fix: emit diagnostic for unsupported extern fn in const evaluation instead of panicking

[orizi]

Summary

Replace unreachable panic with proper error reporting when user-defined extern const functions are used in constant expressions.

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Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

The code previously used unreachable!() when encountering user-defined extern const functions in constant expressions, which would cause the compiler to panic. This is incorrect behavior as user-defined extern functions should be handled gracefully with a proper diagnostic error rather than crashing the compiler.

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What was the behavior or documentation before?

When a user-defined extern const function was used in a constant expression, the compiler would panic with an "Unexpected extern function in constant lowering" message.

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What is the behavior or documentation after?

The compiler now properly reports error E2127 "This expression is not supported as constant" when encountering user-defined extern const functions in constant expressions, providing a clear diagnostic message to the user instead of crashing.

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Related issue or discussion (if any)

Fixes #9799

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Additional context

The fix includes a test case that verifies the proper error reporting behavior when extern const fn f() -> felt252 nopanic; is called within a constant declaration.

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Note

Low Risk
Low risk: replaces an unreachable!() panic with a diagnostic and adds a regression test, affecting only const-evaluation error handling.

Overview
Fixes constant evaluation to gracefully reject user-defined extern const fn calls by returning E2127 (unsupported constant) instead of hitting an unreachable!() panic.

Adds a regression case in expr/test_data/constant asserting the new diagnostic when const X = f(); calls a user-defined extern const fn.

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[orizi]

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• fix: handle mismatched generic param kinds in substitution rewriter instead of panicking #9815
• fix: allow statement-level consts in named argument shorthand instead of panicking #9814
• fix: use Terminal::text() for modifier text to avoid panic on malformed attribute args #9813
• fix: handle missing struct field in const evaluation instead of panicking #9812
• fix: emit diagnostic for unsupported extern fn in const evaluation instead of panicking #9811 👈 (View in Graphite)
• fix: emit diagnostic for non-type numeric literal suffix instead of panicking #9810
• fix: return None for macro rule with missing param kind instead of panicking #9809
• fix: handle missing return type context in loop body instead of panicking #9808
• fix: emit diagnostic for member access on unreduced impl type instead of panicking #9807
• fix: emit diagnostic for unsupported items in statement position instead of panicking #9806
• main

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[eytan-starkware]

@eytan-starkware reviewed 2 files and all commit messages, and made 1 comment.
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[TomerStarkware]

@TomerStarkware made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on orizi).