Fix ICE in normalization during closure capture analysis (#149746) by Delta17920 · Pull Request #149800 · rust-lang/rust

Delta17920 · 2025-12-09T07:15:47Z

This fixes an internal compiler error that occurred when normalizing associated types during closure capture analysis.

The Fix: Modified rustc_middle/src/ty/normalize_erasing_regions.rs to gracefully handle projection normalization failures instead of panicking when analyzing closure captures.

Regression Test: Added tests/ui/associated-types/normalization-ice-issue-149746.rs, a reproduction case involving complex associated type projections (<() as Owner>::Ty) that previously crashed the compiler. Verified it now emits a standard type error (E0277).

Fixes #149746

rustbot · 2025-12-09T07:15:51Z

r? @fee1-dead

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Delta17920 · 2025-12-09T07:21:38Z

r? compiler

rustbot · 2025-12-09T07:21:42Z

fmease is not on the review rotation at the moment.
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rustbot · 2025-12-09T07:23:46Z

compiler-errors is not on the review rotation at the moment.
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lcnr · 2025-12-09T08:17:46Z

compiler/rustc_middle/src/ty/normalize_erasing_regions.rs

+        let input = self.typing_env.as_query_input(arg);
+
+        // 2. Use unwrap_or(arg) to return the original arg if it fails
+        self.tcx.try_normalize_generic_arg_after_erasing_regions(input).unwrap_or(arg)


we're intentionally using a bug! here as we're in normalize_generic_arg_after_erasing_regions.

The issue happens in the caller:

we either have to change the caller to use try_normalize... to avoid this ICE if the error is unavoidable

or prevent this error entirely by not calling normalize_... for types which aren#t well-formed

Thanks for the feedback @lcnr ! I have reverted the global change in rustc_middle and moved the fix to compiler/rustc_hir_typeck/src/upvar.rs as requested. specifically, I updated the needs_drop closure within has_significant_drop_outside_of_captures to use try_normalize_erasing_regions; if normalization fails, it now conservatively returns false (assuming no significant drop) instead of panicking.

Delta17920 · 2025-12-10T16:55:41Z

@lcnr anything else that needs to be changed?

fmease · 2025-12-11T00:07:53Z

tests/ui/associated-types/normalization-ice-issue-149746.rs

@@ -0,0 +1,16 @@
+// edition: 2015..2021


Suggested change

// edition: 2015..2021

//@ edition: 2015..2021

The MCVE I've posted in #149746 (comment) which you're using here as a test, deliberately has an @ there: It's a compiletest directive.

this has not been resolved

Delta17920 · 2025-12-16T17:33:02Z

@lcnr anything else that needs to be changed?

Delta17920 · 2025-12-18T07:34:06Z

r? @fmease

rustbot · 2025-12-18T07:34:10Z

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lcnr · 2025-12-18T11:20:19Z

I am sorry that I did not quickly re-review this PR. For non-critical work, it is expected that reviewers sometimes take 1-2 weeks.

Pinging people after a few days is fine if the issue is blocking other important work or if it fixes a critical issue. Please be more patient going forward.

r? lcnr

Delta17920 · 2025-12-18T11:34:26Z

I am sorry that I did not quickly re-review this PR. For non-critical work, it is expected that reviewers sometimes take 1-2 weeks.

Pinging people after a few days is fine if the issue is blocking other important work or if it fixes a critical issue. Please be more patient going forward.

r? lcnr

sorry sorry, i will take care of this from the next time

lcnr

There's something a lot bigger here, which is why I didn't take the time to look at this PR until now.

At its core, a lot of code in the compiler assumes that if a type is well-formed, so are its fields. This is simply not true, neither currently nor in general:

currently there's #149283
in general, there's lcnr/random-rust-snippets#23

This is separate to the issue in this PR, where the struct itself is not well-formed and we don't emit an error for it. These we could handle by changing normalize_erasing_regions to emit a span_delayed_bug, returning an ty::Error instead. Doing that is kind of whatever however if the issue also just exists for otherwise well-formed programs.

Outside of codegen, using normalize_erasing_regions can pretty much always go wrong. Quickly looking through to source, the following additional calls to normalize_erasing_regions may also result in reachable ICE

rust/compiler/rustc_middle/src/ty/layout.rs

Line 545 in e951f47

let normalized = tcx.normalize_erasing_regions(typing_env, ty);

(we recur into struct fields)
rust/compiler/rustc_middle/src/ty/layout.rs

Line 906 in e951f47

let metadata = tcx.normalize_erasing_regions(

(should ICE for reference to alias as struct field)
pretty much all (indirect) uses of normalize_erasing_regions in CTFE and MIR opts can result in ICE as long as they happen in generic bodies
using normalize_erasing_regions in borrowck for the not explicitly mentioned fields of Foo { ..Default::default() } expressions should ICE
using normalize_erasing_regions in resolve_instance_raw ICEs mir opts and CTFE using it

I would kind of like to figure out a more general strategy here instead of fixing these issues as they crop up. Anyways, fine with this PR for now, but want to start a T-types discussion about this and think about a general solution here.

