move a ui test to coretests unit test

[fee1-dead]

It only tests the usage and formatting of fmt::Pointer.

[rustbot]

r? @petrochenkov

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[rustbot]

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

fmt: checked 6610 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy: Skipping binary file check, read-only filesystem
tidy [triagebot]: triagebot.toml [mentions.*] contains path 'library/stdarch' which doesn't exist
tidy [triagebot]: triagebot.toml [assign.owners] contains path 'library/stdarch' which doesn't exist
tidy [triagebot]: triagebot.toml [autolabel.T-libs] contains trigger_files path 'library/stdarch' which doesn't exist
tidy [triagebot]: FAIL
tidy: The following check failed: triagebot
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 yarn` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1357:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:00:55
  local time: Mon Dec 22 22:17:58 UTC 2025
  network time: Mon, 22 Dec 2025 22:17:58 GMT
##[error]Process completed with exit code 1.