Stabilise inline_const
# Stabilisation Report
## Summary
This PR will stabilise `inline_const` feature in expression position. `inline_const_pat` is still unstable and will *not* be stabilised.
The feature will allow code like this:
```rust
foo(const { 1 + 1 })
```
which is roughly desugared into
```rust
struct Foo;
impl Foo {
const FOO: i32 = 1 + 1;
}
foo(Foo::FOO)
```
This feature is from https://github.com/rust-lang/rfcs/pull/2920 and is tracked in #76001 (the tracking issue should *not* be closed as it needs to track inline const in pattern position). The initial implementation is done in #77124.
## Difference from RFC
There are two major differences (enhancements) as implemented from the RFC. First thing is that the RFC says that the type of an inline const block inferred from the content *within* it, but we currently can infer the type using the information from outside the const block as well. This is a frequently requested feature to the initial implementation (e.g. #89964). The inference is implemented in #89561 and is done by treating inline const similar to a closure and therefore share inference context with its parent body.
This allows code like:
```rust
let v: Vec<i32> = const { Vec::new() };
```
Another enhancement that differs from the RFC is that we currently allow inline consts to reference generic parameters. This is implemented in #96557.
This allows code like:
```rust
fn create_none_array<T, const N: usize>() -> [Option<T>; N] {
[const { None::<T> }; N]
}
```
This enhancement also makes inline const usable as static asserts:
```rust
fn require_zst<T>() {
const { assert!(std::mem::size_of::<T>() == 0) }
}
```
## Documentation
Reference: rust-lang/reference#1295
## Unresolved issues
We still have a few issues that are not resolved, but I don't think it necessarily has to block stabilisation:
* expr fragment specifier issue: #86730
* ~~`const {}` behaves similar to `async {}` but not to `{}` and `unsafe {}` (they are treated as `ExpressionWithoutBlock` rather than `ExpressionWithBlock`): https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/const.20blocks.20differ.20from.20normal.20and.20from.20unsafe.20blocks/near/290229453~~
## Tests
There are a few tests in https://github.com/rust-lang/rust/tree/master/src/test/ui/inline-const
More DefineOpaqueTypes::Yes
This accepts more code on stable. It is now possible to have match arms return a function item `foo::<ConcreteType>` and a function item `foo::<OpaqueTypeInDefiningScope>` in another, and that will constrain `OpaqueTypeInDefiningScope` to have the hidden type `ConcreteType`. So the following function will now compile, but on master it errors with a type mismatch on the second match arm
```rust
// The function item whose generic params we want to merge.
fn foo<T>(t: T) -> T { t }
// Helper ensuring we can constrain `T` on `F` without explicitly specifying it
fn bind<T, F: FnOnce(T) -> T>(_: T, f: F) -> F { f }
fn k() -> impl Sized {
let x = match true {
true => {
// `f` is `FnDef(foo, [infer_var])`
let f = foo;
// Get a value of an opaque type on stable
let t = k();
// this returns `FnDef(foo, [k::return])`
bind(t, f)
}
false => foo::<()>,
};
todo!()
}
```
r? ``@compiler-errors``
cc https://github.com/rust-lang/rust/issues/116652
Test `#[unix_sigpipe = "inherit"]` with both `SIG_DFL` and `SIG_IGN`
Extend our `#[unix_sigpipe = "inherit"]` test so that it detects if `SIGPIPE` wrongly ends up being `SIG_DFL` when the parent has `SIG_IGN`. We have no current test for this particular case.
Tracking issue: https://github.com/rust-lang/rust/issues/97889
delegation: Support renaming, and async, const, extern "ABI" and C-variadic functions
Also allow delegating to functions with opaque types (`impl Trait`).
The delegation item will refer to the original opaque type from the callee, fresh opaque type won't be created, which seems like a reasonable behavior.
(Such delegation items will cause query cycles when used in trait impls, but it can be fixed later.)
Part of https://github.com/rust-lang/rust/issues/118212.
Also allow `impl Trait` in delegated functions.
The delegation item will refer to the original opaque type from the callee, fresh opaque type won't be created.
