Suggest include file for popular standard functions

[tbaederr]

If, e.g. write:

int main() {
  std::cout << "Hello world\n";
}

I think it would be nice for the compiler to suggest importing <iostream>. If I later write

#include <iostream>
int main() {
  std::cout << "Hello world\n";

  int a = 0, b = 0;
  std::memcpy(&a, &b, 4);
}

it would be nice if it suggested importing <cstring>. I think a simple internal list of popular functions would be enough, we don't need to get fancy here.

I don't know if this collides with functionality that clangd or similar tools already provide though.

Popular D compilers have similar functionality:

~ » cat test.d
void main() {
  writeln("hello world");
}

~ » ldc2 test.d
test.d(7): Error: `writeln` is not defined, perhaps `import std.stdio;` is needed?

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Timm Baeder (tbaederr)

If, e.g. write: ```c++ int main() { std::cout << "Hello world\n"; } ``` I think it would be nice for the compiler to suggest importing `<iostream>`. If I later write ```c++ #include <iostream> int main() { std::cout << "Hello world\n";

int a = 0, b = 0;
std::memcpy(&a, &b, 4);
}

it would be nice if it suggested importing `<cstring>`. I think a simple internal list of popular functions would be enough, we don't need to get fancy here.

I don't know if this collides with functionality that clangd or similar tools already provide though.

Popular D compilers have similar functionality:
```console
~ » cat test.d
void main() {
  writeln("hello world");
}

~ » ldc2 test.d
test.d(7): Error: `writeln` is not defined, perhaps `import std.stdio;` is needed?

[zyn0217]

+1 for the idea. IIRC GCC has already offered similar suggestions in diagnostics.

Afaik clangd is maintaining a symbol mapping located in clang/lib/Tooling/Inclusions/Stdlib for its code completion feature. Perhaps the diagnostics can also benefit from that?

[tbaederr]

Yeah would be nice if we could reuse that mapping

[tbaederr]

Adding the good-first-issue label, although I believe this is best for someone who already has some experience in the code base.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Timm Baeder (tbaederr)

If, e.g. write: ```c++ int main() { std::cout << "Hello world\n"; } ``` I think it would be nice for the compiler to suggest importing `<iostream>`. If I later write ```c++ #include <iostream> int main() { std::cout << "Hello world\n";

int a = 0, b = 0;
std::memcpy(&a, &b, 4);
}

it would be nice if it suggested importing `<cstring>`. I think a simple internal list of popular functions would be enough, we don't need to get fancy here.

I don't know if this collides with functionality that clangd or similar tools already provide though.

Popular D compilers have similar functionality:
```console
~ » cat test.d
void main() {
  writeln("hello world");
}

~ » ldc2 test.d
test.d(7): Error: `writeln` is not defined, perhaps `import std.stdio;` is needed?

[jongmyeong-choi]

From a quick inspection, it seems that searching the symbol map and emitting a note diagnostic could be done where err_undeclared_var_use is triggered.
I'll look into implementing this. Could you assign this issue to me?

[ldionne]

FWIW I think this is extremely high value for libc++. It would help people to get their includes clean and would allow libc++ to drop a lot of complexity around maintaining transitive includes for the sole purpose of not breaking people's (technically invalid) code.