[cfe-dev] r347339 - [clang][Parse] Diagnose useless null statements / empty init-statements

Artem Dergachev via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 4 17:07:30 PST 2018



On 12/4/18 5:04 PM, George Karpenkov via cfe-dev wrote:
>
>> On Dec 4, 2018, at 4:47 PM, Aaron Ballman <aaron at aaronballman.com> wrote:
>>
>> On Tue, Dec 4, 2018 at 7:35 PM George Karpenkov <ekarpenkov at apple.com> wrote:
>>> Hi Roman,
>>>
>>> I’m against this new warning.
>>>
>>> A very common idiom is defining a “DEBUG” macro to be a no-op in release, and a logging function otherwise.
>>> The new introduced warning warns on all such cases (e.g. 'DEBUG(“mystring”);')
>> Yeah, that's not good.
>>
>>> As noted in the review, Clang warnings should generally not be used to diagnose style issues — and an extra semicolon does not seem to be a common bug source.
>>>
>>> It is a problem in practice for us since we have projects which enable all available warnings and also enable “-Werror”.
>> Would you be okay with the warning if it was only diagnosed when the
>> source location of the semi-colon is not immediately preceded by a
>> macro expansion location? e.g.,
>>
>> EMPTY_EXPANSION(12); // Not diagnosed
>> EMPTY_EXPANSION; // Not diagnosed
>> ; // Diagnosed
> This could work provided that all empty space preceding the semicolon is ignored (as the macro could be separated by space(s) or newline(s).
> I’m not sure the complexity would be worth it, as I haven’t seen bugs arising from extra semicolons and empty statements,
> but it would take care of my use case.
>

Or rather when the *previous* semicolon is coming from a macro.

>> ~Aaron
>>
>>> Regards,
>>> George
>>>
>>>
>>>> On Nov 20, 2018, at 10:59 AM, Roman Lebedev via cfe-commits <cfe-commits at lists.llvm.org> wrote:
>>>>
>>>> Author: lebedevri
>>>> Date: Tue Nov 20 10:59:05 2018
>>>> New Revision: 347339
>>>>
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=347339&view=rev
>>>> Log:
>>>> [clang][Parse] Diagnose useless null statements / empty init-statements
>>>>
>>>> Summary:
>>>> clang has `-Wextra-semi` (D43162), which is not dictated by the currently selected standard.
>>>> While that is great, there is at least one more source of need-less semis - 'null statements'.
>>>> Sometimes, they are needed:
>>>> ```
>>>> for(int x = 0; continueToDoWork(x); x++)
>>>> ; // Ugly code, but the semi is needed here.
>>>> ```
>>>>
>>>> But sometimes they are just there for no reason:
>>>> ```
>>>> switch(X) {
>>>> case 0:
>>>> return -2345;
>>>> case 5:
>>>> return 0;
>>>> default:
>>>> return 42;
>>>> }; // <- oops
>>>>
>>>> ;;;;;;;;;;; <- OOOOPS, still not diagnosed. Clearly this is junk.
>>>> ```
>>>>
>>>> Additionally:
>>>> ```
>>>> if(; // <- empty init-statement
>>>>   true)
>>>> ;
>>>>
>>>> switch (; // empty init-statement
>>>>        x) {
>>>> ...
>>>> }
>>>>
>>>> for (; // <- empty init-statement
>>>>     int y : S())
>>>> ;
>>>> }
>>>>
>>>> As usual, things may or may not go sideways in the presence of macros.
>>>> While evaluating this diag on my codebase of interest, it was unsurprisingly
>>>> discovered that Google Test macros are *very* prone to this.
>>>> And it seems many issues are deep within the GTest itself, not
>>>> in the snippets passed from the codebase that uses GTest.
>>>>
>>>> So after some thought, i decided not do issue a diagnostic if the semi
>>>> is within *any* macro, be it either from the normal header, or system header.
