reject reborrowing when the target has no lifetime generic arg

[senioria]

suppresses the ICE in #156307 and #156308, but doesn't fix the problem that reborrowing is conflict with moving.

Reborrow traits experiment: #145612

fixes #156307.

[rustbot]

r? @nnethercote

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[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

test [crashes] tests/crashes/98322.rs ... ok
test [crashes] tests/crashes/project-to-simd-array-field.rs ... ok
test [crashes] tests/crashes/reborrow/coerce-shared-alias-projection.rs ... ok
test [crashes] tests/crashes/reborrow/coerce-shared-different-layout.rs ... ok
2026-07-08T17:54:58.117806Z ERROR compiletest::runtest: fatal error, panic: "crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`."
test [crashes] tests/crashes/reborrow/coerce-shared-marker-no-target-lifetime.rs ... FAILED
test [crashes] tests/crashes/reborrow/coerce-shared-multi-field.rs ... ok
test [crashes] tests/crashes/reborrow/coerce-shared-nested.rs ... ok
test [crashes] tests/crashes/reborrow/coerce-shared-tuple-phantom-position.rs ... ok
test [crashes] tests/crashes/reborrow/corrected-field-mismatch-coerce-shared-issue-156315.rs ... ok
---

---- [crashes] tests/crashes/reborrow/coerce-shared-marker-no-target-lifetime.rs stdout ----
------rustc stdout------------------------------

------rustc stderr------------------------------
error: implementing `CoerceShared` does not allow multiple lifetimes or fields to be coerced
##[error]  --> /checkout/tests/crashes/reborrow/coerce-shared-marker-no-target-lifetime.rs:12:10
   |
12 | impl<'a> CoerceShared<CustomMarkerRef> for CustomMarker<'a> {}
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
##[error]  --> /checkout/tests/crashes/reborrow/coerce-shared-marker-no-target-lifetime.rs:19:12
   |
---
   |
note: function defined here
  --> /checkout/tests/crashes/reborrow/coerce-shared-marker-no-target-lifetime.rs:15:4
   |
15 | fn method(_a: CustomMarkerRef) {}
   |    ^^^^^^ -------------------

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0308`.

------------------------------------------

error: crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`.

thread '[crashes] tests/crashes/reborrow/coerce-shared-marker-no-target-lifetime.rs' panicked at src/tools/compiletest/src/runtest/crashes.rs:16:18:
fatal error
stack backtrace:
   8: __rustc::rust_begin_unwind

[nnethercote]

I'm not a good reviewer for this. Let's try...

r? @jackh726