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Macros attempting to expand to multiple items silently only expand to the first one #8012
Description
Users who want a macro that expands to a collection items are required to wrap the RHS in {} in order to get them to expand to a whole block. However, if they fail to do so, they unexpectedly just get the first item of the block, rather than an error message.
macro_rules! print_if_nonzero(
($msg:expr, $($field:ident),*) => ( /*{*/
$(
if t.$field != 0 {
println($msg);
}
)*
/*}*/ )
)
struct T { x: int, y: int }
fn main() {
let t = T { x: 0, y: 0 };
print_if_nonzero!("hello", x, y);
}...expands to...
fn main() { let t = T{x: 0, y: 0,}; if t.x != 0 { println("hello"); }; }
Activity
paulstansifer commentedon Jul 23, 2013
Thanks to
kmcon IRC for reporting this.bstrie commentedon Jul 24, 2013
Perhaps this is really just another plaintive cry for someone to tackle #4375.
huonw commentedon Aug 5, 2013
Smaller testcase:
std: adjust str::test_add so that the macro expands to all 3 items (r…
nejucomo commentedon Sep 29, 2013
I don't understand how this is different than #4375. If not justification for the distinction is given, can we mark this as a duplicate?
paulstansifer commentedon Sep 29, 2013
This source of this problem probably isn't specific to items. I'm worried
that there's a lurking (if minor) usability problem in the lack of erroring
out in this case.
kmcallister commentedon Oct 2, 2013
Yeah,
jason_on IRC provided this example using tuples:syntax: indicate an error when a macro ignores trailing tokens.
auto merge of #9673 : huonw/rust/macros, r=catamorphism
Merge rust-lang#8012