Deduplicate deprecation warning when using unit or tuple structs

[jdonszelmann]

First commit adds a test that's broken currently, 2nd commit fixes it.

Created with @WaffleLapkin

[rustbot]

HIR ty lowering was modified

cc @fmease

[rustbot]

r? @petrochenkov

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[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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

    |
197 | /// supress duplicate warning when importing unit or tuple structs
    |     ^^^^^^^
    |
rerun tidy with `--extra-checks=spellcheck --bless` to fix typos
tidy [extra_checks]: checks with external tool 'typos' failed
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1280:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:34
  local time: Wed Oct 22 16:15:33 UTC 2025
  network time: Wed, 22 Oct 2025 16:15:33 GMT
##[error]Process completed with exit code 1.