Insert checks for enum discriminants when debug assertions are enabled

[1c3t3a]

Similar to the existing null-pointer and alignment checks, this checks for valid enum discriminants on creation of enums through unsafe transmutes. Essentially this sanitizes patterns like the following:

let val: MyEnum = unsafe { std::mem::transmute<u32, MyEnum>(42) };

An extension of this check will be done in a follow-up that explicitly sanitizes for extern enum values that come into Rust from e.g. C/C++.

This check is similar to Miri's capabilities of checking for valid construction of enum values.

This PR is inspired by saethlin@'s PR
#104862. Thank you so much for keeping this code up and the detailed comments!

I also pair-programmed large parts of this together with vabr-g@.

r? @saethlin

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @vakaras

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

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

cc @rust-lang/rust-analyzer

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-2 failed! Check out the build log: (web) (plain)

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

---- collections::btree::map::tests::test_check_ord_chaos stdout ----
---- collections::btree::map::tests::test_check_ord_chaos stderr ----

thread 'main' panicked at alloctests/../alloc/src/collections/btree/map/tests.rs:101:17:
Governed(1, Governor { flipped: Cell { value: true } }) >= Governed(2, Governor { flipped: Cell { value: true } })
stack backtrace:
note: child process exited with signal 5
---- collections::btree::map::tests::test_range_backwards_1 stdout ----
---- collections::btree::map::tests::test_range_backwards_1 stderr ----

---
---- collections::vec_deque::tests::test_rotate_left_panic stdout ----
---- collections::vec_deque::tests::test_rotate_left_panic stderr ----

thread 'main' panicked at alloctests/../alloc/src/collections/vec_deque/mod.rs:2590:9:
assertion failed: n <= self.len()
stack backtrace:
note: child process exited with signal 5
---- collections::vec_deque::tests::test_rotate_right_panic stdout ----
---- collections::vec_deque::tests::test_rotate_right_panic stderr ----

thread 'main' panicked at alloctests/../alloc/src/collections/vec_deque/mod.rs:2633:9:
assertion failed: n <= self.len()
stack backtrace:
note: child process exited with signal 5
---- collections::vec_deque::tests::test_swap_panic stdout ----
---- collections::vec_deque::tests::test_swap_panic stderr ----

thread 'main' panicked at alloctests/../alloc/src/collections/vec_deque/mod.rs:751:9:
assertion failed: j < self.len()
stack backtrace:
note: child process exited with signal 5
---- raw_vec::tests::zst_reserve_exact_panic stdout ----
---- raw_vec::tests::zst_reserve_exact_panic stderr ----

---
    raw_vec::tests::zst_reserve_panic

test result: FAILED. 214 passed; 17 failed; 16 ignored; 0 measured; 0 filtered out; finished in 2.57s

error: test failed, to rerun pass `-p alloctests --lib`
env -u RUSTC_WRAPPER CARGO_ENCODED_RUSTDOCFLAGS="-Csymbol-mangling-version=v0\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}--check-cfg=cfg(llvm_enzyme)\u{1f}-Dwarnings\u{1f}-Wrustdoc::invalid_codeblock_attributes\u{1f}--crate-version\u{1f}1.89.0-nightly\t(fc5ccf4fa\t2025-06-03)" CARGO_ENCODED_RUSTFLAGS="-Csymbol-mangling-version=v0\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}--check-cfg=cfg(llvm_enzyme)\u{1f}-Zmacro-backtrace\u{1f}-Csplit-debuginfo=off\u{1f}-Clink-arg=-L/usr/lib/llvm-19/lib\u{1f}-Cllvm-args=-import-instr-limit=10\u{1f}-Clink-args=-Wl,-z,origin\u{1f}-Clink-args=-Wl,-rpath,$ORIGIN/../lib\u{1f}-Alinker-messages\u{1f}--cap-lints=allow\u{1f}--cfg\u{1f}randomized_layouts" RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools/cg_clif/dist/rustc-clif" RUSTDOC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools/cg_clif/dist/rustdoc-clif" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "test" "--manifest-path" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools/cg_clif/build/sysroot_tests/Cargo.toml" "--target-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools/cg_clif/build/sysroot_tests_target" "--locked" "--target" "aarch64-unknown-linux-gnu" "-p" "coretests" "-p" "alloctests" "--tests" "--" "-q" exited with status ExitStatus(unix_wait_status(25856))
Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:11:16
  local time: Tue Jun  3 12:54:08 UTC 2025
  network time: Tue, 03 Jun 2025 12:54:08 GMT
##[error]Process completed with exit code 1.
Post job cleanup.