>>>>
>>>> Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=39111 | PR39111 ]]
>>>>
>>>> Reviewers: rsmith, aaron.ballman, efriedma
>>>>
>>>> Reviewed By: aaron.ballman
>>>>
>>>> Subscribers: cfe-commits
>>>>
>>>> Differential Revision: https://reviews.llvm.org/D52695
>>>>
>>>> Added:
>>>>    cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp
>>>>    cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp
>>>> Modified:
>>>>    cfe/trunk/docs/ReleaseNotes.rst
>>>>    cfe/trunk/include/clang/Basic/DiagnosticGroups.td
>>>>    cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td
>>>>    cfe/trunk/include/clang/Parse/Parser.h
>>>>    cfe/trunk/lib/Parse/ParseExprCXX.cpp
>>>>    cfe/trunk/lib/Parse/ParseStmt.cpp
>>>>
>>>> Modified: cfe/trunk/docs/ReleaseNotes.rst
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/ReleaseNotes.rst?rev=347339&r1=347338&r2=347339&view=diff
>>>> ==============================================================================
>>>> --- cfe/trunk/docs/ReleaseNotes.rst (original)
>>>> +++ cfe/trunk/docs/ReleaseNotes.rst Tue Nov 20 10:59:05 2018
>>>> @@ -55,6 +55,63 @@ Major New Features
>>>> Improvements to Clang's diagnostics
>>>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>>
>>>> +- ``-Wextra-semi-stmt`` is a new diagnostic that diagnoses extra semicolons,
>>>> +  much like ``-Wextra-semi``. This new diagnostic diagnoses all *unnecessary*
>>>> +  null statements (expression statements without an expression), unless: the
>>>> +  semicolon directly follows a macro that was expanded to nothing or if the
>>>> +  semicolon is within the macro itself. This applies to macros defined in system
>>>> +  headers as well as user-defined macros.
>>>> +
>>>> +  .. code-block:: c++
>>>> +
>>>> +      #define MACRO(x) int x;
>>>> +      #define NULLMACRO(varname)
>>>> +
>>>> +      void test() {
>>>> +        ; // <- warning: ';' with no preceding expression is a null statement
>>>> +
>>>> +        while (true)
>>>> +          ; // OK, it is needed.
>>>> +
>>>> +        switch (my_enum) {
>>>> +        case E1:
>>>> +          // stuff
>>>> +          break;
>>>> +        case E2:
>>>> +          ; // OK, it is needed.
>>>> +        }
>>>> +
>>>> +        MACRO(v0;) // Extra semicolon, but within macro, so ignored.
>>>> +
>>>> +        MACRO(v1); // <- warning: ';' with no preceding expression is a null statement
>>>> +
>>>> +        NULLMACRO(v2); // ignored, NULLMACRO expanded to nothing.
>>>> +      }
>>>> +
>>>> +- ``-Wempty-init-stmt`` is a new diagnostic that diagnoses empty init-statements
>>>> +  of ``if``, ``switch``, ``range-based for``, unless: the semicolon directly
>>>> +  follows a macro that was expanded to nothing or if the semicolon is within the
>>>> +  macro itself (both macros from system headers, and normal macros). This
>>>> +  diagnostic is in the ``-Wextra-semi-stmt`` group and is enabled in
>>>> +  ``-Wextra``.
>>>> +
>>>> +  .. code-block:: c++
>>>> +
>>>> +      void test() {
>>>> +        if(; // <- warning: init-statement of 'if' is a null statement
>>>> +           true)
>>>> +          ;
>>>> +
>>>> +        switch (; // <- warning: init-statement of 'switch' is a null statement
>>>> +                x) {
>>>> +          ...
