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future_not_send using excessive amounts of memory causing OOMs on matrix-sdk under the next trait solver #17812

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#17813
Open
future_not_send using excessive amounts of memory causing OOMs on matrix-sdk under the next trait solver#17812
#17813
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C-bugCategory: Clippy is not doing the correct thingE-mediumCall for participation: Medium difficulty level problem and requires some initial experience.I-ICEIssue: Clippy panicked, giving an Internal Compilation Error (ICE) ❄️L-nurseryLint: Currently in the nursery group

Description

@Hywan
Hywan
opened on Sep 29, 2026

Summary

Hi!

Between nightly-2026-08-21 and nightly-2026-08-22, the memory usage of cargo clippy has increased a lot. It consumes so much memory that Github Actions kills itself when running cargo clippy on our relatively small project (https://github.com/matrix-org/matrix-rust-sdk/).

Reproducer

$ # last nightly (2026-09-28)
$ /usr/bin/time -l rustup run nightly cargo clippy --all-targets --features testing -- -D warnings
[snip]
          3980197888  maximum resident set size

$ # nightly 2026-09-07
$ /usr/bin/time -l rustup run nightly-2026-09-07 cargo clippy --all-targets --features testing -- -D warnings
[snip]
          4348116992  maximum resident set size

$ # nightly 2026-08-22
$ /usr/bin/time -l rustup run nightly-2026-08-22 cargo clippy --all-targets --features testing -- -D warnings
[snip]
          3875028992  maximum resident set size

$ # nightly 2026-08-21
$ /usr/bin/time -l rustup run nightly-2026-08-21 cargo clippy --all-targets --features testing -- -D warnings
[snip]
          1875853312  maximum resident set size

If I put that in a table, in chronological order, with human numbers:

nightly date maximum resident set size
08-21 1.87Gb
08-22 3.88Gb
09-07 4.34Gb
09-28 3.98Gb

Activity

  1. Hywan
    added
    C-bugCategory: Clippy is not doing the correct thing
    on Sep 29, 2026
  2. CommanderStorm
    added
    E-mediumCall for participation: Medium difficulty level problem and requires some initial experience.
    I-ICEIssue: Clippy panicked, giving an Internal Compilation Error (ICE) ❄️
    E-needs-mcveCall for participation: This issue needs a Minimal Complete and Verifiable Example
    on Sep 29, 2026

  3. CommanderStorm commented on Sep 29, 2026

    @CommanderStorm
    CommanderStorm
    on Sep 29, 2026
    Contributor

    tagged as ICE because OOM is a similar-ish category, not because this actually ICEs.

    The issue needs an mcve to where this actually occurs since matrix-sdk is kind of large in terms of

    • which lint/unstable feature and on
    • which code is causing this

    We fixed a few bugs relating to OOMs recently and have 1-2 PRs open regarding this

  4. CommanderStorm
    added
    L-nurseryLint: Currently in the nursery group
    and removed
    E-needs-mcveCall for participation: This issue needs a Minimal Complete and Verifiable Example
    on Sep 29, 2026

  5. CommanderStorm commented on Sep 29, 2026

    @CommanderStorm
    CommanderStorm
    on Sep 29, 2026
    Contributor

    Firstup, can you change the title of the issue to

    future_not_send using excessive amounts of memory causing OOMs on matrix-sdk under the next trait solver

    semi-bisected it.
    Not entirely sure how you get to 4GB RSS, I get more.. xD

    After a bit of an lengthy GDB session, I only see the trait solver and future_not_send being a significant factor in time and memory, which is good.

    Looking into this matrix denying clippy::future_not_send in two places only 😆.

