[clang-tidy] implement utility-function to add 'const' to variables

Summary:
This patch extends the already existing facility to add 'const' to variables
to be more flexible and correct. The previous version did not consider pointers
as value AND pointee. For future automatic introduction for const-correctness
this shortcoming needs to be fixed.
It always allows configuration where the 'const' token is inserted, either on
the left side (if possible) or the right side.
It adds many unit-tests to the utility-function that did not exist before, as
the function was implicitly tested through clang-tidy checks. These
tests were not changed, as the API is still compatible.

Reviewers: aaron.ballman, hokein, alexfh, shuaiwang, lebedev.ri

Reviewed By: aaron.ballman

Subscribers: jdoerfert, mgorny, xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D54395

1 files changed, 194 insertions, 2 deletions

diff --git a/clang-tools-extra/clang-tidy/utils/FixItHintUtils.cpp b/clang-tools-extra/clang-tidy/utils/FixItHintUtils.cpp
index c30a59e99ba..a9c95e9f381 100644
--- a/clang-tools-extra/clang-tidy/utils/FixItHintUtils.cpp
+++ b/clang-tools-extra/clang-tidy/utils/FixItHintUtils.cpp
@@ -9,6 +9,7 @@
 #include "FixItHintUtils.h"
 #include "LexerUtils.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/Type.h"
 
 namespace clang {
 namespace tidy {
@@ -26,10 +27,201 @@ FixItHint changeVarDeclToReference(const VarDecl &Var, ASTContext &Context) {
   return FixItHint::CreateInsertion(AmpLocation, "&");
 }
 
-FixItHint changeVarDeclToConst(const VarDecl &Var) {
-  return FixItHint::CreateInsertion(Var.getTypeSpecStartLoc(), "const ");
+static bool isValueType(const Type *T) {
+  return !(isa<PointerType>(T) || isa<ReferenceType>(T) || isa<ArrayType>(T) ||
+           isa<MemberPointerType>(T) || isa<ObjCObjectPointerType>(T));
 }
+static bool isValueType(QualType QT) { return isValueType(QT.getTypePtr()); }
+static bool isMemberOrFunctionPointer(QualType QT) {
+  return (QT->isPointerType() && QT->isFunctionPointerType()) ||
+         isa<MemberPointerType>(QT.getTypePtr());
+}
+
+static bool locDangerous(SourceLocation S) {
+  return S.isInvalid() || S.isMacroID();
+}
+
+static Optional<SourceLocation>
+skipLParensBackwards(SourceLocation Start, const ASTContext &Context) {
+  if (locDangerous(Start))
+    return None;
+
+  auto PreviousTokenLParen = [&Start, &Context]() {
+    Token T;
+    T = lexer::getPreviousToken(Start, Context.getSourceManager(),
+                                Context.getLangOpts());
+    return T.is(tok::l_paren);
+  };
+
+  while (Start.isValid() && PreviousTokenLParen())
+    Start = lexer::findPreviousTokenStart(Start, Context.getSourceManager(),
+                                          Context.getLangOpts());
+
+  if (locDangerous(Start))
+    return None;
+  return Start;
+}
+
+static Optional<FixItHint> fixIfNotDangerous(SourceLocation Loc,
+                                             StringRef Text) {
+  if (locDangerous(Loc))
+    return None;
+  return FixItHint::CreateInsertion(Loc, Text);
+}
+
+// Build a string that can be emitted as FixIt with either a space in before
+// or after the qualifier, either ' const' or 'const '.
+static std::string buildQualifier(DeclSpec::TQ Qualifier,
+                                  bool WhitespaceBefore = false) {
+  if (WhitespaceBefore)
+    return (llvm::Twine(' ') + DeclSpec::getSpecifierName(Qualifier)).str();
+  return (llvm::Twine(DeclSpec::getSpecifierName(Qualifier)) + " ").str();
+}
+
+static Optional<FixItHint> changeValue(const VarDecl &Var,
+                                       DeclSpec::TQ Qualifier,
+                                       QualifierTarget QualTarget,
+                                       QualifierPolicy QualPolicy,
+                                       const ASTContext &Context) {
+  switch (QualPolicy) {
+  case QualifierPolicy::Left:
+    return fixIfNotDangerous(Var.getTypeSpecStartLoc(),
+                             buildQualifier(Qualifier));
+  case QualifierPolicy::Right:
+    Optional<SourceLocation> IgnoredParens =
+        skipLParensBackwards(Var.getLocation(), Context);
+
+    if (IgnoredParens)
+      return fixIfNotDangerous(*IgnoredParens, buildQualifier(Qualifier));
+    return None;
+  }
+}
+
+static Optional<FixItHint> changePointerItself(const VarDecl &Var,
+                                               DeclSpec::TQ Qualifier,
+                                               const ASTContext &Context) {
+  if (locDangerous(Var.getLocation()))
+    return None;
+
+  Optional<SourceLocation> IgnoredParens =
+      skipLParensBackwards(Var.getLocation(), Context);
+  if (IgnoredParens)
+    return fixIfNotDangerous(*IgnoredParens, buildQualifier(Qualifier));
+  return None;
+}
+
+static Optional<FixItHint>
+changePointer(const VarDecl &Var, DeclSpec::TQ Qualifier, const Type *Pointee,
+              QualifierTarget QualTarget, QualifierPolicy QualPolicy,
+              const ASTContext &Context) {
+  // The pointer itself shall be marked as `const`. This is always to the right
+  // of the '*' or in front of the identifier.
+  if (QualTarget == QualifierTarget::Value)
+    return changePointerItself(Var, Qualifier, Context);
+
+  // Mark the pointee `const` that is a normal value (`int* p = nullptr;`).
+  if (QualTarget == QualifierTarget::Pointee && isValueType(Pointee)) {
+    // Adding the `const` on the left side is just the beginning of the type
+    // specification. (`const int* p = nullptr;`)
+    if (QualPolicy == QualifierPolicy::Left)
+      return fixIfNotDangerous(Var.getTypeSpecStartLoc(),
+                               buildQualifier(Qualifier));
 
