Reflection: get VariantId at runtime by yara-blue · Pull Request #165141 · rust-lang/rust

yara-blue · 2026-10-29T15:55:53Z

We did not yet have a way to use reflection data for enum's at runtime. This adds a method to query the current VariantId give no type information except the TypeId. We do this in two steps:

get the Discriminant given no information about the type at runtime
map the Discriminant to a VariantId

To do 1) we need to generate code for the specific type. Specifically we need to generate std::mem::discriminant. Emitting a function ptr during const-eval is enough to get a monomorphized version of that generated. To get 2) we have an intrinsic that returns a list of VariantId to Discriminant mapping. Getting the discriminant then looking it up in the mapping is packaged into a single function which is what the user actually calls.

Since we have a lot of enum usage in rust the performance of this matters a lot. The current approach is suboptimal, the mapping is done through a linear scan and may not optimize out nicely. I suggest we put some effort in optimizing this after we are sure this will be the API.

rustbot · 2026-10-29T15:55:56Z

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

The reflection data structures are tied exactly to the implementation
in the compiler. Make sure to also adjust rustc_const_eval/src/const_eval/type_info.rs

cc @oli-obk

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

rustbot · 2026-10-29T15:55:59Z

r? @Mark-Simulacrum

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yara-blue · 2026-10-29T15:56:28Z

This PR contains some commits that are currently in a rollup. I'll rebase it once that merges.

yara-blue · 2026-10-29T15:56:53Z

r? oli-obk

oli-obk · 2026-10-29T16:40:53Z

+///
+/// See [`TypeId::variant_getter`] on how to safely use this function.
+#[lang = "variant_getter"]
+unsafe fn variant_getter<T: 'static>(val: *const ()) -> VariantId {


why is the 'static bound needed here? We're in reflection territory, so if it's just for TypeId::of, you can also just use type_info::of::<T>()

View changes since the review

oli-obk · 2026-10-29T16:40:59Z

+    #[unstable(feature = "type_info", issue = "146922")]
+    #[rustc_const_unstable(feature = "type_info", issue = "146922")]
+    #[rustc_comptime]
+    pub fn variant_getter(self) -> fn(*const ()) -> VariantId {


you'll need to make the function pointer an unsafe function pointer, otherwise we have a safe function returning a safe function pointer that when called wrongly can cause UB

View changes since the review

can additionally recommend callers to debug assert the VariantId they are getting back for having a TypeId of the type they expected.

oli-obk · 2026-10-29T16:43:43Z

+pub fn type_id_variant_getter(_type_id: crate::any::TypeId) -> fn(*const ()) -> VariantId;
+
+/// Returns a lookup table for converting between VariantId and Discriminant. This
+/// is use internally by variant getter. **Please do not use this.**


by variant_getter (backticks and underscore)

View changes since the review

oli-obk · 2026-10-29T16:45:22Z

+#[rustc_intrinsic]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[rustc_comptime]
+pub fn type_id_variant_getter(_type_id: crate::any::TypeId) -> fn(*const ()) -> VariantId;


same as TypeId::variant_getter: return fn ptr should be unsafe

View changes since the review

rustbot · 2026-10-29T16:47:24Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2026-10-29T18:02:53Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

rust-log-analyzer · 2026-10-29T18:39:02Z

The job test-x86_64-gnu-gcc-core-tests failed! Check out the build log: (web) (plain enhanced) (plain)

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

[RUSTC-TIMING] corebenches test:true 0.115
error[E0133]: call to unsafe function is unsafe and requires unsafe block
 --> library/coretests/tests/mem/type_info/runtime.rs:9:16
  |
9 |     assert_eq!(variant_getter(as_unit_ptr(test_input)), const { TypeId::of::<T>().variant(0) });
  |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
  |
  = note: consult the function's documentation for information on how to avoid undefined behavior

error[E0133]: call to unsafe function is unsafe and requires unsafe block
  --> library/coretests/tests/mem/type_info/runtime.rs:24:9
   |
24 |         variant_getter(as_unit_ptr(&Kitty::Sleepy)),
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
   |
   = note: consult the function's documentation for information on how to avoid undefined behavior

error[E0133]: call to unsafe function is unsafe and requires unsafe block
  --> library/coretests/tests/mem/type_info/runtime.rs:28:9
   |
28 |         variant_getter(as_unit_ptr(&Kitty::Playful)),
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
   |
   = note: consult the function's documentation for information on how to avoid undefined behavior

error[E0133]: call to unsafe function is unsafe and requires unsafe block
  --> library/coretests/tests/mem/type_info/runtime.rs:32:9
   |
32 |         variant_getter(as_unit_ptr(&Kitty::Cuddly)),
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
   |
   = note: consult the function's documentation for information on how to avoid undefined behavior

warning: unused variable: `variant_getter`
  --> library/coretests/tests/mem/type_info/runtime.rs:94:9
   |
94 |     let variant_getter = const { TypeId::of::<!>().variant_getter() };
   |         ^^^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_variant_getter`
   |
   = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