Rollup of 3 pull requests
Successful merges:
- #124003 (Dellvmize some intrinsics (use `u32` instead of `Self` in some integer intrinsics))
- #124169 (Don't fatal when calling `expect_one_of` when recovering arg in `parse_seq`)
- #124286 (Subtree sync for rustc_codegen_cranelift)
r? `@ghost`
`@rustbot` modify labels: rollup
Don't fatal when calling `expect_one_of` when recovering arg in `parse_seq`
In `parse_seq`, when parsing a sequence of token-separated items, if we don't see a separator, we try to parse another item eagerly in order to give a good diagnostic and recover from a missing separator:
d1a0fa5ed3/compiler/rustc_parse/src/parser/mod.rs (L900-L901)
If parsing the item itself calls `expect_one_of`, then we will fatal because of #58903:
d1a0fa5ed3/compiler/rustc_parse/src/parser/mod.rs (L513-L516)
For `precise_capturing` feature I implemented, we do end up calling `expected_one_of`:
d1a0fa5ed3/compiler/rustc_parse/src/parser/ty.rs (L712-L714)
This leads the compiler to fatal *before* having emitted the first error, leading to absolutely no useful information for the user about what happened in the parser.
This PR makes it so that we stop doing that.
Fixes#124195
Dellvmize some intrinsics (use `u32` instead of `Self` in some integer intrinsics)
This implements https://github.com/rust-lang/compiler-team/issues/693 minus what was implemented in #123226.
Note: I decided to _not_ change `shl`/... builder methods, as it just doesn't seem worth it.
r? ``@scottmcm``
Rollup of 7 pull requests
Successful merges:
- #120929 (Wrap dyn type with parentheses in suggestion)
- #122591 (Suggest using type args directly instead of equality constraint)
- #122598 (deref patterns: lower deref patterns to MIR)
- #123048 (alloc::Layout: explicitly document size invariant on the type level)
- #123993 (Do `check_coroutine_obligations` once per typeck root)
- #124218 (Allow nesting subdiagnostics in #[derive(Subdiagnostic)])
- #124285 (Mark ``@RUSTC_BUILTIN`` search path usage as unstable)
r? `@ghost`
`@rustbot` modify labels: rollup
Do `check_coroutine_obligations` once per typeck root
We only need to do `check_coroutine_obligations` once per typeck root, especially since the new solver can't really (easily) associate which obligations correspond to which coroutines.
This requires us to move the checks for sized coroutine fields into `mir_coroutine_witnesses`, but that's fine imo.
r? lcnr
deref patterns: lower deref patterns to MIR
This lowers deref patterns to MIR. This is a bit tricky because this is the first kind of pattern that requires storing a value in a temporary. Thanks to https://github.com/rust-lang/rust/pull/123324 false edges are no longer a problem.
The thing I'm not confident about is the handling of fake borrows. This PR ignores any fake borrows inside a deref pattern. We are guaranteed to at least fake borrow the place of the first pointer value, which could be enough, but I'm not certain.
Suggest using type args directly instead of equality constraint
When type arguments are written erroneously using an equality constraint we suggest specifying them directly without the equality constraint.
Fixes#122162
Changes the diagnostic in the issue from:
```rust
error[E0229]: associated type bindings are not allowed here
9 | impl std::cmp::PartialEq<Rhs = T> for S {
| ^^^^^^^ associated type not allowed here
|
```
to
```rust
error[E0229]: associated type bindings are not allowed here
9 | impl std::cmp::PartialEq<Rhs = T> for S {
| ^^^^^^^ associated type not allowed here
|
help: to use `T` as a generic argument specify it directly
|
| impl std::cmp::PartialEq<T> for S {
| ~
```
Use fulfillment in method probe, not evaluation
This PR reworks method probing to use fulfillment instead of a `for`-loop of `evaluate_predicate` calls, and moves normalization from method candidate assembly into the `consider_probe`, where it's applied to *all* candidates. This last part coincidentally fixes https://github.com/rust-lang/rust/issues/121643#issuecomment-1975371248.
