Function item should not be used as const arg

[Shourya742]

closes: #138088

r? @BoxyUwU

[rustbot]

HIR ty lowering was modified

cc @fmease

[rustbot]

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[Shourya742]

Previous both free function and associated function, where lowering to zero_sized fn def which would later emit diagnostics, but to lower the assoc it required the parent trait. This now emits an error, instead of lowering.

View changes since the review

[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/136666.rs ... ok
test [crashes] tests/crashes/136766.rs ... ok
test [crashes] tests/crashes/137187.rs ... ok
test [crashes] tests/crashes/137049.rs ... ok
2026-06-16T13:15:29.230202Z 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/138088.rs ... FAILED
test [crashes] tests/crashes/138008.rs ... ok
test [crashes] tests/crashes/138009.rs ... ok
test [crashes] tests/crashes/138156.rs ... ok
test [crashes] tests/crashes/138534.rs ... ok
---

---- [crashes] tests/crashes/138088.rs stdout ----
------rustc stdout------------------------------

------rustc stderr------------------------------
error[E0601]: `main` function not found in crate `138088`
##[error] --> /checkout/tests/crashes/138088.rs:5:2
  |
5 | }
  |  ^ consider adding a `main` function to `/checkout/tests/crashes/138088.rs`

error: function items cannot be used as const arguments
##[error] --> /checkout/tests/crashes/138088.rs:4:26
  |
4 |     fn x(&self) -> [i32; Bar::x];
  |                          ^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0601`.

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

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/138088.rs' panicked at src/tools/compiletest/src/runtest/crashes.rs:16:18:
fatal error
stack backtrace:
   8: __rustc::rust_begin_unwind