    $ touch crates/matrix-sdk/src/lib.rs && RUSTC_WRAPPER= /usr/bin/time -f "peak RSS: %M KB" cargo +clippy clippy -p matrix-sdk --lib -F testing -- -D warnings
peak RSS: 17 462 164 KB
    But with this small sneaky change to crates/matrix-sdk/src/authentication/matrix/login_builder.rs and crates/matrix-sdk/src/client/mod.rs b/crates/matrix-sdk/src/client/mod.rs you can duckt-tape around this temporarily to unblock you 

    --- a/crates/matrix-sdk/src/authentication/matrix/login_builder.rs
+++ b/crates/matrix-sdk/src/authentication/matrix/login_builder.rs
@@ -12,7 +12,6 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-#![cfg_attr(not(target_family = "wasm"), deny(clippy::future_not_send))]
+// TODO: reenable once <<https://github.com/rust-lang/rust-clippy/issues/17812>> is fixed
+// #![cfg_attr(not(target_family = "wasm"), deny(clippy::future_not_send))]

 #[cfg(feature = "sso-login")]
 use std::future::Future;
diff --git a/crates/matrix-sdk/src/client/mod.rs b/crates/matrix-sdk/src/client/mod.rs
index 7770ce15d..3bdb4c719 100644
--- a/crates/matrix-sdk/src/client/mod.rs
+++ b/crates/matrix-sdk/src/client/mod.rs
@@ -1978,7 +1978,6 @@ impl Client {
     /// client.public_rooms(limit, since, server).await;
     /// # };
     /// ```
-    #[cfg_attr(not(target_family = "wasm"), deny(clippy::future_not_send))]
+    // TODO: reenable once <https://github.com/rust-lang/rust-clippy/issues/17812> is fixed
+    // #[cfg_attr(not(target_family = "wasm"), deny(clippy::future_not_send))]
     pub async fn public_rooms(
         &self,
         limit: Option<u32>,

    Unfortunately matrix uses a LOOT of async, which makes the exact cause of this issue a bit hard to debug

    Image Image

    I think this makes the trait solver really work for its 💸💸.

    I assume this is because we need to prove for every async fn, that the whole returned future is Send. To do this (as I read the code) we need to recursively prove every await that it contains.
    Not quite sure why this needs 17GB of RAM to do.

    Given how these rough screenshots look, that might be kind of nested I assume.

    Clippy could sort of duckt-tape over this issue, by using is_lint_allowed before doing the queries, but I am not entirely convinced that this is the ultimate fix. I opened an PR for this.

    At least for matrix-sdk this seems sort of like "the" fix, since they only enable it in a small portion, meaning that we don't actually need to prove everything about that crate...

    @blyxyas given your work on query-fication looking into the incremental system, do you have advice that is not just essentially duckt-tape?

  6. Hywan
    changed the title [-]Memory usage has increased a lot since nightly 2026-08-22[/-] [+]`future_not_send` using excessive amounts of memory causing OOMs on `matrix-sdk` under the next trait solver[/+] on Sep 30, 2026

  7. Hywan commented on Sep 30, 2026

    @Hywan
    Hywan
    on Sep 30, 2026
    Author

    Thanks for digging into this!

  8. CommanderStorm commented on Sep 30, 2026

    @CommanderStorm
    CommanderStorm
    on Sep 30, 2026
    Contributor

    Thanks for reporting 🏓

  9. blyxyas commented on Sep 30, 2026

    @blyxyas
    blyxyas
    on Sep 30, 2026
    Member

    @CommanderStorm try using DHAT the memory profiler (make sure to use --trace-children and go to Clippy's profile)

  10. Hywan commented on Sep 30, 2026

    @Hywan
    Hywan
    on Sep 30, 2026
    Author

    Our CI is unblocked. Thank you very much. Happy to help and to report anything you need! We can revert the hotfix anytime and try something else if it helps.

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    C-bugCategory: Clippy is not doing the correct thingE-mediumCall for participation: Medium difficulty level problem and requires some initial experience.I-ICEIssue: Clippy panicked, giving an Internal Compilation Error (ICE) ❄️L-nurseryLint: Currently in the nursery group

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