ICE: expand: unimplemented

[matthiaskrgr]

auto-reduced (treereduce-rust):

macro_rules! a {
    ( $( { $( [ $( ( $( $foo:ident )* ) )* ] )* } )* ) => {
        (
            $,
            $(u128::{
        const ${concat(FOO, $foo)}: i32 = 2;

    }, ),
            //~^ ERROR depth parameter of meta-variable expression `count` must be less than 4
        )
    };
}

macro_rules! b {
    ( $( { $( [ $( $foo:ident )* ] )* } )* ) => ;
}

fn main() {
    a!( { [ (a) ] [ (b c) ] } );

    c!({ a });
}

original code

original:

#![feature(macro_metavar_expr)]

macro_rules! a {
    ( $( { $( [ $( ( $( $foo:ident )* ) )* ] )* } )* ) => {
        (
            ${count($foo, 0)},
            $(u128::from_le_bytes{
        const ${concat(FOO, $foo)}: i32 = 2;
        //~^ ERROR variable `foo` is not recognized in meta-variable expression
    }, 0x40029000000000000000000000000000),
            //~^ ERROR depth parameter of meta-variable expression `count` must be less than 4
        )
    };
}

macro_rules! b {
    ( $( { $( [ $( $foo:ident )* ] )* } )* ) => {
        (
            $( $( $(
                ${ignore($foo)}
                ${index($p:path |)},
                ${index(10)},
                //~^ ERROR depth parameter of meta-variable expression `index` must be less than 3
            )* )* )*
        )
    };
}

macro_rules! c {
    ( $( { $( $foo:ident )* } )* ) => {
        (
            $( $(
                ${ignore($foo)}
                ${len(0)}
                ${len(10)}
                //~^ ERROR depth parameter of meta-variable expression `len` must be less than 2
            )* )*
        )
    };
}

fn main() {
    a!( { [ (a) ] [ (b c) ] } );
    b!( { [ a b ] } );
    c!({ a });
}

Version information

rustc 1.88.0-nightly (efcbb94dc 2025-04-29)
binary: rustc
commit-hash: efcbb94dccbe0fcc1076f031f88332db4a6a2a52
commit-date: 2025-04-29
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

Possibly related line of code:

rust/compiler/rustc_expand/src/mbe/transcribe.rs

Lines 789 to 801 in efcbb94

	NamedMatch::MatchedSeq(named_matches) => {
	let Some((curr_idx, _)) = repeats.last() else {
	return Err(dcx.struct_span_err(sp.entire(), "invalid syntax"));
	};
	match &named_matches[*curr_idx] {
	// FIXME(c410-f3r) Nested repetitions are unimplemented
	MatchedSeq(_) => unimplemented!(),
	MatchedSingle(pnr) => {
	extract_symbol_from_pnr(dcx, pnr, ident.span)?
	}
	}
	}
	NamedMatch::MatchedSingle(pnr) => {

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

error: expected identifier, found `,`
 --> /tmp/icemaker_global_tempdir.3KWQbQU9CIWz/rustc_testrunner_tmpdir_reporting.JmL2GUsKp85j/mvce.rs:4:14
  |
4 |             $,
  |              ^

error[E0658]: the `concat` meta-variable expression is unstable
 --> /tmp/icemaker_global_tempdir.3KWQbQU9CIWz/rustc_testrunner_tmpdir_reporting.JmL2GUsKp85j/mvce.rs:6:16
  |
6 |         const ${concat(FOO, $foo)}: i32 = 2;
  |                ^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #124225 <https://github.com/rust-lang/rust/issues/124225> for more information
  = help: add `#![feature(macro_metavar_expr_concat)]` to the crate attributes to enable
  = note: this compiler was built on 2025-04-29; consider upgrading it if it is out of date

error: expected one of: `*`, `+`, or `?`
 --> /tmp/icemaker_global_tempdir.3KWQbQU9CIWz/rustc_testrunner_tmpdir_reporting.JmL2GUsKp85j/mvce.rs:8:9
  |
8 |     }, ),
  |         ^

error: macro rhs must be delimited
  --> /tmp/icemaker_global_tempdir.3KWQbQU9CIWz/rustc_testrunner_tmpdir_reporting.JmL2GUsKp85j/mvce.rs:15:49
   |
15 |     ( $( { $( [ $( $foo:ident )* ] )* } )* ) => ;
   |                                                 ^


