Perf: avoid unnecessary allocations when transforming Expr
          #8591
        
          
      
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Which issue does this PR close?
N/A. Addresses a
TODOitem intree_node/expr.rsRationale for this change
Inspired by #7942: currently, the
Exprtransformer'stransform_boxandtransform_vecmethods allocate a newBoxandVec, respectively, with potentially harmful performance implications. We can usestd::mem::replaceto avoid this allocation by reusing the existingBox/Vec.What changes are included in this PR?
Switch to in-place mutations in the
transform_boxandtransform_vecmethods onExprs.Are these changes tested?
Yes, existing tests.
Are there any user-facing changes?
No.