View changes since this review

lcnr · 2025-12-22T15:58:26Z

compiler/rustc_hir_typeck/src/upvar.rs

+            // If normalization fails, we assume it doesn't have a significant drop (false)
+            // to avoid crashing, allowing the compiler to emit the underlying type error later.
+            self.tcx
+                .try_normalize_erasing_regions(typing_env, ty)


can you instead replace the normalize_erasing_regions call in has_significant_drop with its try_ variant. r=me after.

Done! I've moved the fix to has_significant_drop itself, replacing normalize_erasing_regions with try_normalize_erasing_regions and returning false when normalization fails.

lcnr · 2025-12-23T09:52:08Z

compiler/rustc_middle/src/ty/util.rs

+                // FIX: Use try_normalize to avoid crashing. If it fails, return false.
+                tcx.try_normalize_erasing_regions(typing_env, query_ty)
+                    .map(|erased| tcx.has_significant_drop_raw(typing_env.as_query_input(erased)))
+                    .unwrap_or(false)


why false? in case the query_ty has infer we're currently conservatively returning true and we should the the same here

Ah, good point! I completely missed that 😭🤦‍♂️we're already returning true conservatively for the has_infer() case. You're right - we should be consistent here. Updated to return true instead to match that behavior.

rustbot · 2025-12-23T12:59:06Z

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.

lcnr · 2025-12-23T20:01:58Z

@bors r+ rollup

bors · 2025-12-23T20:02:01Z

📌 Commit ac448c9 has been approved by lcnr

It is now in the queue for this repository.

Rollup of 6 pull requests Successful merges: - #149800 (Fix ICE in normalization during closure capture analysis (#149746)) - #150182 (Don't export upstream monomorphizations from compiler-builtins) - #150216 (Tidying up tests/ui/issues 15 tests [6/N]) - #150308 (Update bors configuration) - #150314 (rustc-dev-guide subtree update) - #150319 (use new term in description of --target) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #149800 - Delta17920:fix-ice-149746, r=lcnr Fix ICE in normalization during closure capture analysis (#149746) This fixes an internal compiler error that occurred when normalizing associated types during closure capture analysis. The Fix: Modified rustc_middle/src/ty/normalize_erasing_regions.rs to gracefully handle projection normalization failures instead of panicking when analyzing closure captures. Regression Test: Added tests/ui/associated-types/normalization-ice-issue-149746.rs, a reproduction case involving complex associated type projections (<() as Owner>::Ty<T>) that previously crashed the compiler. Verified it now emits a standard type error (E0277). Fixes #149746

Rollup of 6 pull requests Successful merges: - rust-lang/rust#149800 (Fix ICE in normalization during closure capture analysis (rust-lang/rust#149746)) - rust-lang/rust#150182 (Don't export upstream monomorphizations from compiler-builtins) - rust-lang/rust#150216 (Tidying up tests/ui/issues 15 tests [6/N]) - rust-lang/rust#150308 (Update bors configuration) - rust-lang/rust#150314 (rustc-dev-guide subtree update) - rust-lang/rust#150319 (use new term in description of --target) r? `@ghost` `@rustbot` modify labels: rollup

rustbot assigned fee1-dead Dec 9, 2025

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Delta17920 force-pushed the fix-ice-149746 branch from 3b0c42e to fc87a13 Compare December 9, 2025 09:41

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Delta17920 requested review from fmease and lcnr December 11, 2025 00:38

rustbot assigned fmease and unassigned lcnr Dec 18, 2025

matthiaskrgr mentioned this pull request Dec 18, 2025

when asking a team for review, don't assign to people that are on vacation/off rust-lang/triagebot#2237

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Delta17920 force-pushed the fix-ice-149746 branch from badc3d2 to cce05f0 Compare December 22, 2025 17:51

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lcnr reviewed Dec 23, 2025

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Fix ICE in normalization during closure capture analysis

ac448c9

Delta17920 force-pushed the fix-ice-149746 branch from cce05f0 to ac448c9 Compare December 23, 2025 12:59

bors 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 Dec 23, 2025

matthiaskrgr mentioned this pull request Dec 23, 2025

Rollup of 6 pull requests #150324

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bors merged commit f6c6e84 into rust-lang:main Dec 24, 2025
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rustbot added this to the 1.94.0 milestone Dec 24, 2025

github-actions bot mentioned this pull request Jan 27, 2026

Toolchain upgrade to nightly-2026-01-30 flux-rs/flux#1345

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theemathas mentioned this pull request Feb 6, 2026

ICE : internal error: entered unreachable code #152205

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