fix: early return for module file error in AST parsing

[0xllx0]

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Converts an assert into an early return during AST parsing.

Resolves: #4145

[0xllx0]

Not sure on the exact decorators needed here, or if this test should go into the xfail directory. The test is expected to fail compilation, just with an error message instead of an ICE.

[powerboat9]

grep for dg-error in the testsuite directory -- that should give you some nice examples for expecting error messages

[CohenArthur]

one tiny nitpick but the changes are really good, thank you for the work!

[philberty]

You need to fixup the regressions here those tests probably emit a new error now and they need updated accordingly

[0xllx0]

You need to fixup the regressions here those tests probably emit a new error now and they need updated accordingly

I rebased on the latest master (32ce955), and didn't get a failing test for the gcc/testsuite/rust/compile/issue-4145.rs test. Is there something specific where you are noticing regressions? Can you please provide reproduction steps so I can work on resolving the issue?

[powerboat9]

I think .-1 should be .1, if you want to catch an error on the next line instead of the previous line

[0xllx0]

Thanks, I changed it to .+1, and changed xfail to target since the compiler successfully returns an error (opposed to an ICE). Please let me know if that logic makes sense to you.