[clang] [NFC][clang][AST] Add compile-time dispatch for random access iterators in append() (PR #162000)

[Samarth Narang via cfe-commits]

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@@ -192,9 +204,11 @@ class ASTVector {
       this->grow(C, this->size()+NumInputs);
 
     // Copy the new elements over.
-    // TODO: NEED To compile time dispatch on whether in_iter is a random access
-    // iterator to use the fast uninitialized_copy.
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snarang181 wrote:

@zwuis,thanks for your review.

std::distance and std::uninitialized_copy both rely on iterator tag dispatch, which determines the implementation at overload resolution time rather than through explicit compile-time branching. While std::distance will use an O(1) path for random access iterators, that dispatch happens inside the library and isn’t guaranteed to inline cleanly into templated call sites like append(). Moreover, std::uninitialized_copy always performs an element-by-element loop regardless of iterator category, offering no benefit when the range size (NumInputs) is already known. Using std::uninitialized_copy_n with a precomputed element count avoids the per-iteration sentinel checks and enables the compiler to generate tighter loops.

https://github.com/llvm/llvm-project/pull/162000