Regarding *why* this large rewrite is done: In general, it's an anti-pattern to do `for o in obligations { evaluate(o); }` because it's not compatible with the way that the new solver emits alias-relate obligations which constrain variables that may show up in other predicates.
r? lcnr
Disallow ambiguous attributes on expressions
This implements the suggestion in [#15701](https://github.com/rust-lang/rust/issues/15701#issuecomment-2033124217) to disallow ambiguous outer attributes on expressions. This should resolve one of the concerns blocking the stabilization of `stmt_expr_attributes`.
weak lang items are not allowed to be #[track_caller]
For instance the panic handler will be called via this import
```rust
extern "Rust" {
#[lang = "panic_impl"]
fn panic_impl(pi: &PanicInfo<'_>) -> !;
}
```
A `#[track_caller]` would add an extra argument and thus make this the wrong signature.
The 2nd commit is a consistency rename; based on the docs [here](https://doc.rust-lang.org/unstable-book/language-features/lang-items.html) and [here](https://rustc-dev-guide.rust-lang.org/lang-items.html) I figured "lang item" is more widely used. (In the compiler output, "lang item" and "language item" seem to be pretty even.)
Improve handling of expr->field errors
The current message for "`->` used for field access" is the following:
```rust
error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `->`
--> src/main.rs:2:6
|
2 | a->b;
| ^^ expected one of 8 possible tokens
```
([playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7f8b6f4433aa7866124123575456f54e))
This PR tries to address this by adding a dedicated error message and recovery. The proposed error message is:
```
error: `->` used for field access or method call
--> ./tiny_test.rs:2:6
|
2 | a->b;
| ^^ help: try using `.` instead
|
= help: the `.` operator will dereference the value if needed
```
(feel free to bikeshed it as much as necessary)
Deny gen keyword in `edition_2024_compat` lints
Splits the `keyword_idents` lint into two -- `keyword_idents_2018` and `keyword_idents_2024` -- since each corresponds to a future-compat warning in a different edition. Group these together into a new `keyword_idents` lint group, and add the latter to the `rust_2024_compatibility` so that `gen` is ready for the 2024 edition.
cc `@traviscross` `@ehuss`
Add simple async drop glue generation
This is a prototype of the async drop glue generation for some simple types. Async drop glue is intended to behave very similar to the regular drop glue except for being asynchronous. Currently it does not execute synchronous drops but only calls user implementations of `AsyncDrop::async_drop` associative function and awaits the returned future. It is not complete as it only recurses into arrays, slices, tuples, and structs and does not have same sensible restrictions as the old `Drop` trait implementation like having the same bounds as the type definition, while code assumes their existence (requires a future work).
This current design uses a workaround as it does not create any custom async destructor state machine types for ADTs, but instead uses types defined in the std library called future combinators (deferred_async_drop, chain, ready_unit).
Also I recommend reading my [explainer](https://zetanumbers.github.io/book/async-drop-design.html).
This is a part of the [MCP: Low level components for async drop](https://github.com/rust-lang/compiler-team/issues/727) work.
Feature completeness:
- [x] `AsyncDrop` trait
- [ ] `async_drop_in_place_raw`/async drop glue generation support for
- [x] Trivially destructible types (integers, bools, floats, string slices, pointers, references, etc.)
- [x] Arrays and slices (array pointer is unsized into slice pointer)
- [x] ADTs (enums, structs, unions)
- [x] tuple-like types (tuples, closures)
- [ ] Dynamic types (`dyn Trait`, see explainer's [proposed design](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#async-drop-glue-for-dyn-trait))
- [ ] coroutines (https://github.com/rust-lang/rust/pull/123948)
- [x] Async drop glue includes sync drop glue code
- [x] Cleanup branch generation for `async_drop_in_place_raw`
- [ ] Union rejects non-trivially async destructible fields
- [ ] `AsyncDrop` implementation requires same bounds as type definition
- [ ] Skip trivially destructible fields (optimization)
- [ ] New [`TyKind::AdtAsyncDestructor`](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#adt-async-destructor-types) and get rid of combinators
- [ ] [Synchronously undroppable types](https://github.com/zetanumbers/posts/blob/main/async-drop-design.md#exclusively-async-drop)
- [ ] Automatic async drop at the end of the scope in async context
The current message for "`->` used for field access" is the following:
```rust
error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `->`
--> src/main.rs:2:6
|
2 | a->b;
| ^^ expected one of 8 possible tokens
```
(playground link[1])
This PR tries to address this by adding a dedicated error message and recovery. The proposed error message is:
```
error: `->` used for field access or method call
--> ./tiny_test.rs:2:6
|
2 | a->b;
| ^^ help: try using `.` instead
|
= help: the `.` operator will dereference the value if needed
```
(feel free to bikeshed it as much as necessary)
[1]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7f8b6f4433aa7866124123575456f54e
Signed-off-by: Sasha Pourcelot <sasha.pourcelot@protonmail.com>
Ignore `-C strip` on MSVC
tl;dr - Define `-Cstrip` to only ever affect the binary; no other build artifacts.
