don't ignore ambiguity during MIR type check

[lcnr]

fixes #164054

this was previously fixed in #116530 and later reverted in #118736.

It impacts the new solver more than the old one as the new solver depends significantly more on region identity, see the relevant stabilization report section.

The old solver has significantly less region identity dependence. It does rely on region identity when merging candidates during normalization:

rust/compiler/rustc_trait_selection/src/traits/project.rs

Lines 103 to 107 in 69bccf0

	// Duplicates can happen inside ParamEnv. In the case, we
	// perform a lazy deduplication.
	if current == &candidate {
	return false;
	}

This check results in additional ICE with the old solver while instead triggering the potentially region dependent goals check with the new one

rust/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs

Lines 94 to 95 in 14f0018

	let value = op(&ocx).map_err(|_| {
	infcx.tcx.check_potentially_region_dependent_goals(root_def_id).err().unwrap_or_else(

r? types

[rustbot]

🤖 A dedicated #t-types/nominated topic has been opened for humans to discuss this issue 🤖

[rustbot]

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-log-analyzer]

The job test-x86_64-gnu-miri failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

tests/pass/shims/x86/rounding-error.rs ... ok
tests/pass/shims/x86/intrinsics-x86-gfni.rs ... ok

FAILED TEST: tests/pass/dyn-upcast.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-1pheec" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/miri" "--error-format=json" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0/tests/pass" "tests/pass/dyn-upcast.rs" "--edition" "2021"

error: test got exit status: 1, but expected 0
 = note: compilation failed, but was expected to succeed

error: no output was expected
Execute `./miri test --bless` to update `tests/pass/dyn-upcast.stderr` to the actual output
+++ <stderr output>
error[E0283]: type annotations needed: cannot satisfy `dyn modulo_binder::Trait<&(), &()>: modulo_binder::Supertrait<&()>`
##[error]  --> tests/pass/dyn-upcast.rs:504:10
   |
LL |         .say_hello(&0);
   |          ^^^^^^^^^
   |
   = note: cannot satisfy `dyn modulo_binder::Trait<&(), &()>: modulo_binder::Supertrait<&()>`
help: the trait `modulo_binder::Supertrait<T>` is implemented for `()`
  --> tests/pass/dyn-upcast.rs:493:5
   |
LL |     impl<T> Supertrait<T> for () {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `modulo_binder::Trait::say_hello`
  --> tests/pass/dyn-upcast.rs:495:24
   |
LL |     trait Trait<T, U>: Supertrait<T> + Supertrait<U> {
   |                        ^^^^^^^^^^^^^ required by this bound in `Trait::say_hello`
LL |         fn say_hello(&self, _: &usize) {
---
error: no output was emitted
Execute `./miri test --bless` to remove `tests/pass/dyn-upcast.stdout`

full stderr:
error[E0283]: type annotations needed: cannot satisfy `dyn modulo_binder::Trait<&(), &()>: modulo_binder::Supertrait<&()>`
##[error]  --> tests/pass/dyn-upcast.rs:504:10
   |
LL |         .say_hello(&0);
   |          ^^^^^^^^^
   |
   = note: cannot satisfy `dyn modulo_binder::Trait<&(), &()>: modulo_binder::Supertrait<&()>`
help: the trait `modulo_binder::Supertrait<T>` is implemented for `()`
  --> tests/pass/dyn-upcast.rs:493:5
   |
LL |     impl<T> Supertrait<T> for () {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `modulo_binder::Trait::say_hello`
  --> tests/pass/dyn-upcast.rs:495:24
   |
LL |     trait Trait<T, U>: Supertrait<T> + Supertrait<U> {
   |                        ^^^^^^^^^^^^^ required by this bound in `Trait::say_hello`
LL |         fn say_hello(&self, _: &usize) {
---

Location:
   /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ui_test-0.30.7/src/lib.rs:365

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
error: test failed, to rerun pass `--test ui`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/build/miri/d88a065f2799f036/out/ui-d88a065f2799f036` (exit status: 1)
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo test --target x86_64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout -Zembed-metadata=no --locked --color=always --profile=release --manifest-path /checkout/src/tools/miri/Cargo.toml -- [workdir=/checkout]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:208:21
Executed at: src/bootstrap/src/core/build_steps/test.rs:790:19

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test --stage 2 miri cargo-miri`
Currently active steps:
test::Miri { target: x86_64-unknown-linux-gnu } at src/bootstrap/src/core/build_steps/test.rs:716
Build completed unsuccessfully in 0:27:08
  local time: Mon Oct 12 12:35:42 UTC 2026

[rust-bors]

☔ The latest upstream changes (presumably #164284) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.