Fix ICE segfault on empty static loops

[tommyscholly]

This PR addresses #3618. With this change, the expected error of reaching the iteration limit is reported:

test.rs:1:17: error: ‘constexpr’ loop iteration count exceeds limit of 262144 (use ‘-fconstexpr-loop-limit=’ to increase the limit)
    1 | static _X: () = loop {};
      |                 ^~~~

My handling of t being a NULL_TREE may not be appropriate in eval_constant_expression, seeking guidance there.

Fixes #3618

[philberty]

LGTM but i would slightly tweak this to be:

if (t == NULL_TREE)
  return NULL_TREE;
  
if (CONST_CLASS_P (t))
   ... <continue on with rest of code>

just means we have an explicit guard against null at the start. Just for reference this ia port from the gcc/cp/constexpr.cc implementation which fits well as it operates on GCC tree's

Also for your commit message please prefix with gccrs: so something like:

"gccrs: fix ICE on empty constexpr loops

Empty loops have no body which means this is a NULL_TREE during const
evaluation which needs a check.

Fixes Rust-GCC # issue number

"

[tommyscholly]

Implemented those changes!

[philberty]

looks like you have a stray t in your fixes line:

"Fixes Rust-GCC #3618t"

[philberty]

LGTM

[philberty]

Actually sorry you need to add a test case too

[philberty]

it should be enough to add a test case of :

static _X: () = loop {};

into a file in gcc/testsuite/rust/compile/issue-3618.rs

[tommyscholly]

Fat fingered closing the PR by accident.

Added in the test.

[philberty]

Nice work but i just realised yes this need another change sorry. This null_tree check should still stay but you need another check the error should be:

error[E0658]: `loop` is not allowed in a `static`
 --> <source>:1:17
  |
1 | static _X: () = loop {};
  |                 ^^^^^^^
  |
  = note: see issue #52000 <https://github.com/rust-lang/rust/issues/52000> for more information

error: aborting due to previous error

So what you need to do is add a catch here:

gccrs/gcc/rust/backend/rust-compile-expr.cc

Line 671 in fc6b543

TyTy::BaseType *block_tyty = nullptr;

if (ctx->const_context_p ())
 {
     rich_location r;
     rust_error_at (r, ErrorCode::0658, "%<loop%> is not allowed in const context");
     return;
  }

[tommyscholly]

Oh that's my bad. I was comparing with a 1.86 rustc. On version 1.49 it still gives a "exceeded interpreter step limit (see #[const_eval_limit])" error. Should I not be basing functionality on 1.49?

[philberty]

Oh good point whats really funny is if you test 1.45 you get loop is not allowed but in 1.49 you get the exceeded limit error.

uughh.. I think we should go both of these fixes in one commit. So we emit that error and if we need to we can remove it.

[philberty]

@flip1995 @CohenArthur @P-E-P this was interesting patch because the rustc behaviour has changed so much here. later versions seem to fall back to a const eval limit like we have hit here but earlier versions of rust disable loop being allowed in const/static context.

[philberty]

Hmm maybe we should just open up an issue relating the what behaviour we want since your current PR is more like the latest rustc I bet they let loops return values using break etc now so yeah.. lets go with your patch then.

[philberty]

LGTM

[tommyscholly]

Yeah, even on 1.49, this compiles fine actually:

static _X: () = loop {
    break;
};