thread 'rustc' panicked at compiler/rustc_expand/src/mbe/transcribe.rs:795:54:
not implemented
stack backtrace:
   0:     0x7b592550bf73 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h70d0b0bf7dabcc62
   1:     0x7b5925c059c7 - core::fmt::write::h95cfb50f7a1a4aed
   2:     0x7b5927055811 - std::io::Write::write_fmt::he4f9617dc005cd64
   3:     0x7b592550bdd2 - std::sys::backtrace::BacktraceLock::print::h6e0bf7d3d1f1df33
   4:     0x7b592550fc1a - std::panicking::default_hook::{{closure}}::h91ad0c7cab45c956
   5:     0x7b592550f79f - std::panicking::default_hook::h99b2391f3ee22ec6
   6:     0x7b5924589d63 - std[14bf288ed479d8e3]::panicking::update_hook::<alloc[d722d77ad22a6fc8]::boxed::Box<rustc_driver_impl[6cad6eb0dd6473d4]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7b5925510493 - std::panicking::rust_panic_with_hook::h6c88edccaa127d3c
   8:     0x7b5925510156 - std::panicking::begin_panic_handler::{{closure}}::h8818f97a6b4edced
   9:     0x7b592550c439 - std::sys::backtrace::__rust_end_short_backtrace::h029db3b8bf007b60
  10:     0x7b592550fe4d - __rustc[7b288356a4aef171]::rust_begin_unwind
  11:     0x7b5921e8e590 - core::panicking::panic_fmt::hd1ef7fd41505b481
  12:     0x7b5921e931dc - core::panicking::panic::ha19f52c22e6e3c4e
  13:     0x7b5926fc9f66 - <rustc_expand[b527a14089d49d16]::expand::MacroExpander>::fully_expand_fragment
  14:     0x7b5926f8b1b7 - <rustc_expand[b527a14089d49d16]::expand::MacroExpander>::expand_crate
  15:     0x7b592657f87b - rustc_interface[31c3fa0d7a8298a7]::passes::resolver_for_lowering_raw
  16:     0x7b592657ec8d - rustc_query_impl[5763ea9fb1f3b9e6]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[5763ea9fb1f3b9e6]::query_impl::resolver_for_lowering_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1fd7e3161c5d5d2a]::query::erase::Erased<[u8; 16usize]>>
  17:     0x7b592657ec79 - <rustc_query_impl[5763ea9fb1f3b9e6]::query_impl::resolver_for_lowering_raw::dynamic_query::{closure#2} as core[12175b0b6b0c4721]::ops::function::FnOnce<(rustc_middle[1fd7e3161c5d5d2a]::ty::context::TyCtxt, ())>>::call_once
  18:     0x7b5926ab9051 - rustc_query_system[c3322cd89c137a94]::query::plumbing::try_execute_query::<rustc_query_impl[5763ea9fb1f3b9e6]::DynamicConfig<rustc_query_system[c3322cd89c137a94]::query::caches::SingleCache<rustc_middle[1fd7e3161c5d5d2a]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[5763ea9fb1f3b9e6]::plumbing::QueryCtxt, false>
  19:     0x7b5926ab8bc0 - rustc_query_impl[5763ea9fb1f3b9e6]::query_impl::resolver_for_lowering_raw::get_query_non_incr::__rust_end_short_backtrace
  20:     0x7b5926b2d4f1 - rustc_interface[31c3fa0d7a8298a7]::passes::create_and_enter_global_ctxt::<core[12175b0b6b0c4721]::option::Option<rustc_interface[31c3fa0d7a8298a7]::queries::Linker>, rustc_driver_impl[6cad6eb0dd6473d4]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  21:     0x7b5926cc8eb9 - rustc_interface[31c3fa0d7a8298a7]::interface::run_compiler::<(), rustc_driver_impl[6cad6eb0dd6473d4]::run_compiler::{closure#0}>::{closure#1}
  22:     0x7b5926c8b574 - std[14bf288ed479d8e3]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[31c3fa0d7a8298a7]::util::run_in_thread_with_globals<rustc_interface[31c3fa0d7a8298a7]::util::run_in_thread_pool_with_globals<rustc_interface[31c3fa0d7a8298a7]::interface::run_compiler<(), rustc_driver_impl[6cad6eb0dd6473d4]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  23:     0x7b5926c8b9b4 - <<std[14bf288ed479d8e3]::thread::Builder>::spawn_unchecked_<rustc_interface[31c3fa0d7a8298a7]::util::run_in_thread_with_globals<rustc_interface[31c3fa0d7a8298a7]::util::run_in_thread_pool_with_globals<rustc_interface[31c3fa0d7a8298a7]::interface::run_compiler<(), rustc_driver_impl[6cad6eb0dd6473d4]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[12175b0b6b0c4721]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  24:     0x7b5926c8cdab - std::sys::pal::unix::thread::Thread::new::thread_start::hbfc0b8ce62741752
  25:     0x7b5920aa370a - <unknown>
  26:     0x7b5920b27aac - <unknown>
  27:                0x0 - <unknown>

error: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: rustc 1.88.0-nightly (efcbb94dc 2025-04-29) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [resolver_for_lowering_raw] getting the resolver for lowering
end of query stack
error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0658`.

@rustbot label +F-macro_metavar_expr

[matthiaskrgr]

smaller

macro_rules! a { ( $( { $ [ $b:c ] } )) => ( $(${ concat(d, $b)} ))}
fn e() {
    a!({})
}

[matthiaskrgr]

Regression in nightly-2024-07-22

get_commits_between returning commits, len: 6
commit[0] 2024-07-20: Auto merge of #127663 - Oneirical:fuzzy-testure, r=jieyouxu
commit[1] 2024-07-20: Auto merge of #128007 - Mark-Simulacrum:bump-version, r=Mark-Simulacrum
commit[2] 2024-07-21: Auto merge of #128011 - matthiaskrgr:rollup-0vmf75y, r=matthiaskrgr
commit[3] 2024-07-21: Auto merge of #127715 - klensy:tests-w, r=Mark-Simulacrum
commit[4] 2024-07-21: Auto merge of #127722 - BoxyUwU:new_adt_const_params_limitations, r=compiler-errors
commit[5] 2024-07-21: Auto merge of #127993 - weihanglo:update-cargo, r=weihanglo

[moxian]

• Sounds like [macro_metavar_expr_concat] Allow concat in repetitions #127720

@rustbot label: +A-macros +S-has-mcve +S-has-bisection -needs-triage