+    // Adding the `const` on the right side of the value type requires finding
+    // the `*` token and placing the `const` left of it.
+    // (`int const* p = nullptr;`)
+    if (QualPolicy == QualifierPolicy::Right) {
+      SourceLocation BeforeStar = lexer::findPreviousTokenKind(
+          Var.getLocation(), Context.getSourceManager(), Context.getLangOpts(),
+          tok::star);
+      if (locDangerous(BeforeStar))
+        return None;
+
+      Optional<SourceLocation> IgnoredParens =
+          skipLParensBackwards(BeforeStar, Context);
+
+      if (IgnoredParens)
+        return fixIfNotDangerous(*IgnoredParens,
+                                 buildQualifier(Qualifier, true));
+      return None;
+    }
+  }
+
+  if (QualTarget == QualifierTarget::Pointee && Pointee->isPointerType()) {
+    // Adding the `const` to the pointee if the pointee is a pointer
+    // is the same as 'QualPolicy == Right && isValueType(Pointee)'.
+    // The `const` must be left of the last `*` token.
+    // (`int * const* p = nullptr;`)
+    SourceLocation BeforeStar = lexer::findPreviousTokenKind(
+        Var.getLocation(), Context.getSourceManager(), Context.getLangOpts(),
+        tok::star);
+    return fixIfNotDangerous(BeforeStar, buildQualifier(Qualifier, true));
+  }
+
+  return None;
+}
+
+static Optional<FixItHint>
+changeReferencee(const VarDecl &Var, DeclSpec::TQ Qualifier, QualType Pointee,
+                 QualifierTarget QualTarget, QualifierPolicy QualPolicy,
+                 const ASTContext &Context) {
+  if (QualPolicy == QualifierPolicy::Left && isValueType(Pointee))
+    return fixIfNotDangerous(Var.getTypeSpecStartLoc(),
+                             buildQualifier(Qualifier));
+
+  SourceLocation BeforeRef = lexer::findPreviousAnyTokenKind(
+      Var.getLocation(), Context.getSourceManager(), Context.getLangOpts(),
+      tok::amp, tok::ampamp);
+  Optional<SourceLocation> IgnoredParens =
+      skipLParensBackwards(BeforeRef, Context);
+  if (IgnoredParens)
+    return fixIfNotDangerous(*IgnoredParens, buildQualifier(Qualifier, true));
+
+  return None;
+}
+
+Optional<FixItHint> addQualifierToVarDecl(const VarDecl &Var,
+                                          const ASTContext &Context,
+                                          DeclSpec::TQ Qualifier,
+                                          QualifierTarget QualTarget,
+                                          QualifierPolicy QualPolicy) {
+  assert((QualPolicy == QualifierPolicy::Left ||
+          QualPolicy == QualifierPolicy::Right) &&
+         "Unexpected Insertion Policy");
+  assert((QualTarget == QualifierTarget::Pointee ||
+          QualTarget == QualifierTarget::Value) &&
+         "Unexpected Target");
+
+  QualType ParenStrippedType = Var.getType().IgnoreParens();
+  if (isValueType(ParenStrippedType))
+    return changeValue(Var, Qualifier, QualTarget, QualPolicy, Context);
+
+  if (ParenStrippedType->isReferenceType())
+    return changeReferencee(Var, Qualifier, Var.getType()->getPointeeType(),
+                            QualTarget, QualPolicy, Context);
+
+  if (isMemberOrFunctionPointer(ParenStrippedType))
+    return changePointerItself(Var, Qualifier, Context);
+
+  if (ParenStrippedType->isPointerType())
+    return changePointer(Var, Qualifier,
+                         ParenStrippedType->getPointeeType().getTypePtr(),
+                         QualTarget, QualPolicy, Context);
+
+  if (ParenStrippedType->isArrayType()) {
+    const Type *AT = ParenStrippedType->getBaseElementTypeUnsafe();
+    assert(AT && "Did not retrieve array element type for an array.");
+
+    if (isValueType(AT))
+      return changeValue(Var, Qualifier, QualTarget, QualPolicy, Context);
+
+    if (AT->isPointerType())
+      return changePointer(Var, Qualifier, AT->getPointeeType().getTypePtr(),
+                           QualTarget, QualPolicy, Context);
+  }
+
+  return None;
+}
 } // namespace fixit
 } // namespace utils
 } // namespace tidy