Implement arbitrary casts in custom mir by maxdexh · Pull Request #160860 · rust-lang/rust

maxdexh · 2026-08-10T14:57:49Z

Implement custom casts using an enum in intrinsics::mir that closely matches the CastKind enum in the compiler.

We have 5 different ways to cast in custom MIR, 3 with functions (which hard code a cast kind), 1 implicit coerce (which only does subtyping i think), 1 with as (which does a few more). They are all different and the error messages don't tell you what to do. Some are (i think) not possible at all.

This came up while testing miri. We currently have to guess the casting syntax until we find the one that generates the MIR we want. This PR changes that by adding a custom mir operation that allows picking an arbitrary cast.

r? RalfJung

rustbot · 2026-08-10T14:57:56Z

RalfJung is not on the review rotation at the moment.
They may take a while to respond.

RalfJung · 2026-08-11T07:32:36Z

I am on vacation this week, so this will have to wait a bit. Or maybe @saethlin has time to look at it earlier.

maxdexh · 2026-08-11T07:34:44Z

This is not done yet anyway, I still gotta figure out how to unflatten the enum and write some tests. But thanks for letting me know :)

maxdexh · 2026-08-16T17:45:41Z

I completely forgot about this thing, I'll get back to it later or tomorrow.

rustbot · 2026-08-18T09:46:46Z

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

maxdexh · 2026-08-18T11:00:06Z

+    ClosureFnPointer(Safety),
+    MutToConstPointer,
+    ArrayToPointer,
+    UnsizePointee,


Note: If this is named Unsize, then std::marker::Unsized doesn't get abbreviated to Unsize in diagnostics anymore.

View changes since the review

then std::marker::Unsized doesn't get abbreviated to Unsize in diagnostics anymore.

Did you mean "abbreviated to Unsized"?

Please file an issue for this... it is strange how adding unstable items changes diagnostics for stable code.

to my knowledge we already do this correctly, it affects nightly code, but a stable compiler will not consider unstable items in the diagnostics

RalfJung · 2026-09-08T07:25:14Z

I'm afraid this is a bit too much for me right now, sorry.
r? @oli-obk

oli-obk

@bors r+ rollup

View changes since this review

rust-bors · 2026-09-10T12:34:45Z

📌 Commit a85dda1 has been approved by oli-obk

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 5. This pull request will be tested once the tree is reopened.

Reason for tree closure: git config problem again

Implement arbitrary casts in custom mir [See zulip](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/extending.20custom.20MIR/with/613727602) Implement custom casts using an enum in `intrinsics::mir` that closely matches the `CastKind` enum in the compiler. We have 5 different ways to cast in custom MIR, 3 with functions (which hard code a cast kind), 1 implicit coerce (which only does subtyping i think), 1 with `as` (which does a few more). They are all different and the error messages don't tell you what to do. Some are (i think) not possible at all. This came up [while testing miri](https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/Valdity.20checks.20for.20references/near/613662285). We currently have to guess the casting syntax until we find the one that generates the MIR we want. This PR changes that by adding a custom mir operation that allows picking an arbitrary cast. r? RalfJung

JonathanBrouwer · 2026-09-10T15:08:27Z

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #162596 (comment)

rust-bors · 2026-09-10T15:08:37Z

This pull request was unapproved.

This PR was contained in a rollup (#162596), which was unapproved.

View changes since this unapproval

maxdexh · 2026-09-10T15:30:17Z

Yeah, the symbol numbering stuff again...

maxdexh · 2026-09-10T15:55:38Z

So this is the same thing as what happened earlier: #160860 (comment)

For some reason, adding symbols sometimes causes the output on that test to gain an extra line break (at least that's what I think is happening)

rustbot · 2026-09-10T16:01:01Z

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.

maxdexh · 2026-09-10T16:24:51Z

I just blessed the output, idk what else I can do about this /shrug
@rustbot ready

RalfJung · 2026-09-10T16:36:21Z

That test has

// Don't break whenever Symbol numbering changes
//@ normalize-stdout: "\d+#" -> "0#"

So there's a lot more that changes in that test and it gets normalized away so we don't see it. The normalization is incomplete however so we see the newline change.

oli-obk · 2026-09-11T09:36:43Z

@bors r+

rust-bors · 2026-09-11T09:36:47Z

📌 Commit 676862d has been approved by oli-obk

It is now in the queue for this repository.