>>>> +        }
>>>> +
>>>> +        for (; // <- warning: init-statement of 'range-based for' is a null statement
>>>> +             int y : S())
>>>> +          ;
>>>> +      }
>>>> +
>>>>
>>>> Non-comprehensive list of changes in this release
>>>> -------------------------------------------------
>>>>
>>>> Modified: cfe/trunk/include/clang/Basic/DiagnosticGroups.td
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticGroups.td?rev=347339&r1=347338&r2=347339&view=diff
>>>> ==============================================================================
>>>> --- cfe/trunk/include/clang/Basic/DiagnosticGroups.td (original)
>>>> +++ cfe/trunk/include/clang/Basic/DiagnosticGroups.td Tue Nov 20 10:59:05 2018
>>>> @@ -162,6 +162,8 @@ def GNUEmptyStruct : DiagGroup<"gnu-empt
>>>> def ExtraTokens : DiagGroup<"extra-tokens">;
>>>> def CXX98CompatExtraSemi : DiagGroup<"c++98-compat-extra-semi">;
>>>> def CXX11ExtraSemi : DiagGroup<"c++11-extra-semi">;
>>>> +def EmptyInitStatement : DiagGroup<"empty-init-stmt">;
>>>> +def ExtraSemiStmt : DiagGroup<"extra-semi-stmt", [EmptyInitStatement]>;
>>>> def ExtraSemi : DiagGroup<"extra-semi", [CXX98CompatExtraSemi,
>>>>                                          CXX11ExtraSemi]>;
>>>>
>>>> @@ -768,7 +770,8 @@ def Extra : DiagGroup<"extra", [
>>>>     MissingMethodReturnType,
>>>>     SignCompare,
>>>>     UnusedParameter,
>>>> -    NullPointerArithmetic
>>>> +    NullPointerArithmetic,
>>>> +    EmptyInitStatement
>>>>   ]>;
>>>>
>>>> def Most : DiagGroup<"most", [
>>>>
>>>> Modified: cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td?rev=347339&r1=347338&r2=347339&view=diff
>>>> ==============================================================================
>>>> --- cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td (original)
>>>> +++ cfe/trunk/include/clang/Basic/DiagnosticParseKinds.td Tue Nov 20 10:59:05 2018
>>>> @@ -53,6 +53,10 @@ def ext_extra_semi_cxx11 : Extension<
>>>> def warn_extra_semi_after_mem_fn_def : Warning<
>>>>   "extra ';' after member function definition">,
>>>>   InGroup<ExtraSemi>, DefaultIgnore;
>>>> +def warn_null_statement : Warning<
>>>> +  "empty expression statement has no effect; "
>>>> +  "remove unnecessary ';' to silence this warning">,
>>>> +  InGroup<ExtraSemiStmt>, DefaultIgnore;
>>>>
>>>> def ext_thread_before : Extension<"'__thread' before '%0'">;
>>>> def ext_keyword_as_ident : ExtWarn<
>>>> @@ -554,6 +558,9 @@ def ext_for_range_init_stmt : ExtWarn<
>>>> def warn_cxx17_compat_for_range_init_stmt : Warning<
>>>>   "range-based for loop initialization statements are incompatible with "
>>>>   "C++ standards before C++2a">, DefaultIgnore, InGroup<CXXPre2aCompat>;
>>>> +def warn_empty_init_statement : Warning<
>>>> +  "empty initialization statement of '%select{if|switch|range-based for}0' "
>>>> +  "has no effect">, InGroup<EmptyInitStatement>, DefaultIgnore;
>>>>
>>>> // C++ derived classes
>>>> def err_dup_virtual : Error<"duplicate 'virtual' in base specifier">;
>>>>
>>>> Modified: cfe/trunk/include/clang/Parse/Parser.h
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Parse/Parser.h?rev=347339&r1=347338&r2=347339&view=diff
>>>> ==============================================================================
>>>> --- cfe/trunk/include/clang/Parse/Parser.h (original)
>>>> +++ cfe/trunk/include/clang/Parse/Parser.h Tue Nov 20 10:59:05 2018
>>>> @@ -1888,6 +1888,7 @@ private:
>>>>   StmtResult ParseCompoundStatement(bool isStmtExpr,
>>>>                                     unsigned ScopeFlags);
>>>>   void ParseCompoundStatementLeadingPragmas();
