[-Wunsafe-buffer-usage] Implement fixits for const size array assigned to pointer

Introducing CArrayToPtrAssignment gadget and implementing fixits for some cases
of array being assigned to pointer.

Key observations:
- const size array can be assigned to std::span and bounds are propagated
- const size array can't be on LHS of assignment
This means array to pointer assignment has no strategy implications.

Fixits are implemented for cases where one of the variables in the assignment is
safe. For assignment of a safe array to unsafe pointer we know that the RHS will
never be transformed since it's safe and can immediately emit the optimal fixit.
Similarly for assignment of unsafe array to safe pointer.
(Obviously this is not and can't be future-proof in regards to what
variables we consider unsafe and that is fine.)

Fixits for assignment from unsafe array to unsafe pointer (from Array to Span
strategy) are not implemented in this patch as that needs to be properly designed
first - we might possibly implement optimal fixits for partially transformed
cases, put both variables in a single fixit group or do something else.

Author: jkorous-apple

clang/include/clang/Analysis/Analyses/UnsafeBufferUsageGadgets.def

@@ -46,6 +46,7 @@ FIXABLE_GADGET(UPCStandalonePointer)
 FIXABLE_GADGET(UPCPreIncrement)            // '++Ptr' in an Unspecified Pointer Context
 FIXABLE_GADGET(UUCAddAssign)            // 'Ptr += n' in an Unspecified Untyped Context
 FIXABLE_GADGET(PtrToPtrAssignment)
+FIXABLE_GADGET(CArrayToPtrAssignment)
 FIXABLE_GADGET(PointerInit)
 
 #undef FIXABLE_GADGET

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -7,11 +7,14 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Analysis/Analyses/UnsafeBufferUsage.h"
+#include "clang/AST/ASTContext.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/AST/Stmt.h"
 #include "clang/AST/StmtVisitor.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
 #include "clang/Basic/CharInfo.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Lex/Lexer.h"
@@ -849,6 +852,59 @@ class PtrToPtrAssignmentGadget : public FixableGadget {
   }
 };
 
+/// An assignment expression of the form:
+///  \code
+///  ptr = array;
+///  \endcode
+/// where `p` is a pointer and `array` is a constant size array.
+class CArrayToPtrAssignmentGadget : public FixableGadget {
+private:
+  static constexpr const char *const PointerAssignLHSTag = "ptrLHS";
+  static constexpr const char *const PointerAssignRHSTag = "ptrRHS";
+  const DeclRefExpr * PtrLHS;         // the LHS pointer expression in `PA`
+  const DeclRefExpr * PtrRHS;         // the RHS pointer expression in `PA`
+
+public:
+  CArrayToPtrAssignmentGadget(const MatchFinder::MatchResult &Result)
+      : FixableGadget(Kind::CArrayToPtrAssignment),
+    PtrLHS(Result.Nodes.getNodeAs<DeclRefExpr>(PointerAssignLHSTag)),
+    PtrRHS(Result.Nodes.getNodeAs<DeclRefExpr>(PointerAssignRHSTag)) {}
+
+  static bool classof(const Gadget *G) {
+    return G->getKind() == Kind::CArrayToPtrAssignment;
+  }
+
+  static Matcher matcher() {
+    auto PtrAssignExpr = binaryOperator(allOf(hasOperatorName("="),
+      hasRHS(ignoringParenImpCasts(declRefExpr(hasType(hasCanonicalType(constantArrayType())),
+                                               toSupportedVariable()).
+                                   bind(PointerAssignRHSTag))),
+                                   hasLHS(declRefExpr(hasPointerType(),
+                                                      toSupportedVariable()).
+                                          bind(PointerAssignLHSTag))));
+
+    return stmt(isInUnspecifiedUntypedContext(PtrAssignExpr));
+  }
+
+  virtual std::optional<FixItList>
+  getFixits(const FixitStrategy &S) const override;
+
+  virtual const Stmt *getBaseStmt() const override {
+    // FIXME: This should be the binary operator, assuming that this method
+    // makes sense at all on a FixableGadget.
+    return PtrLHS;
+  }
+
+  virtual DeclUseList getClaimedVarUseSites() const override {
+    return DeclUseList{PtrLHS, PtrRHS};
+  }
+
+  virtual std::optional<std::pair<const VarDecl *, const VarDecl *>>
+  getStrategyImplications() const override {
+    return {};
+  }
+};
+
 /// A call of a function or method that performs unchecked buffer operations
 /// over one of its pointer parameters.
 class UnsafeBufferUsageAttrGadget : public WarningGadget {
@@ -1494,6 +1550,22 @@ PtrToPtrAssignmentGadget::getFixits(const FixitStrategy &S) const {
 /// \returns fixit that adds .data() call after \DRE.
 static inline std::optional<FixItList> createDataFixit(const ASTContext& Ctx, const DeclRefExpr * DRE);
 
+std::optional<FixItList>
+CArrayToPtrAssignmentGadget::getFixits(const FixitStrategy &S) const {
+  const auto *LeftVD = cast<VarDecl>(PtrLHS->getDecl());
+  const auto *RightVD = cast<VarDecl>(PtrRHS->getDecl());
+  if (S.lookup(LeftVD) == FixitStrategy::Kind::Span) {
+    if (S.lookup(RightVD) == FixitStrategy::Kind::Wontfix) {
+      return FixItList{};
+    }
+  } else if (S.lookup(LeftVD) == FixitStrategy::Kind::Wontfix) {
+    if (S.lookup(RightVD) == FixitStrategy::Kind::Array) {
+      return createDataFixit(RightVD->getASTContext(), PtrRHS);
+    }
+  }
+  return std::nullopt;
+}
+
 std::optional<FixItList>
 PointerInitGadget::getFixits(const FixitStrategy &S) const {
   const auto *LeftVD = PtrInitLHS;

clang/test/SemaCXX/warn-unsafe-buffer-usage-fixits-array-assign-to-ptr.cpp

@@ -0,0 +1,43 @@
+// RUN: %clang_cc1 -std=c++20 -Wunsafe-buffer-usage \
+// RUN:            -fsafe-buffer-usage-suggestions \
+// RUN:            -fdiagnostics-parseable-fixits %s 2>&1 | FileCheck %s
+
+void safe_array_assigned_to_safe_ptr(unsigned idx) {
+  int buffer[10];
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:
+  int* ptr;
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:
+  ptr = buffer;
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:
+}
+
+void safe_array_assigned_to_unsafe_ptr(unsigned idx) {
+  int buffer[10];
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:
+  int* ptr;
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:3-[[@LINE-1]]:11}:"std::span<int> ptr"
+  ptr = buffer;
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:
+  ptr[idx] = 0;
+}
+
+void unsafe_array_assigned_to_safe_ptr(unsigned idx) {
+  int buffer[10];
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:3-[[@LINE-1]]:17}:"std::array<int, 10> buffer"
+  int* ptr;
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:
+  ptr = buffer;
+  // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:15-[[@LINE-1]]:15}:".data()"
+  buffer[idx] = 0;
+}
+
+void unsafe_array_assigned_to_unsafe_ptr(unsigned idx) {
+  int buffer[10];
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:{{.*}}
+  int* ptr;
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:{{.*}}
+  ptr = buffer;
+  // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]:{{.*}}
+  buffer[idx] = 0;
+  ptr[idx] = 0;
+}