[clang-tools-extra] [clang-tidy] Adds readability-redundant-const check (PR #189733)

[Berkay Sahin via cfe-commits]

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@@ -0,0 +1,100 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "RedundantConstCheck.h"
+#include "../utils/LexerUtils.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include <optional>
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::readability {
+
+static std::optional<Token>
+findConstToRemove(const VarDecl *VD, const MatchFinder::MatchResult &Result) {
+  const SourceManager &SM = *Result.SourceManager;
+
+  const SourceLocation NameBeginLoc = VD->getQualifier()
+                                          ? VD->getQualifierLoc().getBeginLoc()
+                                          : VD->getLocation();
+
+  const bool IsPointer =
+      VD->getType()->isPointerType() || VD->getType()->isMemberPointerType();
+
+  // If the 'findPreviousTokenKind' below fails,
+  // we know it is a pointer but cannot find the start token.
+  // This can happen when either type is aliased or `auto` was used.
+  // e.g: constexpr const auto const str = "hello";
+  // In cases like this, Clang already warns about the use of const
+  // as duplicate, so we can safely ignore these cases.
+  const SourceLocation ConstSearchStartLoc =
+      !IsPointer
+          ? VD->getBeginLoc()
+          : utils::lexer::findPreviousTokenKind(
+                NameBeginLoc, SM, Result.Context->getLangOpts(), tok::star);
+
+  if (ConstSearchStartLoc.isInvalid())
+    return std::nullopt;
+
+  const SourceLocation PrevSemi = utils::lexer::findPreviousAnyTokenKind(
+      NameBeginLoc, SM, Result.Context->getLangOpts(), tok::semi);
+
+  // Verify that there is no semicolon between ConstSearchStartLoc and
+  // NameBeginLoc. This is to limit search area for our variable decl only
+  if (!PrevSemi.isInvalid() &&
+      SM.isBeforeInTranslationUnit(ConstSearchStartLoc, PrevSemi))
+    return std::nullopt;
+
+  const CharSourceRange FileRange = Lexer::makeFileCharRange(
+      CharSourceRange::getCharRange(ConstSearchStartLoc, NameBeginLoc), SM,
+      Result.Context->getLangOpts());
+
+  if (FileRange.isInvalid())
+    return std::nullopt;
+
+  return utils::lexer::getQualifyingToken(tok::kw_const, FileRange,
+                                          *Result.Context, SM);
+}
+
+RedundantConstCheck::RedundantConstCheck(StringRef Name,
+                                         ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context) {}
+
+void RedundantConstCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      varDecl(isConstexpr(), unless(hasType(referenceType()))).bind("var_decl"),
+      this);
+}
+
+void RedundantConstCheck::check(const MatchFinder::MatchResult &Result) {
+  const auto *VD = Result.Nodes.getNodeAs<VarDecl>("var_decl");
+
+  // Since we cannot tell the difference between `constexpr const` and
+  // `constexpr` from the AST only, if we cannot find the actual `const` token,
+  // we cannot do anything
+  const std::optional<Token> Tok = findConstToRemove(VD, Result);
+  if (!Tok)
+    return;
+
+  diag(Tok->getLocation(),
+       "redundant use of 'const'; 'constexpr' already implies 'const'")
----------------
berkaysahiin wrote:

Feel like if we expand this more than constexpr at some point we can keep the first sentence same and change the reason like:

`redundant use of 'const'; here is the reason`

https://github.com/llvm/llvm-project/pull/189733