Compiling ambiguous macro invocation hangs forever

[stevenblenkinsop]

Trying to compile this code containing an ambiguous macro causes the compiler to hang forever and max out memory:

macro_rules! make_vec(
    (a $e1:expr $($(, a $e2:expr)*)*) => ([$e1 $($(, $e2)*)*]);
)

fn main() {
    let _ = make_vec!(a 1, a 2, a 3);
}

rustc 0.5
host: x86_64-apple-darwin

[catamorphism]

Reproduced as of 4a5e8c5

Backtrace:

#0  0x0000000100a937c5 in glue_drop_19989 ()
#1  0x0000000100a93aa2 in glue_drop_20005 ()
#2  0x0000000100a93a16 in glue_free_20001 ()
#3  0x0000000100a939a1 in glue_drop_19999 ()
#4  0x0000000100b4c3d8 in ext::tt::macro_parser::parse::anon::expr_fn_22797 ()
#5  0x00000001008780fc in uint::range_step::_dbf93249fa86a4bd::_06 ()
#6  0x0000000100877fa9 in uint::range::_555af5271cf0ae9::_06 ()
#7  0x0000000100b474f4 in ext::tt::macro_parser::parse::_4cb9211e6da3397::_06 ()
#8  0x0000000100b53045 in ext::tt::macro_rules::add_new_extension::generic_extension::anon::expr_fn_22947 ()
#9  0x0000000100b52c96 in iter::eachi_22942::anon::expr_fn_22945 ()
#10 0x000000010087436b in vec::__extensions__::each_6884::anon::expr_fn_6886 ()
#11 0x0000000100872e7b in vec::each_6846::anon::expr_fn_6852 ()
#12 0x0000000100872d6e in vec::as_imm_buf_6849::_76567a8977b7947f::_06 ()
#13 0x0000000100872c1a in vec::each_6846::_c97c9de192e2ac95::_06 ()
#14 0x00000001008742f5 in vec::__extensions__::each_6884::_42d0327b1fcd8913::_06 ()
#15 0x0000000100b52c13 in iter::eachi_22942::_43ae4c53d7f2785::_06 ()
#16 0x0000000100b52b93 in vec::__extensions__::eachi_22939::_d7f9bbd7ce525cae::_06 ()
#17 0x0000000100b5278b in ext::tt::macro_rules::add_new_extension::generic_extension::_612cddf6d24ae4::_06 ()
#18 0x0000000100b56e9b in ext::tt::macro_rules::add_new_extension::anon::expr_fn_23067 ()
#19 0x0000000100afa850 in ext::expand::expand_expr::_3612288c626ca4a::_06 ()
#20 0x0000000100b0483e in ext::expand::expand_crate::anon::expr_fn_21721 ()
#21 0x00000001009905b4 in fold::__extensions__::meth_15920::fold_expr::_a9118ad64aa217e::_06 ()
#22 0x0000000100980de8 in fold::noop_fold_local::anon::expr_fn_15276 ()
#23 0x0000000100967d82 in option::map_14843::_10edfb7beead866d::_06 ()
#24 0x0000000100967ce3 in option::__extensions__::map_14840::_ba1ce0cc6134b934::_06 ()
#25 0x0000000100980c04 in fold::noop_fold_local::_4005d84b6236af3::_06 ()
#26 0x000000010098dc87 in fold::wrap_15853::anon::expr_fn_15856 ()
#27 0x00000001009918df in fold::__extensions__::meth_15941::fold_local::_2326fa4da97afec9::_06 ()
#28 0x000000010096f86b in fold::noop_fold_decl::anon::expr_fn_14996 ()
#29 0x0000000100876934 in vec::map_6941::anon::expr_fn_6944 ()
#30 0x0000000100872e7b in vec::each_6846::anon::expr_fn_6852 ()
#31 0x0000000100872d6e in vec::as_imm_buf_6849::_76567a8977b7947f::_06 ()
#32 0x0000000100872c1a in vec::each_6846::_c97c9de192e2ac95::_06 ()
#33 0x000000010087680d in vec::map_6941::_e529897178cd16c::_06 ()
#34 0x000000010096f331 in fold::noop_fold_decl::_835bf0d0d6aef58::_06 ()
#35 0x000000010098c207 in fold::wrap_15761::anon::expr_fn_15764 ()
#36 0x00000001009901cf in fold::__extensions__::meth_15917::fold_decl::_ddb0f9c66531d997::_06 ()
#37 0x000000010096ad0f in fold::noop_fold_stmt::_cdae6f3fce5642c2::_06 ()
#38 0x000000010098b4c7 in fold::wrap_15715::anon::expr_fn_15718 ()
#39 0x0000000100b009ac in ext::expand::expand_stmt::_b5ba7a6ff0557ece::_06 ()
#40 0x0000000100b058fe in ext::expand::expand_crate::anon::expr_fn_21757 ()
#41 0x000000010098f782 in fold::__extensions__::meth_15908::fold_stmt::_132f4f3220116793::_06 ()
#42 0x000000010096aa38 in fold::noop_fold_block::anon::expr_fn_14926 ()
#43 0x0000000100876934 in vec::map_6941::anon::expr_fn_6944 ()
#44 0x0000000100872e7b in vec::each_6846::anon::expr_fn_6852 ()
#45 0x0000000100872d6e in vec::as_imm_buf_6849::_76567a8977b7947f::_06 ()
#46 0x0000000100872c1a in vec::each_6846::_c97c9de192e2ac95::_06 ()
#47 0x000000010087680d in vec::map_6941::_e529897178cd16c::_06 ()
#48 0x00000001008766fe in vec::__extensions__::map_6938::_dddce24f9d4b3050::_06 ()
#49 0x000000010096a6b9 in fold::noop_fold_block::_3ce2ed96932f52d::_06 ()
#50 0x000000010098ae27 in fold::wrap_15692::anon::expr_fn_15695 ()
#51 0x000000010098f437 in fold::__extensions__::meth_15905::fold_block::_bcc5eee69f78d19e::_06 ()
#52 0x0000000100964f86 in fold::noop_fold_item_underscore::_ee3665e2af9ebe80::_06 ()
#53 0x000000010098f15f in fold::__extensions__::meth_15899::fold_item_underscore::_20b45acd839eff24::_06 ()
#54 0x000000010096396c in fold::noop_fold_item::_1b9a3be6faf8edd7::_06 ()
#55 0x0000000100afd2a9 in ext::expand::expand_item::_b1fa382e62bda90::_06 ()
#56 0x0000000100b0585e in ext::expand::expand_crate::anon::expr_fn_21755 ()
#57 0x000000010098ecff in fold::__extensions__::meth_15893::fold_item::_5921b8e062e01364::_06 ()
#58 0x000000010097d4b8 in fold::noop_fold_mod::anon::expr_fn_15180 ()
#59 0x0000000100877222 in vec::filter_mapped_6955::anon::expr_fn_6958 ()
#60 0x0000000100872e7b in vec::each_6846::anon::expr_fn_6852 ()
#61 0x0000000100872d6e in vec::as_imm_buf_6849::_76567a8977b7947f::_06 ()
#62 0x0000000100872c1a in vec::each_6846::_c97c9de192e2ac95::_06 ()
#63 0x0000000100877108 in vec::filter_mapped_6955::_71cc5ed03fa06ab7::_06 ()
#64 0x000000010097d394 in fold::noop_fold_mod::_1161a4ca3fa6e5bb::_06 ()
#65 0x0000000100afbe0f in ext::expand::expand_mod_items::_8c4164b358186694::_06 ()
#66 0x0000000100b057ce in ext::expand::expand_crate::anon::expr_fn_21753 ()
#67 0x0000000100990f55 in fold::__extensions__::meth_15926::fold_mod::_3e418619a34db868::_06 ()
#68 0x0000000100962173 in fold::noop_fold_crate::_99468a714c2466aa::_06 ()
#69 0x000000010098a787 in fold::wrap_15669::anon::expr_fn_15672 ()
#70 0x000000010098e321 in fold::__extensions__::meth_15876::fold_crate::_78bac320468be4d2::_06 ()
#71 0x0000000100b0445d in ext::expand::expand_crate::_3d56f396e40d45d::_06 ()
#72 0x0000000101ad1406 in driver::driver::compile_rest::anon::expr_fn_81354 ()
#73 0x0000000101ad0b1c in driver::driver::time_81337::_ca8b201e95b5826::_06 ()
#74 0x0000000101acb78e in driver::driver::compile_rest::_4c5957856ca98447::_06 ()
#75 0x0000000101ad6619 in driver::driver::compile_upto::_7ae65df72940ea29::_06 ()
#76 0x0000000101ad6a4c in driver::driver::compile_input::_a9a269979a1247c::_06 ()
#77 0x0000000101b22f18 in run_compiler::_3eed7ed095be4467::_06 ()
#78 0x0000000101b44777 in main::anon::expr_fn_83837 ()
#79 0x0000000101b40660 in monitor::anon::expr_fn_83683 ()
#80 0x0000000101b36dca in task::__extensions__::try_83253::anon::expr_fn_83520 ()
#81 0x00000001000cfe5c in task::spawn::spawn_raw::make_child_wrapper::anon::expr_fn_11434 ()
#82 0x0000000102106a05 in task_start_wrapper (a=0x0) at rust_task.cpp:162
(gdb) 

