[clang-tools-extra] [clangd] Track IWYU pragmas for non-preamble includes (PR #75612)

[kadir çetinkaya via cfe-commits]

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@@ -113,7 +113,7 @@ class PragmaIncludes {
   llvm::DenseSet<llvm::sys::fs::UniqueID> ShouldKeep;
 
   /// Owns the strings.
-  llvm::BumpPtrAllocator Arena;
+  std::shared_ptr<llvm::BumpPtrAllocator> Arena;
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kadircet wrote:

you're right in theory, but in practice we actually never build ASTs concurrently from a preamble in clangd (we've a single ASTWorker peer per preamble). Moreover mutating the same arena bit is actually never the case, but quite subtle. We build the arena specifically in PPCallbacks and just move it to PI at the end. Hence in practice we have 2 separate PIs, one inside preamble, whose arena is never mutated. Another in ParsedAST, which can refer to strings both inside its own Arena, but also to the arena "kept" alive by the preamble. so I think this was safe for clangd, but too subtle to keep working in the long run I guess.

storing a vector of const arenas in PI instead, and changing the `record` to just push each arena to the end, rather than overwriting the existing Arena (while extending all other structures).

https://github.com/llvm/llvm-project/pull/75612