MIR move elimination [1/6]: MIR preparation by Amanieu · Pull Request #163335 · rust-lang/rust

Amanieu · 2026-09-25T15:41:48Z

This PR contains preparatory work for supporting the new MIR semantics from rust-lang/rfcs#3943.

Adds -Zmir-move-elimination which both opts-in to the new semantics and enables the new optimization pass.
Updates doc comments with the new semantics of MIR under the flag.
Adds the StorageAlloc MIR statement. This is used by the MIR inliner when it needs to borrow a destination place that hasn't been initialized.
Fixes several cases where ZST unit values are not properly initialized.
Fixes async drop elaboration which incorrectly used a move instead of a copy, leading to a use-after-move.

r? tmiasko

rustbot · 2026-09-25T15:41:52Z

Some changes occurred to constck

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Amanieu · 2026-09-25T15:58:55Z

LLM disclosure: this PR is in large part the result of an LLM-assisted audit of all existing MIR transformations against the new MIR semantics as implemented in Miri in #163340. LLMs were also used to generate the boilerplate for the MIR tests, but each test was manually reviewed and adjusted before committing.

tmiasko · 2026-09-25T19:28:01Z

We started with a single pull request with 65 files changed, and now ended up with multiple "smaller" pull requests, where the first one has 115 files changed. This is not exactly what I expected. Could you move some parts to smaller pull requests still. For example parts of "Fix missing ZST initialization in MIR passes" which don't depend on -Zmir-move-elimination at all.

Amanieu · 2026-09-25T20:02:40Z

I'm currently working to make none of the changes in this PR depend on -Zmir-move-elimination. That way we always have MIR that works with both the old and new semantics, and we don't need to make this a target modifier.

The main concern is that this means restricting RemoveZsts to never remove assignments, which is causing a lot of churn in the mir-opt test snapshots. Most of these assignments are later removed by DSE, but most mir-opt tests only run a single MIR pass.

I will then move all of the ZST-related changes to a separate PR since they no longer depend on the flag.

Amanieu · 2026-09-25T22:07:01Z

I've moved the ZST-related changes to #163359.

We started with a single pull request with 65 files changed, and now ended up with multiple "smaller" pull requests, where the first one has 115 files changed. This is not exactly what I expected.

Most of the difference come from this preparation work which adjusts the rest of the compiler to work with the new semantics, which ends up touching a lot of tests. The actual compiler code changes are not that big.

Amanieu · 2026-09-25T23:21:15Z

I moved the MIR validity changes to the Miri PR.

tmiasko · 2026-09-26T21:58:01Z

                (!debuginfo_locals.contains(place.local)).then_some(**place)
            }
            StatementKind::FakeRead(_)
+            | StatementKind::StorageAlloc(_)


Do you know a specific reason why StorageAlloc cannot be removed? (If yes, a comment and a test case would be nice to have).

View changes since the review

tmiasko · 2026-09-26T22:11:39Z

@@ -226,6 +227,7 @@ impl<'tcx> Analysis<'tcx> for MaybeRequiresStorage {
            StatementKind::SetDiscriminant { place, .. } => {
                state.gen_(place.local);
            }
+            StatementKind::StorageAlloc(local) => state.gen_(*local),


Can we turn those StorageAlloc arms into bug!?

View changes since the review

tmiasko · 2026-09-26T22:17:04Z

+            StatementKind::StorageAlloc(l) => {
+                if let Some(final_locals) = self.replacements.place_fragments(l.into()) {
+                    for (_, _, fl) in final_locals {
+                        self.patch.add_statement(location, StatementKind::StorageAlloc(fl));
+                    }
+                    statement.make_nop(true);
+                }
+                return;
+            }


Please add a test case.

View changes since the review

tmiasko · 2026-09-26T22:20:24Z

    pub(crate) fn remove_unused_storage_annotations<'tcx>(&self, body: &mut Body<'tcx>) {
        for data in body.basic_blocks.as_mut_preserves_cfg() {
-            // Remove unnecessary StorageLive and StorageDead annotations.
+            // Remove storage statements for unused locals.
            for statement in data.statements.iter_mut() {
                let keep_statement = match &statement.kind {
-                    StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
-                        self.locals.contains(*local)
-                    }
+                    StatementKind::StorageLive(local)
+                    | StatementKind::StorageDead(local)
+                    | StatementKind::StorageAlloc(local) => self.locals.contains(*local),


Please add a test case.

View changes since the review

tmiasko · 2026-09-26T22:27:11Z

-        if !context.is_use() {
+        // StorageAlloc requires the local to be live.
+        if !context.is_use() && context != PlaceContext::NonUse(NonUseContext::StorageAlloc) {


Please add a test case.

View changes since the review

Add the experimental -Zmir-move-elimination option

a087870

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 25, 2026

rustbot assigned tmiasko Sep 25, 2026

rustbot added T-clippy Relevant to the Clippy team. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Sep 25, 2026

tmiasko reviewed Sep 25, 2026

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Comment thread compiler/rustc_borrowck/src/def_use.rs Outdated

oli-obk added the llm-assisted An LLM-assisted PR as defined by the LLM policy. Requires ahead-of-time consent by assignee. label Sep 25, 2026

tmiasko reviewed Sep 25, 2026

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Amanieu mentioned this pull request Sep 25, 2026

MIR move elimination [1.5/6]: Ensure ZSTs are always initialized #163359

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Amanieu force-pushed the move-elimination/mir-preparation branch from a4912a3 to 43c741c Compare September 25, 2026 22:05

Amanieu mentioned this pull request Sep 25, 2026

Implement RFC 3943: MIR move elimination #157943

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Amanieu added 4 commits September 26, 2026 00:10

Document move-elimination semantics

9012aed

Add MIR StorageAlloc statement

c6556ba

Use StorageAlloc before borrowing uninitialized locals

8d9f7bd

Use Copy for resume_place in async drop elaboration

7d28b47

Amanieu force-pushed the move-elimination/mir-preparation branch from 43c741c to 7d28b47 Compare September 25, 2026 23:20

This was referenced Sep 26, 2026

MIR move elimination [2/6]: TailCopyToMove #163336

Open

MIR move elimination [3/6]: PreciseLiveness #163337

Draft

MIR move elimination [6/6]: Miri support #163340

Draft

MIR move elimination rust-lang/goals#396

Open

tmiasko reviewed Sep 26, 2026

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