Creeping macro expansion - possible exponential time?
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A-macros
C-bug
E-needs-mcve
I-hang
T-compiler
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- Rust
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Description
I am working on the cloudmqtt crate with the test-dsl crate to create some tests. The test-dsl recently got an update to include some fancy macros for argument construction (see here). I am using this macro in here.
Compiling that code never finishes (see further down for rustc log output). It seems to me that the macro expansion, which is recursive, gets slower and slower, as the rustc debug output gets slower and slower.
To get the code and compile it, this should be sufficient:
git clone https://github.com/matthiasbeyer/cloudmqtt
cd cloudmqtt
git fetch origin --tags
git checkout rustc-bug-report-2025-04-03
cargo build --tests
I expected to see this happen: Code compiles.
Instead, this happened: Compilation never finishes.
Meta
rustc --version --verbose:
rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: x86_64-unknown-linux-gnu
release: 1.85.0
LLVM version: 19.1.7
There is no backtrace, because there's no crash.
The last lines of RUSTC_LOG=debug cargo build --tests --verbose are:
rustc_expand::mbe::macro_rules::expand_macro sp=crates/cloudmqtt/tests/check_cases.rs:14:1: 16:2 (#0), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:9:1: 9:21 (#0), node_id=NodeId(4294967040), name=harness#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:17:17: 17:91 (#9), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:43:1: 43:29 (#0), node_id=NodeId(4294967040), name=harness_collect#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:51:9: 55:10 (#10), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:43:1: 43:29 (#0), node_id=NodeId(4294967040), name=harness_collect#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:91:9: 95:10 (#11), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:43:1: 43:29 (#0), node_id=NodeId(4294967040), name=harness_collect#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:131:9: 135:10 (#12), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/datatest-stable-0.3.2/src/macros.rs:43:1: 43:29 (#0), node_id=NodeId(4294967040), name=harness_collect#0
rustc_expand::mbe::macro_rules::expand_macro sp=crates/cloudmqtt/tests/check_cases.rs:45:9: 51:10 (#0), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#15), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:106:17: 106:75 (#16), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:108:17: 108:87 (#16), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:120:9: 120:83 (#15), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:125:17: 125:79 (#15), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:127:20: 127:87 (#15), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=crates/cloudmqtt/tests/check_cases.rs:56:9: 61:12 (#0), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#27), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#28), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#29), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#30), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#31), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#32), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#33), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#34), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#35), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#36), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#37), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#38), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#39), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#40), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#41), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#42), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#43), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#44), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#45), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#46), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
rustc_expand::mbe::macro_rules::expand_macro sp=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:117:13: 117:98 (#47), def_span=/home/m/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/test-dsl-0.3.1/src/macros.rs:67:1: 67:35 (#0), node_id=NodeId(4294967040), name=named_parameters_verb#0
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Reproduce the hang from cloudmqtt/tests/check_cases.rs with cargo build --tests, then inspect the reported rustc_expand::mbe::macro_rules::expand_macro entries and the referenced test-dsl/src/macros.rs definitions. Done means determining whether recursive named_parameters_verb expansion causes the non-terminating build and establishing a compiler-level regression or fix.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 25/100