>>>> +  bool ConsumeNullStmt(StmtVector &Stmts);
>>>>   StmtResult ParseCompoundStatementBody(bool isStmtExpr = false);
>>>>   bool ParseParenExprOrCondition(StmtResult *InitStmt,
>>>>                                  Sema::ConditionResult &CondResult,
>>>>
>>>> Modified: cfe/trunk/lib/Parse/ParseExprCXX.cpp
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Parse/ParseExprCXX.cpp?rev=347339&r1=347338&r2=347339&view=diff
>>>> ==============================================================================
>>>> --- cfe/trunk/lib/Parse/ParseExprCXX.cpp (original)
>>>> +++ cfe/trunk/lib/Parse/ParseExprCXX.cpp Tue Nov 20 10:59:05 2018
>>>> @@ -1773,7 +1773,13 @@ Sema::ConditionResult Parser::ParseCXXCo
>>>>     //   if (; true);
>>>>     if (InitStmt && Tok.is(tok::semi)) {
>>>>       WarnOnInit();
>>>> -      SourceLocation SemiLoc = ConsumeToken();
>>>> +      SourceLocation SemiLoc = Tok.getLocation();
>>>> +      if (!Tok.hasLeadingEmptyMacro() && !SemiLoc.isMacroID()) {
>>>> +        Diag(SemiLoc, diag::warn_empty_init_statement)
>>>> +            << (CK == Sema::ConditionKind::Switch)
>>>> +            << FixItHint::CreateRemoval(SemiLoc);
>>>> +      }
>>>> +      ConsumeToken();
>>>>       *InitStmt = Actions.ActOnNullStmt(SemiLoc);
>>>>       return ParseCXXCondition(nullptr, Loc, CK);
>>>>     }
>>>>
>>>> Modified: cfe/trunk/lib/Parse/ParseStmt.cpp
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Parse/ParseStmt.cpp?rev=347339&r1=347338&r2=347339&view=diff
>>>> ==============================================================================
>>>> --- cfe/trunk/lib/Parse/ParseStmt.cpp (original)
>>>> +++ cfe/trunk/lib/Parse/ParseStmt.cpp Tue Nov 20 10:59:05 2018
>>>> @@ -930,6 +930,34 @@ void Parser::ParseCompoundStatementLeadi
>>>>
>>>> }
>>>>
>>>> +/// Consume any extra semi-colons resulting in null statements,
>>>> +/// returning true if any tok::semi were consumed.
>>>> +bool Parser::ConsumeNullStmt(StmtVector &Stmts) {
>>>> +  if (!Tok.is(tok::semi))
>>>> +    return false;
>>>> +
>>>> +  SourceLocation StartLoc = Tok.getLocation();
>>>> +  SourceLocation EndLoc;
>>>> +
>>>> +  while (Tok.is(tok::semi) && !Tok.hasLeadingEmptyMacro() &&
>>>> +         Tok.getLocation().isValid() && !Tok.getLocation().isMacroID()) {
>>>> +    EndLoc = Tok.getLocation();
>>>> +
>>>> +    // Don't just ConsumeToken() this tok::semi, do store it in AST.
>>>> +    StmtResult R = ParseStatementOrDeclaration(Stmts, ACK_Any);
>>>> +    if (R.isUsable())
>>>> +      Stmts.push_back(R.get());
>>>> +  }
>>>> +
>>>> +  // Did not consume any extra semi.
>>>> +  if (EndLoc.isInvalid())
>>>> +    return false;
>>>> +
>>>> +  Diag(StartLoc, diag::warn_null_statement)
>>>> +      << FixItHint::CreateRemoval(SourceRange(StartLoc, EndLoc));
>>>> +  return true;
>>>> +}
>>>> +
>>>> /// ParseCompoundStatementBody - Parse a sequence of statements and invoke the
>>>> /// ActOnCompoundStmt action.  This expects the '{' to be the current token, and
>>>> /// consume the '}' at the end of the block.  It does not manipulate the scope
>>>> @@ -992,6 +1020,9 @@ StmtResult Parser::ParseCompoundStatemen
>>>>       continue;
>>>>     }
>>>>
>>>> +    if (ConsumeNullStmt(Stmts))
>>>> +      continue;
>>>> +
>>>>     StmtResult R;
>>>>     if (Tok.isNot(tok::kw___extension__)) {
>>>>       R = ParseStatementOrDeclaration(Stmts, ACK_Any);
>>>> @@ -1588,10 +1619,15 @@ StmtResult Parser::ParseForStatement(Sou
>>>>   ParsedAttributesWithRange attrs(AttrFactory);
>>>>   MaybeParseCXX11Attributes(attrs);
>>>>
>>>> +  SourceLocation EmptyInitStmtSemiLoc;
>>>> +
>>>>   // Parse the first part of the for specifier.
