parallel compiler: async fns not reproducible

I tried this code:

async fn test1(_: &'_ u8) {}
async fn test2<'s>(_: &'s u8,) {}
fn main() {}

rustc --edition=2024 -Zremap-cwd-prefix=reproducible_dir -Ccodegen-units=1 -Zthreads=30 --crate-type=lib --crate-name=test -o out ; md5sum out

I expected to see this happen:
constant md5sum

Instead, this happened:
the md5sum is flipflopping between d1afc31db4a80828c5c3937fae627a66 and 56d12483eea2eaaca0cd435f83439b59

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rustc --version --verbose:

rustc 1.100.0-nightly (5db7f4be8 2026-09-01)
binary: rustc
commit-hash: 5db7f4be8a36c1b8ae19299469e2be2b0f052c21
commit-date: 2026-09-01
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.0

hmm the problem may be that RPIT already is not reproducible 🤔

trait Foo {
    fn test() -> impl IntoIterator<Item = ()> + Send;
}

struct A;
impl Foo for A {
    fn test() -> impl IntoIterator<Item = ()> + Send {
        []
    }
}

struct B;
impl Foo for B {
    fn test() -> impl IntoIterator<Item = ()> + Send {
        []
    }
}

fn main() {}

c264c8aa81fb4ed39014b7cda7ad1246  out
13aa0ad3cdf07b23e8982f2adde770ba  out
c264c8aa81fb4ed39014b7cda7ad1246  out
c264c8aa81fb4ed39014b7cda7ad1246  out
13aa0ad3cdf07b23e8982f2adde770ba  out
13aa0ad3cdf07b23e8982f2adde770ba  out
13aa0ad3cdf07b23e8982f2adde770ba  out
13aa0ad3cdf07b23e8982f2adde770ba  out
c264c8aa81fb4ed39014b7cda7ad1246  out

I tracked down an issue with RPIT, not sure about the solution yet

The DefIndex of these anon associated types is nondeterministic

 DefIndex(18) DefId(0:18 ~ test[d7b8]::{impl#0}::test::{anon_assoc#0}), def_key DefKey { parent: Some(DefIndex(8)), disambiguated_data: DisambiguatedDefPathData { data: AnonAssocTy("test"), disambiguator: 0 } }, def_path_hash DefPathHash(Fingerprint(15544225586418147366, 3099408607842894160))
 DefIndex(18) DefId(0:18 ~ test[d7b8]::{impl#1}::test::{anon_assoc#0}), def_key DefKey { parent: Some(DefIndex(13)), disambiguated_data: DisambiguatedDefPathData { data: AnonAssocTy("test"), disambiguator: 0 } }, def_path_hash DefPathHash(Fingerprint(15544225586418147366, 13768408182880456915))

The DefIndex of RPITIT types is created lazily in associated_type_for_impl_trait_in_impl called from the associated_types_for_impl_traits_in_trait_or_impl query which is called from associated_item_def_ids. Some of these are likely called in parallel during type checking, racing which assoc item gets DefIndex 18.

There are more cases where DefId-s are synthesized in queries: coroutine_by_move_body_def_id and intern_as_new_static both call create_def.
coroutine_by_move_body_def_id and the RPITIT ones seem solvable by calling these queries in deterministic order.
inter_as_new_static is different. The function is part of const eval and takes an AllocId as an input which is not deterministic AFAIK. There is a good chance this is related to #162203

Rollup merge of rust-lang#162580 - susitsm:parallel-frontend-by-move-…

Unrolled build for #162580 in rollup 162640

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parallel compiler: async fns not reproducible #162202

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parallel compiler: async fns not reproducible #162202

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matthiaskrgr commented on Sep 2, 2026

susitsm commented on Sep 8, 2026

susitsm commented on Sep 8, 2026

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