[clangd] allow extracting to variable for complete lambda expressions

[5chmidti]

Summary

Support for extracting complete lambda expressions, e.g. extracting a lambda from a callexpr (e.g. algorithms/ranges) to a named variable.

[5chmidti]

Add change to release notes

[nridge]

LGTM, thanks!

clang-tools-extra/docs/ReleaseNotes.rst

86

Maybe add a "Code Actions" section for this?

[kadircet]

branch cut is upon us, so it'd be great if you can land this soon (or let us know if you don't have commit access)

clang-tools-extra/clangd/refactor/tweaks/ExtractVariable.cpp

176–178

nit: can we re-write this as: !isa<LambdaExpr>(Stmt) || InsertionPoint->Selected == SelectionTree::Complete ?

clang-tools-extra/docs/ReleaseNotes.rst

86

+1

[5chmidti]

1. addressed comments

2 bugfix (sorry, should've said something)
Local variables inside the lambda were previously added to the ReferencedDecls vector and would block the action inside of exprIsValidOutside (declarations are inside of the lambda).
Now only consider DeclRefExprs in the lambda captures.

3. I don't have commit access, someone else would need to commit this.

I don't know if you want to delay this patch due to the bugfix or not, either way is fine with me.

[kadircet]

sorry i'll need to take a look at this again.

[nridge]

clang-tools-extra/clangd/refactor/tweaks/ExtractVariable.cpp

84

What about variables referenced implicitly via a capture-default?

For example, consider:

int main() {
  if (int a = 1)
    if ([[ [&](){ return a + 1; } ]]() == 4)
      a = a + 1;
}

Here, the attempted extraction point is before the outer if, which is before the declaration of a, and the fact that the lambda references a should prevent that.

[nridge]

clang-tools-extra/clangd/refactor/tweaks/ExtractVariable.cpp

86

On the other hand, the exclusion of local variables declared inside lambdas should also apply to lambdas which are subexpressions of the selected expression. Consider:

template <typename T>
auto call(T t) { return t(); }

int main() {
  return [[ call([](){ int a = 1; return a + 1; }) ]] + 5;
}

Here, the presence of the local variable a inside the lambda should not interfere with the ability to extract the call(...) expression.

[5chmidti]

You're right, I'll incorporate the logic for lambdas into FindDeclRefsVisitor and change the docs/comments/commit message to reflect this. I will try to think of more edge cases to test as well. Though I'll be busy until mid march with uni, so there will be no new revisions until then.