>>>>   if (Tok.is(tok::semi)) {  // for (;
>>>>     ProhibitAttributes(attrs);
>>>>     // no first part, eat the ';'.
>>>> +    SourceLocation SemiLoc = Tok.getLocation();
>>>> +    if (!Tok.hasLeadingEmptyMacro() && !SemiLoc.isMacroID())
>>>> +      EmptyInitStmtSemiLoc = SemiLoc;
>>>>     ConsumeToken();
>>>>   } else if (getLangOpts().CPlusPlus && Tok.is(tok::identifier) &&
>>>>              isForRangeIdentifier()) {
>>>> @@ -1723,6 +1759,11 @@ StmtResult Parser::ParseForStatement(Sou
>>>>                    : diag::ext_for_range_init_stmt)
>>>>               << (FirstPart.get() ? FirstPart.get()->getSourceRange()
>>>>                                   : SourceRange());
>>>> +          if (EmptyInitStmtSemiLoc.isValid()) {
>>>> +            Diag(EmptyInitStmtSemiLoc, diag::warn_empty_init_statement)
>>>> +                << /*for-loop*/ 2
>>>> +                << FixItHint::CreateRemoval(EmptyInitStmtSemiLoc);
>>>> +          }
>>>>         }
>>>>       } else {
>>>>         ExprResult SecondExpr = ParseExpression();
>>>>
>>>> Added: cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp?rev=347339&view=auto
>>>> ==============================================================================
>>>> --- cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp (added)
>>>> +++ cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt-in-init-statement.cpp Tue Nov 20 10:59:05 2018
>>>> @@ -0,0 +1,64 @@
>>>> +// RUN: %clang_cc1 -fsyntax-only -Wextra -std=c++2a -verify %s
>>>> +// RUN: %clang_cc1 -fsyntax-only -Wextra-semi-stmt -std=c++2a -verify %s
>>>> +// RUN: %clang_cc1 -fsyntax-only -Wempty-init-stmt -std=c++2a -verify %s
>>>> +// RUN: cp %s %t
>>>> +// RUN: %clang_cc1 -x c++ -Wempty-init-stmt -std=c++2a -fixit %t
>>>> +// RUN: %clang_cc1 -x c++ -Wempty-init-stmt -std=c++2a -Werror %t
>>>> +
>>>> +struct S {
>>>> +  int *begin();
>>>> +  int *end();
>>>> +};
>>>> +
>>>> +void naive(int x) {
>>>> +  if (; true) // expected-warning {{empty initialization statement of 'if' has no effect}}
>>>> +    ;
>>>> +
>>>> +  switch (; x) { // expected-warning {{empty initialization statement of 'switch' has no effect}}
>>>> +  }
>>>> +
>>>> +  for (; int y : S()) // expected-warning {{empty initialization statement of 'range-based for' has no effect}}
>>>> +    ;
>>>> +
>>>> +  for (;;) // OK
>>>> +    ;
>>>> +}
>>>> +
>>>> +#define NULLMACRO
>>>> +
>>>> +void with_null_macro(int x) {
>>>> +  if (NULLMACRO; true)
>>>> +    ;
>>>> +
>>>> +  switch (NULLMACRO; x) {
>>>> +  }
>>>> +
>>>> +  for (NULLMACRO; int y : S())
>>>> +    ;
>>>> +}
>>>> +
>>>> +#define SEMIMACRO ;
>>>> +
>>>> +void with_semi_macro(int x) {
>>>> +  if (SEMIMACRO true)
>>>> +    ;
>>>> +
>>>> +  switch (SEMIMACRO x) {
>>>> +  }
>>>> +
>>>> +  for (SEMIMACRO int y : S())
>>>> +    ;
>>>> +}
>>>> +
>>>> +#define PASSTHROUGHMACRO(x) x
>>>> +
>>>> +void with_passthrough_macro(int x) {
