Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Add memoization for const function evaluations #66294

Merged
merged 2 commits into from
Nov 28, 2019
Merged

Add memoization for const function evaluations #66294

Show file tree
Hide file tree
Changes from 1 commit
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
5398139
Add memoization for const function evaluations
davidhewitt Nov 11, 2019
a28fbd4
Correct typo in src/librustc_mir/const_eval.rs
davidhewitt Nov 27, 2019
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
5 changes: 4 additions & 1 deletion src/librustc/mir/interpret/mod.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -123,7 +123,10 @@ use rustc_data_structures::tiny_list::TinyList;
use rustc_macros::HashStable;
use byteorder::{WriteBytesExt, ReadBytesExt, LittleEndian, BigEndian};

/// Uniquely identifies a specific constant or static.
/// Uniquely identifies one of the following:
/// - A constant
/// - A static
/// - A const fn where all arguments (if any) are zero-sized types
#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, RustcEncodable, RustcDecodable)]
#[derive(HashStable, Lift)]
pub struct GlobalId<'tcx> {
Expand Down
11 changes: 10 additions & 1 deletion src/librustc_mir/const_eval.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -146,7 +146,16 @@ fn eval_body_using_ecx<'mir, 'tcx>(
let name = ty::tls::with(|tcx| tcx.def_path_str(cid.instance.def_id()));
let prom = cid.promoted.map_or(String::new(), |p| format!("::promoted[{:?}]", p));
trace!("eval_body_using_ecx: pushing stack frame for global: {}{}", name, prom);
assert!(body.arg_count == 0);

// Assert all args (if any) are zero-sized types; `eval_body_using_ecx` doesn't
// make sense if the body is expecting nontrival arguments.
davidhewitt marked this conversation as resolved.
Show resolved Hide resolved
// (The alternative would be to use `eval_fn_call` with an args slice.)
for arg in body.args_iter() {
let decl = body.local_decls.get(arg).expect("arg missing from local_decls");
let layout = ecx.layout_of(decl.ty.subst(tcx, cid.instance.substs))?;
assert!(layout.is_zst())
};

ecx.push_stack_frame(
cid.instance,
body.span,
Expand Down
33 changes: 32 additions & 1 deletion src/librustc_mir/interpret/terminator.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use syntax::source_map::Span;
use rustc_target::spec::abi::Abi;

use super::{
InterpResult, PointerArithmetic,
GlobalId, InterpResult, PointerArithmetic,
InterpCx, Machine, OpTy, ImmTy, PlaceTy, MPlaceTy, StackPopCleanup, FnVal,
};

Expand Down Expand Up @@ -284,6 +284,18 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
ty::InstanceDef::DropGlue(..) |
ty::InstanceDef::CloneShim(..) |
ty::InstanceDef::Item(_) => {
// If this function is a `const fn` then as an optimization we can query this
// evaluation immediately.
//
// For the moment we only do this for functions which take no arguments
// (or all arguments are ZSTs) so that we don't memoize too much.
if self.tcx.is_const_fn_raw(instance.def.def_id()) &&
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This means we do that even in the Miri interpreter, when such functions are run at "run-time", right? I think that's wrong: the function, even without any arguments, can access global state and cause aliasing violations on each call. So I think this PR as-is will regress Miri's ability to find UB.

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I propose to instead move memoization into const_eval's find_fn implementation.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Is there a situation where a const fn can access global state which itself is not constant? This seems like the only situation to me where aliasing may be an issue? (But I think you know much more about Miri than me! 😄)

Copy link
Contributor Author

@davidhewitt davidhewitt Nov 27, 2019 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I propose to instead move memoization into const_eval's find_fn implementation.

How would you like such an implementation to work? Memoize the evaluation of the body and then return that from find_fn? Or memoize something else entirely?

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Well we might have to expand what find_fn can return, and the name might have to change reflecting that.

Is there a situation where a const fn can access global state which itself is not constant?

Right now, probably not, but as const fn get more powerful and allow unsafe code, that seems very possible to me. #66302 is a step in that direction. At that point CTFE relies on dynamic checks (at CTFE evaluation time), which Miri of course doesn't do as it runs run-time code.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Right now, probably not, but as const fn get more powerful and allow unsafe code, that seems very possible to me.

Yikes - I wasn't aware we'll one day have unsafe code in const fn!

Well we might have to expand what find_fn can return, and the name might have to change reflecting that.

Ok. I'm going to be unavailable for the next week but will follow up on rust-lang/miri#1081 on my return.

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Right now, probably not, but as const fn get more powerful and allow unsafe code, that seems very possible to me

If we ever allowed (without it being UB) you to mutate or read mutable state in const fn, we couldn't ever do a MIR optimization that evals a const fn call where all args are known at compile-time. Also it would allow constants to read from global mutable state in its original state.