This is necessary to improve cross-platform behavior consistency: if someone wanted debug information to be contained only in separate files on all platforms, they would set `-Cstrip=symbols` and `-Csplit-debuginfo=packed`, but this would result in no PDB files on MSVC.
Resolves#114215
fix normalizing in different `ParamEnv`s with the same `InferCtxt`
This PR changes the key of the projection cache from just `AliasTy` to `(AliasTy, ParamEnv)` to allow normalizing in different `ParamEnv`s without resetting caches. Previously, normalizing the same alias in different param envs would always reuse the cached result from the first normalization, which is incorrect if the projection clauses in the param env have changed.
Fixing this bug allows us to get rid of `InferCtxt::clear_caches`, which was only used by the `AutoTraitFinder`, because it requires normalizing in different param envs.
r? `@fmease`
Fix trait solver overflow with `non_local_definitions` lint
This PR fixes the trait solver overflow with the `non_local_definitions` lint reported in https://github.com/rust-lang/rust/issues/123573 using the suggestion from `@lcnr:` https://github.com/rust-lang/rust/issues/123573#issuecomment-2041348320 to use the next trait solver.
~~I have not (yet) tried to create a minimized repro~~ ``@compiler-errors`` did the minimization (thanks you) but I have manually tested on the `starlark-rust` project that it fixes the issue.
Fixes#123573
r? `@lcnr`
Flip spans for precise capturing syntax not capturing a ty/const param, and for implicit captures of lifetime params
Make the primary span point to the opaque, rather than the param which might be very far away (e.g. in an impl header hundreds of lines above).
This handles using deref patterns to choose the correct match arm. This
does not handle bindings or guards.
Co-authored-by: Deadbeef <ent3rm4n@gmail.com>
Fix capturing duplicated lifetimes via parent in `precise_captures` (`impl use<'...>`)
For technical reasons related to the way that `Self` and `T::Assoc` are lowered from HIR -> `rustc_middle::ty`, an opaque may mention in its bounds both the original early-bound lifetime from the parent `impl`/`fn`, *and* the *duplicated* early-bound lifetime on the opaque.
This is fine -- and has been fine since `@cjgillot` rewrote the way we handled opaque lifetime captures, and we went further to allow this behavior explicitly in https://github.com/rust-lang/rust/pull/115659. It's worthwhile to read this PR's technical section to recall how this duplication works and when it acts surprisingly.
The problem here is that the check that make sure that `impl use<'a, 'b>` lists all of the opaque's captured lifetimes wasn't smart enough to consider both these captured lifetimes and the original lifetimes they're duplicated from to be equal. This PR fixes that.
r? oli-obk
Implement Modified Condition/Decision Coverage
This is an implementation based on llvm backend support (>= 18) by `@evodius96` and branch coverage support by `@Zalathar.`
### Major changes:
* Add -Zcoverage-options=mcdc as switch. Now coverage options accept either `no-branch`, `branch`, or `mcdc`. `mcdc` also enables `branch` because it is essential to work.
* Add coverage mapping for MCDCBranch and MCDCDecision. Note that MCDCParameter evolves from llvm 18 to llvm 19. The mapping in rust side mainly references to 19 and is casted to 18 types in llvm wrapper.
* Add wrapper for mcdc instrinc functions from llvm. And inject associated statements to mir.
* Add BcbMappingKind::Decision, I'm not sure is it proper but can't find a better way temporarily.
* Let coverage-dump support parsing MCDCBranch and MCDCDecision from llvm ir.
* Add simple tests to check whether mcdc works.
* Same as clang, currently rustc does not generate instrument for decision with more than 6 condtions or only 1 condition due to considerations of resource.
