gccrs:fix ICE with continue/break/return in while condition

[Harishankar14]

Fixes ICE when continue/break/return/loop are used as while conditions.

Fixes #3977

The predicate expression must be evaluated before type checking to ensure side effects occur even when the predicate has Never type. This prevents skipping function calls, panics, or other side effects in diverging predicates.

Proof of fix using -fdump-tree-gimple

As requested, here is the GIMPLE dump for the test case while loop {} {}. It confirms that the infinite loop side-effect is preserved correctly:

__attribute__((cdecl))
struct () test::main ()
{
  struct () D.107;

  <D.101>:
  <D.103>:
  {
    <D.104>:
    {
      <D.102>:
      goto <D.102>; // Side-effect correctly preserved
      if (0 != 0) goto <D.105>; else goto <D.106>;
      <D.106>:
      {

      }
    }
    goto <D.104>;
    <D.105>:
  }
  goto <D.103>;
  return D.107;
}

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[powerboat9]

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=f4db2628116a78ca94e09a3cf1bcfb21

Having a predicate of type ! should work -- it gets coerced into a boolean. See https://doc.rust-lang.org/reference/types/never.html, though don't get confused by the last paragraph

[Harishankar14]

@powerboat9 Thanks for the feedback! I dont really know how did i miss this.. I've updated the fix to properly handle never-type coercion. Instead of rejecting continue as a value, the compiler now:

Detects when the while predicate has type ! (never)
Coerces it to bool by treating it as true (infinite loop)
Issues a warning about unreachable code

[Harishankar14]

@powerboat9, @philberty

[philberty]

i think this can be simplified a fair bit but good job.

[Harishankar14]

@philberty LMK, if any changes required. Applied all the ones which you pointed out. :)

[Harishankar14]

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=f4db2628116a78ca94e09a3cf1bcfb21

Having a predicate of type ! should work -- it gets coerced into a boolean. See https://doc.rust-lang.org/reference/types/never.html, though don't get confused by the last paragraph

@powerboat9 This should do, lmk your thoughts.

[philberty]

nearly there

[Harishankar14]

nearly there

Yeah completed as per your suggestions,waiting for your review @philberty

[philberty]

you need to fix the commit format CI is failing for you.

[Harishankar14]

you need to fix the commit format CI is failing for you.

Yep, done @philberty

[philberty]

yeah that condition isnt correct you still need to make the loop evaluate the expression you should be using -fdump-tree-gimple to verify the logic

[philberty]

i dont think this is right to just override to boolean_true_node it means the never expression will never get evaluated

[powerboat9]

I think ctx->add_statement takes care of that

[philberty]

OK I think this is ready but before we merge can you show an example of -fdump-tree-gimple in the git commit message of what we are doing here for this senario? But also put it in the github description

[Harishankar14]

OK I think this is ready but before we merge can you show an example of -fdump-tree-gimple in the git commit message of what we are doing here for this senario? But also put it in the github description

Sure will do.