Forbid outer variables in const evaluations

[ironcev]

Description

This PR fixes #7520 by forbidding const evaluation of expressions that contain variables from outer scope. Forbidding outer variables also partially tackles #7521 but also exposes an issue of SROA panicking on out of bounds array access. We will treat this issue fully in a separate PR.

This PR is a prerequisite for #7489 because optimizing initialization of aggregates requires valid const evaluation for certain cases.

• Fixes Const eval does not forbid using variables from outer scope #7520.

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).
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• I have added tests that prove my fix is effective or that my feature works.
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[ironcev]

👍

[codspeed-hq]

CodSpeed Performance Report

Merging #7522 will not alter performance

_{Comparing ironcev/outer-vars-in-const-eval (1284e88) with master (73533a4)}

Summary

✅ 25 untouched

[cursor]

PR Summary

Prevents invalid constant evaluation that captured outer-scope variables and strengthens negative test coverage for array bounds.

• Add contains_outer_vars AST walker and codeblock_contains_outer_vars to detect outer-scope variable usage in const expressions
• Gate compile_constant_expression_to_constant and constant handling in code blocks with the outer-var check; skip evaluation when detected
• Import ProjectionKind and HashSet to support traversal and indexing checks
• Tests: add should_fail/const_eval_outer_vars with many outer-var cases; add array_oob_const_index and array_oob_reassignment failing tests; keep array_oob_variable_index with disabled snapshot; remove older array_oob and array_oob_global_const_index tests

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[xunilrj]

Any reason to call this "outer variables"? I was thinking we were forbidding variable declarations "outside" of the const expression, even other const expressions.

Why not something like "forbid non-const variables" or something to that effect...

[ironcev]

Any reason to call this "outer variables"? I was thinking we were forbidding variable declarations "outside" of the const expression, even other const expressions.

Why not something like "forbid non-const variables" or something to that effect...

I am not particularly happy with the term "outer" either, but that's the best I figured out so far 😄 Suggestions are welcome!

The reason for "outer" was that they are coming from a lexical scope(s) outer to the const evaluated expression itself. In other words, they are not declared inside of the expression being const evaluated.

Also, we strictly distinguish between variables and constants, so I don't see that the "outer variable" would imply constants as well.

What would be "non-const variables"? Like, immutable variables? Note that we ban both mutable and immutable variables because we don't know even if immutable are changed between the definition and usage in const. Having "immutable" e.g. b256 changed in asm blocks is a very common pattern. Also, expectedly Rust also bans it.