[clang-tidy] clang-analyzer-* checks should be disabled by default

[carlosgalvezp]

Currently, clang-analyzer-* checks are enabled by default. They are not special compared to other checks, so I see no reason why they should have special treatment. They are also heavier than most regular checks so there's a hidden performance penalty by default.

Also, it leads to clang-tidy running silently when no checks are requested. It's better to fail early and tell people that they must explicitly request checks of their choice for clang-tidy to work. Otherwise people may think they have enabled checks, but they haven't.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Carlos Galvez (carlosgalvezp)

Currently, `clang-analyzer-*` checks are enabled by default. They are not special compared to other checks, so I see no reason why they should have special treatment. They are also heavier than most regular checks so there's a hidden performance penalty by default.

Also, it leads to clang-tidy running silently when no checks are requested. It's better to fail early and tell people that they must explicitly request checks of their choice for clang-tidy to work. Otherwise people may think they have enabled checks, but they haven't.

[vbvictor]

After disabling default checkers, we should think about creating a default config that first-time users can just use without much thinking. It will lower entry threshold for projects to integrate clang-tidy in their pipeline, thus more people will use the tool.

When I introduced clang-tidy to a new project, the first question was "what is the default config, let's run it". Luckily for me, I had already config in mind. However, a totally new user can be overwhelmed by big variety of checks, so having a some default config that show "widely known issue" would immediately show how good clang-tidy is.

Maybe we could create a new flag clang-tidy --dump-default-config that will create a .clang-tidy file.

[vbvictor]

Furthermore, if no checks are enabled, we could print a message like this:
"No checks are enabled, please use --check to specify checks or run clang-tidy --dump-default-config to create a default config for clang-tidy".

[carlosgalvezp]

Good ideas! Maybe open a separate ticket for those?

[vbvictor]

I could create a ticket and an RFC for that in near future. In RFC, we should discuss what default checkers we want people to have. I suppose it would be a mixture of bugprone, modernize, misc and performace.

[vbvictor]

Also, maybe we don't need clang-diagnostic-* errors, too.
When running --dump-config on LLVM config, the output is

Checks: 'clang-diagnostic-*,clang-analyzer-*,-*,clang-diagnostic-*,llvm-*,misc-*,

As a user, if I write Checks: '-*,clang-diagnostic-*,llvm-*,misc-*, I would not expect any new check to appear.

Also, some users are confused with clang-diagnostic- when they come from compiler flags, especially when code usually compiled with GCC instead of Clang.

[nicovank]

Furthermore, if no checks are enabled, we could print a message like this: [...]

IMO, if no checks are enabled, the default config should just be used directly. Maybe adding warnings saying 1. the default config/checks will be used and 2. use --dump-default-config to get started with a custom configuration file in that directory. I think it's better user experience to just run with the default config immediately rather than add one setup step.