### Implementation Details
1. To get information about conditions and decisions, `MCDCState` in `BranchInfoBuilder` is used during hir lowering to mir. For expressions with logical op we call `Builder::visit_coverage_branch_operation` to record its sub conditions, generate condition ids for them and save their spans (to construct the span of whole decision). This process mainly references to the implementation in clang and is described in comments over `MCDCState::record_conditions`. Also true marks and false marks introduced by branch coverage are used to detect where the decision evaluation ends: the next id of the condition == 0.
2. Once the `MCDCState::decision_stack` popped all recorded conditions, we can ensure that the decision is checked over and push it into `decision_spans`. We do not manually insert decision span to avoid complexity from then_else_break in nested if scopes.
3. When constructing CoverageSpans, add condition info to BcbMappingKind::Branch and decision info to BcbMappingKind::Decision. If the branch mapping has non-zero condition id it will be transformed to MCDCBranch mapping and insert `CondBitmapUpdate` statements to its evaluated blocks. While decision bcb mapping will insert `TestVectorBitmapUpdate` in all its end blocks.
### Usage
```bash
echo "[build]\nprofiler=true" >> config.toml
./x build --stage 1
./x test tests/coverage/mcdc_if.rs
```
to build the compiler and run tests.
```shell
export PATH=path/to/llvm-build:$PATH
rustup toolchain link mcdc build/host/stage1
cargo +mcdc rustc --bin foo -- -Cinstrument-coverage -Zcoverage-options=mcdc
cd target/debug
LLVM_PROFILE_FILE="foo.profraw" ./foo
llvm-profdata merge -sparse foo.profraw -o foo.profdata
llvm-cov show ./foo -instr-profile=foo.profdata --show-mcdc
```
to check "foo" code.
### Problems to solve
For now decision mapping will insert statements to its all end blocks, which may be optimized by inserting a final block of the decision. To do this we must also trace the evaluated value at each end of the decision and join them separately.
This implementation is not heavily tested so there should be some unrevealed issues. We are going to check our rust products in the next. Please let me know if you had any suggestions or comments.
Don't repeatedly duplicate TAIT lifetimes for each subsequently nested TAIT
Make it so that nested TAITs inherit the lifetimes from their parent item, not their parent TAIT. This is because we don't need to re-duplicate lifetimes for nested TAITs over and over, since the only lifetimes they can capture are from the parent item anyways.
This mirrors how RPITs work. This is **not** a functional change that should be observable, since the whole point of duplicating lifetimes and marking the shadowed ones (and uncaptured ones) as bivariant is designed to *not* be observable.
r? oli-obk
Don't inline integer literals when they overflow - new attempt
Basically #116633 but I implemented the suggested changes.
Fixes#115423. Fixes#116631.
This is my first contribution to this repo so please let me know if I'm supposed to change something :)
Properly handle emojis as literal prefix in macros
Do not accept the following
```rust
macro_rules! lexes {($($_:tt)*) => {}}
lexes!(🐛"foo");
```
Before, invalid emoji identifiers were gated during parsing instead of lexing in all cases, but this didn't account for macro pre-expansion of literal prefixes.
Fix#123696.
Match ergonomics 2024: miscellaneous code cleanups
- Store `ByRef` instead of `BindingAnnotation` in `PatInfo`
- Rename `BindingAnnotation` to `BindingMode`
r? ``@Nadrieril``
cc #123076
``@rustbot`` label A-patterns
Delay interning errors to after validation
fixes https://github.com/rust-lang/rust/issues/122398fixes#122548
This improves diagnostics since validation errors are usually more helpful compared with interning errors that just make broad statements about the entire constant
r? `@RalfJung`
Codegen ZSTs without an allocation
This makes sure that &[] is equivalent to unsafe code (from_raw_parts(dangling, 0)). No new stable guarantee is intended about whether or not we do this, this is just an optimization.
This regressed in #67000 (no comments I can see about that regression in the PR, though it did change the test modified here). We had previously performed this optimization since #63635.
Remove libc from rust_get_test_int uses
`rust_test_helpers.c` has a few unfortunate signatures which have made some of our UI tests _technically_ need the `libc` crate. This is my attempt to evict the need of `libc` for `rust_get_test_int`.
I've deleted `tests/ui/abi/foreign/foreign-no-abi.rs` because the test was originally written to check that `native mod` will compile without an ABI specifier. `native mod` was removed years before 1.0 and the test hasn't checked for anything for a long time.