>>>> +  if (PASSTHROUGHMACRO(;) true)
>>>> +    ;
>>>> +
>>>> +  switch (PASSTHROUGHMACRO(;) x) {
>>>> +  }
>>>> +
>>>> +  for (PASSTHROUGHMACRO(;) int y : S())
>>>> +    ;
>>>> +}
>>>>
>>>> Added: cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp?rev=347339&view=auto
>>>> ==============================================================================
>>>> --- cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp (added)
>>>> +++ cfe/trunk/test/Parser/extra-semi-resulting-in-nullstmt.cpp Tue Nov 20 10:59:05 2018
>>>> @@ -0,0 +1,96 @@
>>>> +// RUN: %clang_cc1 -fsyntax-only -Wextra-semi-stmt -verify %s
>>>> +// RUN: cp %s %t
>>>> +// RUN: %clang_cc1 -x c++ -Wextra-semi-stmt -fixit %t
>>>> +// RUN: %clang_cc1 -x c++ -Wextra-semi-stmt -Werror %t
>>>> +
>>>> +#define GOODMACRO(varname) int varname
>>>> +#define BETTERMACRO(varname) GOODMACRO(varname);
>>>> +#define NULLMACRO(varname)
>>>> +
>>>> +enum MyEnum {
>>>> +  E1,
>>>> +  E2
>>>> +};
>>>> +
>>>> +void test() {
>>>> +  ; // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
>>>> +  ;
>>>> +
>>>> +  // This removal of extra semi also consumes all the comments.
>>>> +  // clang-format: off
>>>> +  ;;;
>>>> +  // clang-format: on
>>>> +
>>>> +  // clang-format: off
>>>> +  ;NULLMACRO(ZZ);
>>>> +  // clang-format: on
>>>> +
>>>> +  {}; // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
>>>> +
>>>> +  {
>>>> +    ; // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
>>>> +  }
>>>> +
>>>> +  if (true) {
>>>> +    ; // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
>>>> +  }
>>>> +
>>>> +  GOODMACRO(v0); // OK
>>>> +
>>>> +  GOODMACRO(v1;) // OK
>>>> +
>>>> +  BETTERMACRO(v2) // OK
>>>> +
>>>> +  BETTERMACRO(v3;) // Extra ';', but within macro expansion, so ignored.
>>>> +
>>>> +  BETTERMACRO(v4); // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
>>>> +
>>>> +  BETTERMACRO(v5;); // expected-warning {{empty expression statement has no effect; remove unnecessary ';' to silence this warning}}
>>>> +
>>>> +  NULLMACRO(v6) // OK
>>>> +
>>>> +  NULLMACRO(v7); // OK. This could be either GOODMACRO() or BETTERMACRO() situation, so we can't know we can drop it.
>>>> +
>>>> +  if (true)
>>>> +    ; // OK
>>>> +
>>>> +  while (true)
>>>> +    ; // OK
>>>> +
>>>> +  do
>>>> +    ; // OK
>>>> +  while (true);
>>>> +
>>>> +  for (;;) // OK
>>>> +    ;      // OK
>>>> +
>>>> +  MyEnum my_enum;
>>>> +  switch (my_enum) {
>>>> +  case E1:
>>>> +    // stuff
>>>> +    break;
>>>> +  case E2:; // OK
>>>> +  }
>>>> +
>>>> +  for (;;) {
>>>> +    for (;;) {
>>>> +      goto contin_outer;
>>>> +    }
>>>> +  contin_outer:; // OK
>>>> +  }
>>>> +}
>>>> +
>>>> +;
>>>> +
>>>> +namespace NS {};
>>>> +
>>>> +void foo(int x) {
>>>> +  switch (x) {
>>>> +  case 0:
>>>> +    [[fallthrough]];
>>>> +  case 1:
>>>> +    return;
>>>> +  }
>>>> +}
>>>> +
>>>> +[[]];
>>>>
>>>>
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