[RUSTC-TIMING] coretests test:true 54.290
rustc exited with signal: 6 (SIGABRT) (core dumped)
error: `coretests` (test "coretests") generated 1 warning
error: could not compile `coretests` (test "coretests") due to 4 previous errors; 1 warning emitted

Caused by:
  process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc /checkout/obj/build/bootstrap/debug/rustc --crate-name coretests --edition=2024 library/coretests/tests/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat,unused-externs-silent --emit=dep-info,link -C opt-level=3 -C embed-bitcode=no -C codegen-units=1 --warn=unexpected_cfgs --check-cfg 'cfg(target_has_reliable_f16)' --check-cfg 'cfg(target_has_reliable_f16_math)' --check-cfg 'cfg(target_has_reliable_f128)' --check-cfg 'cfg(target_has_reliable_f128_math)' -Cembed-bitcode=yes -Zunstable-options -Cforce-frame-pointers=non-leaf -C overflow-checks=on --test --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=8e648b9a86af2a25 -C extra-filename=-87d8502a0950c483 --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/coretests/87d8502a0950c483/out --target x86_64-unknown-linux-gnu -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rand_core/e6ca3b32e65dc6c8/out -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/dist/build/coretests/87d8502a0950c483/out --extern coretests=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/coretests/2725d6e8dc02b311/out/libcoretests-2725d6e8dc02b311.rlib --extern coretests=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/coretests/2725d6e8dc02b311/out/libcoretests-2725d6e8dc02b311.rmeta --extern rand=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rand/fdbf1810afc26db5/out/librand-fdbf1810afc26db5.rlib --extern rand=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rand/fdbf1810afc26db5/out/librand-fdbf1810afc26db5.rmeta --extern rand_xorshift=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rand_xorshift/934927e553085397/out/librand_xorshift-934927e553085397.rlib --extern rand_xorshift=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-unknown-linux-gnu/dist/build/rand_xorshift/934927e553085397/out/librand_xorshift-934927e553085397.rmeta -Zannotate-moves '--check-cfg=cfg(feature,values(any()))' -Zunstable-options -Zmacro-backtrace -Csplit-debuginfo=off -Cprefer-dynamic -Zinline-mir -Zinline-mir-preserve-debug -Zmir_strip_debuginfo=locals-in-tiny-functions -Clink-args=-Wl,-z,origin '-Clink-args=-Wl,-rpath,$ORIGIN/../lib' -Alinker-messages '-Zcrate-attr=doc(html_root_url="https://doc.rust-lang.org/nightly/")' -Cpanic=abort -Zpanic-abort-tests -Z binary-dep-depinfo --force-warn=unused_crate_dependencies` (exit status: 254)
Bootstrap failed while executing `--stage 1 test library/coretests`
Currently active steps:
test::Crate { build_compiler: Compiler { stage: 1, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, mode: Std, crates: ["coretests"] } at src/bootstrap/src/core/build_steps/test.rs:3623
Build completed unsuccessfully in 0:13:16
  local time: Thu Oct 29 18:21:41 UTC 2026

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For more information how to resolve CI failures of this job, visit this link.

rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 29, 2026

rustbot assigned Mark-Simulacrum Oct 29, 2026

rustbot added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 29, 2026

rustbot assigned oli-obk and unassigned Mark-Simulacrum Oct 29, 2026

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yara-blue force-pushed the reflection-type-id-variant-getter branch from e285bfa to 9cc6dd5 Compare October 29, 2026 16:44

oli-obk requested changes Oct 29, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 29, 2026

yara-blue added 4 commits October 29, 2026 17:55

reflection: add TypeId::name

ef5be98

reflection: move field-representing-field intrinsics

fc84f78

reflection: add type_name to tests

7a7157d

reflection: add variant_getter (gets variant at runtime)

5204d40

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reflection: add coretests for variant_getter

c414965

yara-blue force-pushed the reflection-type-id-variant-getter branch from 9cc6dd5 to c414965 Compare October 29, 2026 18:02

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