There's no need for dynamic checks if we do post monomorphization checks for this situation.

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

If we ever allowed (without it being UB) you to mutate or read mutable state in const fn, we couldn't ever do a MIR optimization that evals a const fn call where all args are known at compile-time. Also it would allow constants to read from global mutable state in its original state.

We certainly shouldn't allow that during CTFE, but that's not what I talked about. I am talking about Miri-the-standalone-tool.

There's no need for dynamic checks if we do post monomorphization checks for this situation.

Now I am very confused, the dynamic checks are post-monomorphization? Both are the same thing to me in this context. Also I thought we all agreed that we should check everything dynamically so that even "Miri unleashed" is sound?

I thought we had consensus that the dynamic check described all the way at the bottom here should definitely be added. (Not that that is very relevant to this PR, but you brought it up.)

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Right, I don't remember what I meant by that.

args.iter().all(|a| a.layout.is_zst())
{
let gid = GlobalId { instance, promoted: None };
return self.eval_const_fn_call(gid, ret);
}

// We need MIR for this fn
let body = match M::find_fn(self, instance, args, ret, unwind)? {
Some(body) => body,
Expand Down Expand Up @@ -449,6 +461,25 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
}
}

/// Evaluate a const function where all arguments (if any) are zero-sized types.
/// The evaluation is memoized thanks to the query system.
fn eval_const_fn_call(
&mut self,
gid: GlobalId<'tcx>,
ret: Option<(PlaceTy<'tcx, M::PointerTag>, mir::BasicBlock)>,
) -> InterpResult<'tcx> {
trace!("eval_const_fn_call: {:?}", gid);

let place = self.const_eval_raw(gid)?;
let dest = ret.ok_or_else(|| err_ub!(Unreachable))?.0;
oli-obk marked this conversation as resolved.
Show resolved Hide resolved

self.copy_op(place.into(), dest)?;

self.return_to_block(ret.map(|r| r.1))?;
self.dump_place(*dest);
return Ok(())
}

fn drop_in_place(
&mut self,
place: PlaceTy<'tcx, M::PointerTag>,
Expand Down
14 changes: 14 additions & 0 deletions src/test/ui/consts/const-fn-zst-args.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
// build-pass

// Check that the evaluation of const-functions with
// zero-sized types as arguments compiles successfully

struct Zst {}

const fn foo(val: Zst) -> Zst { val }

const FOO: Zst = foo(Zst {});

fn main() {
const _: Zst = FOO;
}
5 changes: 5 additions & 0 deletions src/test/ui/consts/const-size_of-cycle.stderr
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,11 @@ note: ...which requires const-evaluating + checking `Foo::bytes::{{constant}}#0`
LL | bytes: [u8; std::mem::size_of::<Foo>()]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires const-evaluating `Foo::bytes::{{constant}}#0`...
--> $DIR/const-size_of-cycle.rs:5:17
|
LL | bytes: [u8; std::mem::size_of::<Foo>()]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires const-evaluating `std::mem::size_of`...
--> $SRC_DIR/libcore/mem/mod.rs:LL:COL
|
LL | intrinsics::size_of::<T>()
Expand Down
6 changes: 3 additions & 3 deletions src/test/ui/consts/uninhabited-const-issue-61744.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,11 +1,11 @@
// compile-fail

pub const unsafe fn fake_type<T>() -> T {
hint_unreachable() //~ ERROR any use of this value will cause an error
hint_unreachable()
}

pub const unsafe fn hint_unreachable() -> ! {
fake_type()
fake_type() //~ ERROR cycle detected when const-evaluating `hint_unreachable` [E0391]
}

trait Const {
Expand All @@ -15,5 +15,5 @@ trait Const {
impl <T> Const for T {}

pub fn main() -> () {
dbg!(i32::CONSTANT); //~ ERROR erroneous constant used
dbg!(i32::CONSTANT);
}
80 changes: 14 additions & 66 deletions src/test/ui/consts/uninhabited-const-issue-61744.stderr
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,73 +1,21 @@
error: any use of this value will cause an error
error[E0391]: cycle detected when const-evaluating `hint_unreachable`
--> $DIR/uninhabited-const-issue-61744.rs:8:5
|
LL | fake_type()
| ^^^^^^^^^^^
|
note: ...which requires const-evaluating `fake_type`...
--> $DIR/uninhabited-const-issue-61744.rs:4:5
|
LL | hint_unreachable()
| ^^^^^^^^^^^^^^^^^^
| |
| reached the configured maximum number of stack frames
| inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<i32>` at $DIR/uninhabited-const-issue-61744.rs:12:36
...
LL | const CONSTANT: i32 = unsafe { fake_type() };
| ---------------------------------------------
|
= note: `#[deny(const_err)]` on by default

