feature(semantic): Recognize Span<T> in fixed-size array patterns

[ilyalesokhin-starkware]

Teach the semantic layer to accept [a, b, c] patterns when matching on Span<T>, converting the pattern type to @[T; N]. The lowering layer reports an unsupported-type diagnostic for now; full lowering support will follow in a subsequent PR.

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Note

Medium Risk
Changes semantic pattern typing so [a, b, ...] may type-check against Span<T>, which can affect type inference and diagnostics in match/let-else. Lowering explicitly rejects these patterns for now, so there’s limited runtime impact but potential for new compile-time behaviors/regressions.

Overview
Enables fixed-size array patterns like [a, b] to be semantically valid when the matched expression has type Span<T>, by treating the match as a fixed-size array of length N (with snapshot-wrapped element type) and updating tuple-like pattern validation logic accordingly.

Lowering is not implemented yet: when matching on a struct type, lowering now detects FixedSizeArray patterns (Span destructures) and emits an UnsupportedMatchedType match diagnostic instead of attempting struct deconstruction. Diagnostics and test expectations were updated (notably E2106) and new tests cover valid Span destructuring plus common error cases (non-Span structs, Array<T> vs Span<T>, and trait inference failures).

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[reviewable-StarkWare]

This change is 

[ilyalesokhin-starkware]

crates/cairo-lang-semantic/src/expr/test_data/let_statement line 156 at r1 (raw file):

                    ^^^^^^^^^

error[E2106]: Unexpected type for fixed size array pattern. "(<missing>, <missing>, <missing>)" is not a fixed size array or a span.

The error go a bit wierd, what do you think about doing something like:
#9825

Code quote:

error[E2106]: Unexpected type for fixed size array pattern. "(<missing>, <missing>, <missing>)" is not a fixed size array or a span.

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[ilyalesokhin-starkware]

• feature(cairo): Support Span<T> destructuring via fixed-size array pa… #9823
• feature(semantic): Recognize Span<T> in fixed-size array patterns #9826 👈 (View in Graphite)
• main

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[orizi]

@orizi reviewed 5 files and all commit messages, and made 1 comment.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on eytan-starkware and ilyalesokhin-starkware).

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crates/cairo-lang-semantic/src/expr/test_data/let_statement line 156 at r1 (raw file):

Previously, ilyalesokhin-starkware wrote…

The error go a bit wierd, what do you think about doing something like:
#9825

lgtmed it.

[ilyalesokhin-starkware]

@ilyalesokhin-starkware resolved 2 discussions.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on eytan-starkware).

[ilyalesokhin-starkware]

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

[orizi]

@orizi reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ilyalesokhin-starkware).

[orizi]

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