bug#20268: 25.0.50; pcase-lambda broken

[Stefan Monnier <monnier@iro.umontreal.ca>]

> Why not raise an error instead if there is no match

That would be an acceptable semantics, yes.  I happen to dislike it
because of the performance cost, where in most use-cases you know the
pattern will match and just want to replace a bunch of car/cdr/aref with
a single pattern.

I.e. the code you'd have written without pcase-let would have been

      (let ((a (foo))
            (b (car foo))
            (c (nth 2 foo)))

which would naturally turn into a pattern like `(,b ,_ ,c)
but if you want to signal an error when the pattern fails to match, then
you get 3 additional consp tests plus a null test, at which point the
performance impact can become noticeable.  You can get rid of the null
test with a pattern like `(,b ,_ ,c . ,_) but you can't get rid of the
3 consp tests.


        Stefan