------- Comment #3 from jakub at gcc dot gnu dot org 2009-11-24 08:10 ------- -Wunused-variable is checking the TREE_USED flag, which is set quite early during the parsing (in C FE build_external_ref, in C++ FE mark_used called from finish_id_expression). At that point the parser doesn't track whether it is LHS of an assignment or RHS in a way that could be easily checked in those routines unfortunately (and just setting a some parser flag e.g. around lhs = c_parser_conditional_expression (parser, after); in c_parser_expr_no_commas is insufficient, we want that only for the actual LHS, but not other variables referenced in that, say in x[i] = 5; i must be marked as used, etc.). So implementing this isn't just a one-liner.
On the other side, I agree the warning is very useful in some cases, not so much when var is only used on LHS where RHS has no side-effects, that is just optimized out, but if an otherwise unused variable is initialized using some expensive function call the user might find out that the call is just a waste of time and remove it, even when it is not marked pure/const and thus the compiler can't optimize it itself. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36390