Implement arbitrary casts in custom mir [See zulip](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/extending.20custom.20MIR/with/613727602) Implement custom casts using an enum in `intrinsics::mir` that closely matches the `CastKind` enum in the compiler. We have 5 different ways to cast in custom MIR, 3 with functions (which hard code a cast kind), 1 implicit coerce (which only does subtyping i think), 1 with `as` (which does a few more). They are all different and the error messages don't tell you what to do. Some are (i think) not possible at all. This came up [while testing miri](https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/Valdity.20checks.20for.20references/near/613662285). We currently have to guess the casting syntax until we find the one that generates the MIR we want. This PR changes that by adding a custom mir operation that allows picking an arbitrary cast. r? RalfJung

…uwer Rollup of 11 pull requests Successful merges: - #162520 (Refactor `HygieneEncodeContext`) - #154373 (Skip linting unused braces for FunctionArg and MethodArg context for 2024 later ) - #160023 (Refactor the way cg_ssa handles indirect returns (returns via `sret`) — Take 2) - #160860 (Implement arbitrary casts in custom mir) - #161482 (Use attribute parser for `#[non_exhaustive]` attribute check) - #161867 (deeper `rustc_builtin_macros` cleanups) - #162099 (small refactor of doc attribute arguments warnings) - #162541 (split `macroless_generic_const_args` in two) - #162549 (The fuchsia team maintains `riscv64gc-unknown-fuchsia`) - #162577 (Fix `i686-pc-windows-msvc` platform support docs and target spec metadata) - #162624 (regression test for opaque field projection in closure capture)

Rollup merge of #160860 - maxdexh:custom-mir-casts, r=oli-obk Implement arbitrary casts in custom mir [See zulip](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/extending.20custom.20MIR/with/613727602) Implement custom casts using an enum in `intrinsics::mir` that closely matches the `CastKind` enum in the compiler. We have 5 different ways to cast in custom MIR, 3 with functions (which hard code a cast kind), 1 implicit coerce (which only does subtyping i think), 1 with `as` (which does a few more). They are all different and the error messages don't tell you what to do. Some are (i think) not possible at all. This came up [while testing miri](https://rust-lang.zulipchat.com/#narrow/channel/136281-t-opsem/topic/Valdity.20checks.20for.20references/near/613662285). We currently have to guess the casting syntax until we find the one that generates the MIR we want. This PR changes that by adding a custom mir operation that allows picking an arbitrary cast. r? RalfJung

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maxdexh force-pushed the custom-mir-casts branch from bb0a712 to a596f67 Compare August 10, 2026 16:26

RalfJung reviewed Aug 16, 2026

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Comment thread tests/ui/hygiene/unpretty-debug-lifetimes.stdout

maxdexh force-pushed the custom-mir-casts branch from a596f67 to 48141ff Compare August 18, 2026 07:52

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maxdexh force-pushed the custom-mir-casts branch 2 times, most recently from 1b4e3de to a85dda1 Compare August 18, 2026 09:24

maxdexh marked this pull request as ready for review August 18, 2026 09:46

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 18, 2026

maxdexh commented Aug 18, 2026

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rustbot assigned oli-obk and unassigned RalfJung Sep 8, 2026

oli-obk approved these changes Sep 10, 2026

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 10, 2026

JonathanBrouwer mentioned this pull request Sep 10, 2026

Rollup of 4 pull requests #162596

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rust-bors Bot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 10, 2026

maxdexh added 2 commits September 10, 2026 17:56

Implement arbitrary casts in custom mir

225a061

Add extra linebreak to weird test

676862d

maxdexh force-pushed the custom-mir-casts branch from a85dda1 to 676862d Compare September 10, 2026 16:00

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 10, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 11, 2026

JonathanBrouwer mentioned this pull request Sep 11, 2026

Rollup of 11 pull requests #162631

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Zalathar mentioned this pull request Sep 11, 2026

Rollup of 11 pull requests #162632

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rust-bors Bot merged commit 9e9dc37 into rust-lang:main Sep 11, 2026
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rustbot added this to the 1.100.0 milestone Sep 11, 2026

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