Introduce TerminatorValue to force handling of divergence

[jjcnn]

Description

This PR refactors how Value is used during IR conversion. The PR introduces TerminatorValue which forces the converter to handle diverging expressions properly. There were checks for divergence scattered around the code before, but it's now been made more systematic and thus more robust.

Additionally, this change allows for early returns from some functions. For instance, if the condition of a while diverges, then we don't need to generate code for the body of the loop. In fact, if an expression does diverge then the algorithm assumes that a terminator value (e.g., a branch or a return) is returned, but this was not always the case, and led to a number of spurious checks that probably didn't quite work.

The heavy lifting is done by the macro return_on_termination_or_extract, which acts as destructor of TerminatorValues.

Most changes are straightforward, but do review the functions compile_lazy_op, compile_fn_call, compile_if, compile_while_loop, compile_var_decl and compile_array_expr extra carefully, as they contain changes to the conversion algorithm. Mostly the changes are small, but they need an extra pair of eyes. In a few cases the algorithm was and still is a bit convoluted, but that's because the typechecker unhelpfully assigns Unknown instead of Never to diverging expressions, so the first sub-expression needs to be compiled and divergence checked before we can generate IR for anything that relies on the type of the expression.

Finally, I have taken the liberty of removing Value::is_diverging() and using its alias Value::is_terminator() everywhere. The concept of divergence lives in the source language, whereas the concept of terminators lives in the IR, so is_terminator seems to be more appropriate at this stage of the compilation.

Fixes #2051.

Partial fix of #5438 (the call to deterministically_aborts() is removed).

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[jjcnn]

@IGI-111 : This PR isn't actually ready for review yet. I just opened it as a draft to trigger the CI checks.

Sorry, I should have written it in the PR description.

[IGI-111]

Yeah no problem, I understand it's not finished. I try to comment on drafts too so we waste no time in the proper review, but don't feel compelled by it if it's still WIP 😄

[jjcnn]

I've made a small change to compile_if to improve readability.

[IGI-111]

LGTM

[tritao]

In a few cases the algorithm was and still is a bit convoluted, but that's because the typechecker unhelpfully assigns Unknown instead of Never to diverging expressions, so the first sub-expression needs to be compiled and divergence checked before we can generate IR for anything that relies on the type of the expression.

Is this something we should be looking to fix in the future? Should we create an issue to track this improvement as well?

[ironcev]

Awesome job!! The code is much more cleaner and robust now. Not to mention that the bugs are fixed 😄

Just a few comments worth addressing to avoid terminology clashes and confusions.

[jjcnn]

In a few cases the algorithm was and still is a bit convoluted, but that's because the typechecker unhelpfully assigns Unknown instead of Never to diverging expressions, so the first sub-expression needs to be compiled and divergence checked before we can generate IR for anything that relies on the type of the expression.

Is this something we should be looking to fix in the future? Should we create an issue to track this improvement as well?

I think so. I actually thought there was an issue on it already, but I can't find it. I'll open one up.