Remove unneeded drop_in_place calls

[scottmcm]

Might as well pull this out from #144561 because this is still used in things like Vec::truncate where it'd be nice to allow it be removed if inlined enough to see that the type is Copy.

So long as perf says it's ok, at least 🤞

[rustbot]

r? @nnethercote

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[scottmcm]

@bors try @rust-timer queue

[rust-bors]

⌛ Trying commit 952f142 with merge 641adcb…

To cancel the try build, run the command @bors try cancel.

[rust-log-analyzer]

The job 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/fail/tree_borrows/reserved/cell-protected-write.rs ... ok
tests/fail/weak_memory/weak_uninit.rs ... ok

FAILED TEST: tests/fail/stacked_borrows/drop_in_place_retag.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-GUclM8" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/miri" "--error-format=json" "--sysroot=/checkout/obj/build/x86_64-unknown-linux-gnu/miri-sysroot" "-Dwarnings" "-Dunused" "-Ainternal_features" "-Zui-testing" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0/tests/fail/stacked_borrows" "tests/fail/stacked_borrows/drop_in_place_retag.rs" "--edition" "2021"

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

error: no output was emitted
Execute `./miri test --bless` to remove `tests/fail/stacked_borrows/drop_in_place_retag.stderr`

error: `/retag .* for Unique permission .* only grants SharedReadOnly permission/` does not match diagnostics outside the testfile
##[error] --> tests/fail/stacked_borrows/drop_in_place_retag.rs:4:26
  |
4 | //@error-in-other-file: /retag .* for Unique permission .* only grants SharedReadOnly permission/
  |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected because of this pattern
  |

full stderr:

---

Location:
   /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ui_test-0.30.2/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/stage1-tools/x86_64-unknown-linux-gnu/release/deps/ui-48471e55e94b1667` (exit status: 1)
Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:39:33
  local time: Mon Aug  4 06:27:11 UTC 2025
  network time: Mon, 04 Aug 2025 06:27:12 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 641adcb (641adcb8fe61a1a2f4cc317723476b9d171e2813, parent: 07b7dc90ee4df5815dbb91ef8e98cb93571230f5)

[rust-timer]

Finished benchmarking commit (641adcb): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary -2.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 467.146s -> 468.977s (0.39%)
Artifact size: 377.00 MiB -> 377.01 MiB (0.00%)