43ba15ca38
In a pattern like
(let loop ([l l])
(define v (syntax-e l))
(cond
[(null? v) 'done]
[else
(loop (datum->syntax #f (cdr v)))]))
the running time was O(N^2) for a list of syntax objects of length N.
That pattern is relevant for traversals that use `syntax-case` like
(let loop ([l l])
(syntax-case l ()
[(a . b) (loop #'b)]
[() 'done]))
Avoid bad behavior by recording in an weak hash table certain pairs
that were previously been produced by `datum->syntax` internally so
that they can be used as-is.
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