Cleanup: remove useless parameter
Test for issue #6338.
Pointers to bound variables are stored after all patterns are matched.

Pointers to bound variables shouldn't be stored before checking pattern,
otherwise piped patterns can conflict with each other (issue #6338).

Closes #6338.
auto merge of #8544 : dim-an/rust/fix-match-pipes, r=pcwalton

Pointers to bound variables shouldn't be stored before checking pattern,
otherwise piped patterns can conflict with each other (issue #6338).

Closes #6338.