---
 .clang-tidy                                      |  7 ++++
 subprojects/lix-clang-tidy/CharPtrCast.cc        | 45 ++++++++++++++++++++++++
 subprojects/lix-clang-tidy/CharPtrCast.hh        | 32 +++++++++++++++++
 subprojects/lix-clang-tidy/LixClangTidyChecks.cc |  2 ++
 subprojects/lix-clang-tidy/meson.build           |  3 +-
 5 files changed, 88 insertions(+), 1 deletion(-)
 create mode 100644 subprojects/lix-clang-tidy/CharPtrCast.cc
 create mode 100644 subprojects/lix-clang-tidy/CharPtrCast.hh

diff --git a/.clang-tidy b/.clang-tidy
index ccfdf9e7d..87f6d0404 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -22,6 +22,13 @@ Checks:
   - cppcoreguidelines-avoid-capturing-lambda-coroutines
     # crimes must be appropriately declared as crimes
   - cppcoreguidelines-pro-type-cstyle-cast
+  - lix-*
+    # This can not yet be applied to Lix itself since we need to do source
+    # reorganization so that lix/ include paths work.
+  - -lix-fixincludes
+    # This lint is included as an example, but the lib function it replaces is
+    # already gone.
+  - -lix-hasprefixsuffix
 
 
 CheckOptions:
diff --git a/subprojects/lix-clang-tidy/CharPtrCast.cc b/subprojects/lix-clang-tidy/CharPtrCast.cc
new file mode 100644
index 000000000..e76797f5d
--- /dev/null
+++ b/subprojects/lix-clang-tidy/CharPtrCast.cc
@@ -0,0 +1,45 @@
+#include "CharPtrCast.hh"
+#include <clang/AST/ExprCXX.h>
+#include <clang/Basic/Diagnostic.h>
+#include <clang/Tooling/Transformer/SourceCode.h>
+
+namespace nix::clang_tidy {
+using namespace clang::ast_matchers;
+using namespace clang;
+
+void CharPtrCastCheck::registerMatchers(ast_matchers::MatchFinder *Finder) {
+  Finder->addMatcher(
+      traverse(clang::TK_IgnoreUnlessSpelledInSource,
+               cxxReinterpretCastExpr(allOf(
+                   hasDestinationType(qualType(pointsTo(isAnyCharacter()))),
+                   has(expr(hasType(qualType(pointsTo(isAnyCharacter()))))))))
+          .bind("reinterpret-cast-expr"),
+      this);
+}
+
+void CharPtrCastCheck::check(
+    const ast_matchers::MatchFinder::MatchResult &Result) {
+  const auto ReinterpretCastExpr =
+      Result.Nodes.getNodeAs<CXXReinterpretCastExpr>("reinterpret-cast-expr");
+  const auto ToTypeSpan = ReinterpretCastExpr->getAngleBrackets();
+  const auto & SM = Result.Context->getSourceManager();
+
+  auto Diag =
+      diag(ReinterpretCastExpr->getExprLoc(),
+           "reinterpret_cast used for trivially safe character pointer cast");
+  Diag << ReinterpretCastExpr->getSourceRange();
+
+  auto Inside = tooling::getText(*ReinterpretCastExpr->getSubExprAsWritten(),
+                                 *Result.Context);
+
+  Diag << Inserter.createIncludeInsertion(SM.getFileID(ReinterpretCastExpr->getExprLoc()), "charptr-cast.hh");
+
+  llvm::Twine Replacement =
+      "charptr_cast" +
+      tooling::getText(CharSourceRange(ToTypeSpan, true), *Result.Context) +
+      "(" + Inside + ")";
+  Diag << FixItHint::CreateReplacement(ReinterpretCastExpr->getSourceRange(),
+                                       Replacement.str());
+}
+
+} // namespace nix::clang_tidy
diff --git a/subprojects/lix-clang-tidy/CharPtrCast.hh b/subprojects/lix-clang-tidy/CharPtrCast.hh
new file mode 100644
index 000000000..66883d055
--- /dev/null
+++ b/subprojects/lix-clang-tidy/CharPtrCast.hh
@@ -0,0 +1,32 @@
+#pragma once
+///@file
+
+#include <clang-tidy/ClangTidyCheck.h>
+#include <clang-tidy/utils/IncludeInserter.h>
+#include <clang/ASTMatchers/ASTMatchFinder.h>
+#include <llvm/ADT/StringRef.h>
+
+namespace nix::clang_tidy {
+
+using namespace clang;
+using namespace clang::tidy;
+
+class CharPtrCastCheck : public ClangTidyCheck {
+  tidy::utils::IncludeInserter Inserter{
+      Options.getLocalOrGlobal("IncludeStyle",
+                               tidy::utils::IncludeSorter::IS_Google),
+      false};
+
+public:
+  CharPtrCastCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+
+  void registerPPCallbacks(const SourceManager &, Preprocessor *PP,
+                           Preprocessor *) override {
+    Inserter.registerPreprocessor(PP);
+  }
+
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+} // namespace nix::clang_tidy
diff --git a/subprojects/lix-clang-tidy/LixClangTidyChecks.cc b/subprojects/lix-clang-tidy/LixClangTidyChecks.cc
index b3503dd3a..283e0e7c8 100644
--- a/subprojects/lix-clang-tidy/LixClangTidyChecks.cc
+++ b/subprojects/lix-clang-tidy/LixClangTidyChecks.cc
@@ -2,6 +2,7 @@
 #include <clang-tidy/ClangTidyModuleRegistry.h>
 #include "FixIncludes.hh"
 #include "HasPrefixSuffix.hh"
+#include "CharPtrCast.hh"
 
 namespace nix::clang_tidy {
 using namespace clang;
@@ -12,6 +13,7 @@ class NixClangTidyChecks : public ClangTidyModule {
         void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
             CheckFactories.registerCheck<HasPrefixSuffixCheck>("lix-hasprefixsuffix");
             CheckFactories.registerCheck<FixIncludesCheck>("lix-fixincludes");
+            CheckFactories.registerCheck<CharPtrCastCheck>("lix-charptrcast");
         }
 };
 
diff --git a/subprojects/lix-clang-tidy/meson.build b/subprojects/lix-clang-tidy/meson.build
index ef0226420..43648a1c8 100644
--- a/subprojects/lix-clang-tidy/meson.build
+++ b/subprojects/lix-clang-tidy/meson.build
@@ -5,9 +5,10 @@ project('lix-clang-tidy', ['cpp', 'c'],
 
 llvm = dependency('Clang', version: '>= 17', modules: ['libclang'])
 sources = files(
+  'CharPtrCast.cc',
+  'FixIncludes.cc',
   'HasPrefixSuffix.cc',
   'LixClangTidyChecks.cc',
-  'FixIncludes.cc',
 )
 
 lix_clang_tidy = shared_module('lix-clang-tidy', sources,
-- 
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