[catamorphism]

Seems non-critical for 0.7. Nominating for milestone 1 (well-defined).

[graydon]

Accepted for well-defined. we need to write down what is and is not macro-parsable.

[pnkfelix]

visited for bug triage 2013 june 27. still a problem.

It would be good to define whether our goal is to eliminate all such macro definitions, or if it would be acceptable to fail instead at the point of expansion, when the macro expander determines that there is more than one possible parse. (The latter would integrate well with a proper Earley parsing scheme and probably with any reasonable GLR parsing scheme as well.)

[emberian]

Still a problem, unfortunately. I have no knowledge about this, but what @pnkfelix says sounds impressive.

[reem]

Triage: still a problem, no updates.

[mitaa]

This does not compile anymore (PR #20563)
playpen

<anon>:2:17: 2:36 error: `$e1:expr` is followed by a sequence repetition, which is not allowed for `expr` fragments
<anon>:2     (a $e1:expr $($(, a $e2:expr)*)*) => ([$e1 $($(, $e2)*)*]);
                         ^~~~~~~~~~~~~~~~~~~

[Stebalien]

triage: still an issue (on nightly).

[ghost]

Still occurs using this updated code:

macro_rules! make_vec(
    (a $e1:expr $($(, a $e2:expr)*)*) => ([$e1 $($(, $e2)*)*]);
);

fn main() {
    let _ = make_vec!(a 1, a 2, a 3);
}

[jonas-schievink]

This code is enough to trigger the bug:

macro_rules! m {
    ( $()* ) => {}
}

m!();

I guess repetitions should check that their innards can not be not empty.

[dtolnay]

Still an issue in 1.12.0-nightly using the minimal repro in the previous comment.