error[E0080]: erroneous constant used
--> $DIR/uninhabited-const-issue-61744.rs:18:10
= note: ...which again requires const-evaluating `hint_unreachable`, completing the cycle
note: cycle used when const-evaluating `fake_type`
--> $DIR/uninhabited-const-issue-61744.rs:4:5
|
LL | dbg!(i32::CONSTANT);
| ^^^^^^^^^^^^^ referenced constant has errors
LL | hint_unreachable()
| ^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors
error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.
For more information about this error, try `rustc --explain E0391`.
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Does removing the use of i32::CONSTANT prevent this hard error? Previously, using #![allow(const_err)] made this supressible.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Removing the use of i32::CONSTANT does indeed prevent the error. It would be nice if the error trace included mention of i32::CONSTANT so that users might figure this out.

I've found a similar case on stable, which I've opened as a separate issue: #66332

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

If the constant were public, the error would persist. So this is a breaking change, where a deny by default lint was changed into an error, but only if using the constant would have errored anyway

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

iirc there's a way to mark a query so that its cycles do something other than report a hard error, but I don't remember the details and whether it would help us here. I'll have a look tomorrow when I'm on a computer

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Great, thanks

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Sorry. This got lost in my todo list.

So We only have cycle_delay_bug, which is not really what we want here, because it will still cause the compilation to fail. We could specifically add a cycle_future_incompat_lint that works exactly like cycle_delay_bug except that it emits a future incompat lint instead.

cc @michaelwoerister I'm not sure if the query system can handle a query succeeding when a cycle occurs.

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Any cycle occurring should be fatal for the compilation session. I don't think we ever quite clarified if cycles can leave incr. comp. in an invalid state and sidestep the issue by never committing such possibly invalid state.
(cc @nikomatsakis, correct me if I'm wrong)

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Recovering from a query cycle is also problematic for a parallel compiler since the location we can recover from is not deterministic.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Seems like the conclusion is that we will leave this cycle error as a failure. This is technically a breaking change (move from unusable-but-valid-compile to compile-fail). It's a weird edge case that didn't seem likely to be in use in the wild; indeed no regressions were found in the crater run.

2 changes: 1 addition & 1 deletion src/test/ui/infinite/infinite-recursion-const-fn.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
//https://github.com/rust-lang/rust/issues/31364

const fn a() -> usize { b() } //~ ERROR evaluation of constant value failed
const fn a() -> usize { b() } //~ ERROR cycle detected when const-evaluating `a` [E0391]
const fn b() -> usize { a() }
const ARR: [i32; a()] = [5; 6];

Expand Down
69 changes: 12 additions & 57 deletions src/test/ui/infinite/infinite-recursion-const-fn.stderr
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,66 +1,21 @@
error[E0080]: evaluation of constant value failed
error[E0391]: cycle detected when const-evaluating `a`
--> $DIR/infinite-recursion-const-fn.rs:3:25
|
LL | const fn a() -> usize { b() }
| ^^^
| |
| reached the configured maximum number of stack frames
| inside call to `b` at $DIR/infinite-recursion-const-fn.rs:3:25
|
note: ...which requires const-evaluating `b`...
--> $DIR/infinite-recursion-const-fn.rs:4:25
|
LL | const fn b() -> usize { a() }
| ---
| |
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| ^^^
= note: ...which again requires const-evaluating `a`, completing the cycle
note: cycle used when const-evaluating `ARR::{{constant}}#0`
--> $DIR/infinite-recursion-const-fn.rs:5:18
|
LL | const ARR: [i32; a()] = [5; 6];
| --- inside call to `a` at $DIR/infinite-recursion-const-fn.rs:5:18
| ^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.
For more information about this error, try `rustc --explain E0391`.
8 changes: 6 additions & 2 deletions src/test/ui/invalid_const_promotion.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -17,12 +17,16 @@ use std::env;
use std::process::{Command, Stdio};

// this will panic in debug mode and overflow in release mode
//
// NB we give bar an unused argument because otherwise memoization
// of the const fn kicks in, causing a different code path in the
// compiler to be executed (see PR #66294).
#[stable(feature = "rustc", since = "1.0.0")]
#[rustc_promotable]
const fn bar() -> usize { 0 - 1 }
const fn bar(_: bool) -> usize { 0 - 1 }

fn foo() {
let _: &'static _ = &bar();
let _: &'static _ = &bar(true);
}

#[cfg(unix)]
Expand Down