miri: require (almost) all 1-ZST arguments to be actually passed by RalfJung · Pull Request #156085 · rust-lang/rust

RalfJung · 2026-05-02T12:29:15Z

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We can't ignore all of them since the compiler itself relies on non-capturing closure arguments being ignored.

Fixes rust-lang/miri#4993

Cc @folkertdev since it also changes the checks for variadics.

rustbot · 2026-05-02T12:29:18Z

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @oli-obk, @lcnr

rustbot · 2026-05-02T12:29:20Z

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saethlin · 2026-05-02T16:26:48Z

@bors try @rust-timer queue

rust-timer · 2026-05-02T16:26:51Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

rust-bors · 2026-05-02T16:26:53Z

⌛ Trying commit 403e9b0 with merge 583ebe0…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/25256422338

miri: require (almost) all 1-ZST arguments to be actually passed

saethlin · 2026-05-02T16:44:36Z

+    /// Determine whether this argument can be safely ignored.
+    /// We have to ignore closures that didn't capture anything to ensure that their
+    /// coercion to function pointers is sound. We don't want to ignore anything else
+    /// since we don't actually guarantee that anything is ever ignored.
+    fn is_ignored_arg(arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool {
+        let can_ignore = matches!(arg.layout.ty.kind(), ty::Closure (_def, closure_args) if {
+            closure_args.as_closure().upvar_tys().is_empty()
+        });


I feel like this should mention that normally ZSTs are ignored. But maybe you disagree and think that's base knowledge for Rust argument passing, which would be fine with me.

The connection to closure coercion is turning my brain into a pretzel. Does this mean that we still ignore the argument in this?

fn func<T>(_: T) {} func(|| {});

View changes since the review

I have extended the comment, does that help?

The connection to closure coercion is turning my brain into a pretzel. Does this mean that we still ignore the argument in this?

Yes. That's unfortunate, ideally we'd still report UB if that argument gets ignored. However, I'm not sure it's worth trying to restrict this. It seems non-trivial to figure out whether the callee is actually a closure. But maybe you have a good idea for how to do that?

Conceptually, I think of the closure captures being implicitly passed to closure calls, not literally self. But of course the FnOnce trait takes self so I suspect trying to implement my mental model would be really fiddly, if even possible.

In MIR closures are just normal functions -- closure conversion already happened at that point. So we can't really treat them as anything else.

The best we could do is check the DefId of the callee and figure out if it is a closure, and then treat the first parameter different.

saethlin · 2026-05-02T17:00:37Z

The new explanation is much easier for me to follow. Thanks.

r? saethlin
@bors r+

rust-bors · 2026-05-02T17:00:39Z

📌 Commit 3427f02 has been approved by saethlin

It is now in the queue for this repository.

saethlin · 2026-05-02T17:01:06Z

@bors try-

rust-bors · 2026-05-02T17:01:09Z

Unknown command "try-". Run @bors help to see available commands.

saethlin · 2026-05-02T17:01:34Z

@bors try cancel

rust-bors · 2026-05-02T17:01:36Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/25256422338

rust-bors · 2026-05-02T17:45:00Z

⚠️ A new commit ec802b2e72a59eda62cefbb9c0de81695a860de8 was pushed.

This pull request was unapproved.

RalfJung · 2026-05-07T10:26:03Z

@saethlin what do you think about the latest version of this?

RalfJung · 2026-05-07T10:30:57Z

@theemathas thinking about your example again I think it is in fact covered by our documented ABI guarantees since we say that all 1-ZST are ABI compatible, with both () and the closure environment type are 1-ZST. It just kind of "leaks" the fact that there is actually an argument there that implicitly gets removed by the closure-to-fn-ptr coercion.

saethlin · 2026-05-31T20:47:04Z

@saethlin what do you think about the latest version of this?

I wish our semantics were easier to implement.

@bors r+ rollup=iffy

rust-bors · 2026-05-31T20:47:08Z

📌 Commit 5d2de1b has been approved by saethlin

It is now in the queue for this repository.

RalfJung · 2026-05-31T21:07:23Z

I wish our semantics were easier to implement.

💯

We could avoid this complication by having the closure-to-fn-ptr coercion insert a shim that does the ABI adjustment. But that's outside the parts of the compiler I am comfortable in...

@folkertdev

…hlin miri: require (almost) all 1-ZST arguments to be actually passed We can't ignore *all* of them since the compiler itself relies on non-capturing closure arguments being ignored. Fixes rust-lang/miri#4993 Cc @folkertdev since it also changes the checks for variadics.

@folkertdev

…hlin miri: require (almost) all 1-ZST arguments to be actually passed We can't ignore *all* of them since the compiler itself relies on non-capturing closure arguments being ignored. Fixes rust-lang/miri#4993 Cc @folkertdev since it also changes the checks for variadics.

jhpratt · 2026-06-01T02:30:17Z

@bors r-

#157225 (comment)

rust-bors · 2026-06-01T02:30:23Z

This pull request was unapproved.

This PR was contained in a rollup (#157225), which was unapproved.

View changes since this unapproval

rustbot · 2026-06-01T06:40:12Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rust-bors · 2026-06-01T06:40:12Z

⚠️ A new commit a96cc82f159d32a00e6169074689d2a0f126a206 was pushed.

This PR was contained in a rollup (#157225), which was closed.

RalfJung · 2026-06-01T06:53:34Z

@bors r=saethlin

rust-bors · 2026-06-01T06:53:37Z

📌 Commit a96cc82 has been approved by saethlin

It is now in the queue for this repository.

@folkertdev

…hlin miri: require (almost) all 1-ZST arguments to be actually passed We can't ignore *all* of them since the compiler itself relies on non-capturing closure arguments being ignored. Fixes rust-lang/miri#4993 Cc @folkertdev since it also changes the checks for variadics.

@folkertdev

…hlin miri: require (almost) all 1-ZST arguments to be actually passed We can't ignore *all* of them since the compiler itself relies on non-capturing closure arguments being ignored. Fixes rust-lang/miri#4993 Cc @folkertdev since it also changes the checks for variadics.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 2, 2026

rustbot assigned nikomatsakis May 2, 2026

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 2, 2026

rust-bors Bot pushed a commit that referenced this pull request May 2, 2026

Auto merge of #156085 - RalfJung:miri-ignored-args, r=<try>

583ebe0

miri: require (almost) all 1-ZST arguments to be actually passed

saethlin reviewed May 2, 2026

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Comment thread compiler/rustc_const_eval/src/interpret/call.rs Outdated

RalfJung force-pushed the miri-ignored-args branch from 403e9b0 to e7467e2 Compare May 2, 2026 16:30

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Rollup of 6 pull requests #157225

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Rollup of 7 pull requests #157227

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miri: require (almost) all 1-ZST arguments to be actually passed

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RalfJung force-pushed the miri-ignored-args branch from 5d2de1b to a96cc82 Compare June 1, 2026 06:40

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Rollup of 10 pull requests #157246

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