[PATCH] D44826: Add -Wunused-using, a warning that finds unused using declarations.

[Carlos Alberto Enciso via Phabricator via cfe-commits]

CarlosAlbertoEnciso created this revision.
CarlosAlbertoEnciso added reviewers: rsmith, erichkeane, probinson, dblaikie.
CarlosAlbertoEnciso added a project: clang.
Herald added subscribers: cfe-commits, JDevlieghere, aprantl.

Add -Wunused-using, a warning that finds unused using declarations.
Also added an alias -Wunused-usings.

This patch uses a similar approach as the work done for the typedefs
as discussed in:

https://reviews.llvm.org/rC217298

It uses the 'used' and 'referenced' available bits for the declarations.

As consequence of a previous work on Debug Information done by:

https://reviews.llvm.org/D6173

The size of the Debug Information increased by a considerable factor,
as discussed in:

http://clang-developers.42468.n3.nabble.com/r222220-causes-real-debug-info-bloat-td4045257.html

For the below test:

  namespace nsp {
    void foo();
  }
  
  using foo;

A debug information entries are generated for the namespace
and for the using declaration.

In order to reduce the debug information for those specific cases,
the work is divided in 2 parts:

- Emit a warning for the unused using
- Do not generate debug information for the unused using

The current patch deals with the first part and it covers global
and local detection of unused using declarations.

For the below test, I have marked the generated warnings

  namespace nsp {
    void foo();
    int var;
  }
  
  using foo;     <-- warning
  using var;     <-- warning  
  
  void bar() {
    using foo;     <-- warning
    using var;     <-- warning
  }

However, there is a specific case, where an unused using
escapes the warning.

Adding the following function 'bar'

  void bar() {
    nsp::var = 1;
  }

it causes the warning to the global unused 'using var'
not being issued, as the assignment sets the 'used' bit,
despite of using the qualified 'var'.

  namespace nsp {
    void foo();
    int var;
  }using var
  
  using foo;     <-- warning
  using var;     <-- MISSING warning  
  
  void bar() {
    using foo;     <-- warning
    using var;     <-- warning
  }
  
  void bar() {
    nsp::var = 1;
  }

Thanks for your view on this issue and on the general approach.


Repository:
  rC Clang

https://reviews.llvm.org/D44826

Files:
  include/clang/Basic/DiagnosticGroups.td
  include/clang/Basic/DiagnosticSemaKinds.td
  include/clang/Sema/ExternalSemaSource.h
  include/clang/Sema/MultiplexExternalSemaSource.h
  include/clang/Sema/Sema.h
  include/clang/Serialization/ASTBitCodes.h
  include/clang/Serialization/ASTReader.h
  lib/Sema/MultiplexExternalSemaSource.cpp
  lib/Sema/Sema.cpp
  lib/Sema/SemaDecl.cpp
  lib/Sema/SemaDeclCXX.cpp
  lib/Serialization/ASTReader.cpp
  lib/Serialization/ASTWriter.cpp
  test/FixIt/fixit.cpp
  test/Modules/Inputs/module.map
  test/Modules/Inputs/warn-unused-using.h
  test/Modules/warn-unused-using.cpp
  test/SemaCXX/coreturn.cpp
  test/SemaCXX/warn-unused-using-serialize.cpp
  test/SemaCXX/warn-unused-using.cpp

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