Validate transmute in CTFE

[oli-obk]

fixes #142230

let's see what perf says, maybe we need to restrict it to literal transmutes, and not all the implicit ones happening in mir interpreter internal situations

r? @ghost

[rustbot]

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[oli-obk]

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit d032919 with merge e6b0b5d…

To cancel the try build, run the command @bors2 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/pass/shims/x86/intrinsics-x86-avx2.rs ... ok
tests/pass/shims/x86/intrinsics-x86-gfni.rs ... ok

FAILED TEST: tests/pass/enum_discriminant_ptr_value.rs
command: MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-yr7JWE" 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/pass" "tests/pass/enum_discriminant_ptr_value.rs" "-Zmiri-disable-stacked-borrows" "-Zmiri-disable-validation" "--edition" "2021"

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

error: no output was expected
Execute `./miri test --bless` to update `tests/pass/enum_discriminant_ptr_value.stderr` to the actual output
+++ <stderr output>
error: Undefined Behavior: constructing invalid value at .<enum-variant(Some)>.0: encountered a dangling reference (going beyond the bounds of its allocation)
##[error]  --> tests/pass/enum_discriminant_ptr_value.rs:7:38
   |
LL |     let val: Option<&i32> = unsafe { std::mem::transmute((&x as *const i32).wrapping_offset(2)) };
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE:
   = note: inside `main` at tests/pass/enum_discriminant_ptr_value.rs:7:38: 7:96
---
full stderr:
error: Undefined Behavior: constructing invalid value at .<enum-variant(Some)>.0: encountered a dangling reference (going beyond the bounds of its allocation)
##[error]  --> tests/pass/enum_discriminant_ptr_value.rs:7:38
   |
LL |     let val: Option<&i32> = unsafe { std::mem::transmute((&x as *const i32).wrapping_offset(2)) };
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE:
   = note: inside `main` at tests/pass/enum_discriminant_ptr_value.rs:7:38: 7:96
---

Location:
   /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ui_test-0.29.2/src/lib.rs:369

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-0bacf351ef83efc6` (exit status: 1)
Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:40:25
  local time: Wed Jul 16 17:08:28 UTC 2025
  network time: Wed, 16 Jul 2025 17:08:28 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

[rust-bors]

☀️ Try build successful (CI)
Build commit: e6b0b5d (e6b0b5d69a1aea3e0e367e405c676642a0b35313, parent: 1c6de215099bbe33668de762f9591187f6c25eef)

[rust-timer]

Finished benchmarking commit (e6b0b5d): comparison URL.

Overall result: ❌ regressions - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.2%, 1.7%]	17
Regressions ❌ (secondary)	38.4%	[0.5%, 149.4%]	28
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.2%, 1.7%]	17

Max RSS (memory usage)

Results (primary -2.0%, secondary 0.7%)

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)	12.3%	[12.3%, 12.3%]	1
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-2.2%	[-4.1%, -1.0%]	4
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Cycles

Results (primary 2.5%, secondary 348.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.5%, 2.5%]	1
Regressions ❌ (secondary)	426.9%	[336.3%, 574.7%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-3.5%, -2.8%]	2
All ❌✅ (primary)	2.5%	[2.5%, 2.5%]	1

Binary size

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

Bootstrap: 464.891s -> 467.195s (0.50%)
Artifact size: 374.78 MiB -> 374.45 MiB (-0.09%)