Delay span bug when `Self` kw resolves to `DefKind::{Mod,Trait}`
Catch the case where `kw::Self` is recovered in the parser and causes us to subsequently resolve `&self`'s implicit type to something that's not a type.
This check could be made more accurate, though I'm not sure how hard we have to try here.
Fixes#123988
This makes sure that &[] is just as efficient as indirecting through
unsafe code (from_raw_parts). No new stable guarantee is intended about
whether or not we do this, this is just an optimization.
Co-authored-by: Ralf Jung <post@ralfj.de>
Use `/* value */` as a placeholder
The expression `value` isn't a valid suggestion; let's use `/* value */` as a placeholder (which is also invalid) since it more clearly signals to the user that they need to fill it in with something meaningful. This parallels the suggestions we have in a couple other places, like arguments.
We could also print the type name instead of `/* value */`, especially if it's suggestable, but I don't care strongly about that.
Taint const qualifs if a static is referenced that didn't pass wfcheck
It is correct to only check the signature here, as the ICE is caused by `USE_WITH_ERROR` trying to allocate memory to store the result of `WITH_ERROR` before evaluating it.
fixes#123153
Stabilize checking of cfgs at compile-time: `--check-cfg` option
This PR stabilize the `--check-cfg` CLI option of `rustc` (and `rustdoc`) 🎉.
In particular this PR does two things:
1. it makes the `--check-cfg` option stable
2. and it moves the documentation to the stable books
FCP: https://github.com/rust-lang/rust/issues/82450#issuecomment-1965328542Resolves#82450
``@rustbot`` labels +S-blocked +F-check-cfg
r? ``@petrochenkov``
Implement syntax for `impl Trait` to specify its captures explicitly (`feature(precise_capturing)`)
Implements `impl use<'a, 'b, T, U> Sized` syntax that allows users to explicitly list the captured parameters for an opaque, rather than inferring it from the opaque's bounds (or capturing *all* lifetimes under 2024-edition capture rules). This allows us to exclude some implicit captures, so this syntax may be used as a migration strategy for changes due to #117587.
We represent this list of captured params as `PreciseCapturingArg` in AST and HIR, resolving them between `rustc_resolve` and `resolve_bound_vars`. Later on, we validate that the opaques only capture the parameters in this list.
We artificially limit the feature to *require* mentioning all type and const parameters, since we don't currently have support for non-lifetime bivariant generics. This can be relaxed in the future.
We also may need to limit this to require naming *all* lifetime parameters for RPITIT, since GATs have no variance. I have to investigate this. This can also be relaxed in the future.
r? `@oli-obk`
Tracking issue:
- https://github.com/rust-lang/rust/issues/123432
Trait predicates for types which have errors may still
evaluate to OK leading to downstream ICEs. Now we return
a selection error for such types in candidate assembly and
thereby prevent such issues
Fix pretty HIR for anon consts in diagnostics
This removes the `NoAnn` printer which skips over nested bodies altogether, which is confusing, and requires users of `{ty|qpath|pat}_to_string` to pass in `&tcx` which now impleemnts `hir_pretty::PpAnn`.
There's one case where this "regresses" by actually printing out the body of the anon const -- we could suppress that, but I don't expect people to actually get anon consts like that unless they're fuzzing, tbh.
r? estebank
Don't even parse an intrinsic unless the feature gate is enabled
Don't return true in `tcx.is_intrinsic` if the function is defined locally and `#![feature(intrinsics)]` is not enabled. This is a slightly more general fix than #123526, since #123587 shows that we have simplifying assumptions about intrinsics elsewhere in the compiler.
This will make the code ICE again if the user **enables** `#[feature(intrinsics)]`, but I kind of feel like if we want to fix that, we should make the `INTERNAL_FEATURES` lint `Deny` again. Perhaps we could do that on non-nightly compilers. Or we should just stop compilation altogether if they have `#![feature]` enabled on a non-nightly compiler.
As for the UX of *real* cases of hitting these ICEs, I believe pretty strongly that if a compiler/stdlib dev is modifying internal intrinsics (intentionally, like when making a change to rustc) we have no guarantee to make the ICE better looking for them. Honestly, *not* spitting out a stack trace is probably a disservice to the people who hit those ICEs in that case.
r? `@Nilstrieb` `@estebank`
