feature(cairo): Support Span<T> destructuring via fixed-size array pa…

[ilyalesokhin-starkware]

…tterns

Add support for let [a, b] = span else { ... } syntax by introducing a SliceDestructure flow control node that calls tuple_from_span to convert Span to [T; N] at runtime.

---

Note

Medium Risk
Touches semantic type-checking and match/let lowering by adding a new flow-control node and runtime conversion path; bugs here could affect pattern matching correctness and generated code for matches involving Span<T> and fixed-size array patterns.

Overview
Adds support for matching/destructuring Span<T> with fixed-size array patterns (e.g. let [a, b] = span else { ... } and match span { [a,b] => ... }).

Lowering now builds a size-dispatch chain for Span<T> patterns via a new FlowControlNode::SliceDestructure, which calls corelib tuple_from_span (with FixedSizedArrayInfoImpl) and handles success/failure branching. Semantic pattern computation is updated to accept fixed-size array patterns on Span types, and diagnostics/tests are extended to cover the new behavior.

^{Reviewed by Cursor Bugbot for commit dbd4689. Bugbot is set up for automated code reviews on this repo. Configure here.}

[reviewable-StarkWare]

This change is 

[ilyalesokhin-starkware]

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

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[ilyalesokhin-starkware]

@ilyalesokhin-starkware resolved 1 discussion.
Reviewable status: 0 of 11 files reviewed, 1 unresolved discussion (waiting on orizi).

[orizi]

@orizi made 2 comments.
Reviewable status: 0 of 11 files reviewed, 3 unresolved discussions (waiting on ilyalesokhin-starkware).

---

corelib/src/test/language_features/match_test.cairo line 26 at r3 (raw file):

            assert_eq!(b, @20);
            assert_eq!(c, @30);
        },

Suggestion:

        [a, b, c] => {
            assert_eq!((*a, *b, *c), (10, 20, 30));
        },

---

crates/cairo-lang-lowering/src/lower/flow_control/test_data/match line 2603 at r3 (raw file):

    match s {
        [a, b] => *a + *b,
        [a, b] => *a + *b,

add also:

Suggestion:

    match s {
        [a, b] | [a, b, _] => *a + *b,

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 465 at r5 (raw file):

                        group.filter.add(idx);
                        group.patterns.push(None);
                    }

Not sure if I should do that like in create_node_for_enum, or just use break here.

Code quote:

                    opt_wildcard_idx = Some(idx);
                    for group in size_groups.values_mut() {
                        group.filter.add(idx);
                        group.patterns.push(None);
                    }

[ilyalesokhin-starkware]

corelib/src/test/language_features/match_test.cairo line 26 at r3 (raw file):

            assert_eq!(b, @20);
            assert_eq!(c, @30);
        },

Done.

[ilyalesokhin-starkware]

@ilyalesokhin-starkware made 1 comment and resolved 1 discussion.
Reviewable status: 0 of 11 files reviewed, 3 unresolved discussions (waiting on orizi).

---

crates/cairo-lang-lowering/src/lower/flow_control/test_data/match line 2603 at r3 (raw file):

Previously, orizi wrote…

add also:

Done.

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 465 at r5 (raw file):

Previously, ilyalesokhin-starkware wrote…

Not sure if I should do that like in create_node_for_enum, or just use break here.

see r5 <-> r6.

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 503 at r10 (raw file):

Previously, orizi wrote…

just the goto at the end the arm to the continuation of the flow.

We don't generated code for the unreacheable arms, just diagnostics

[ilyalesokhin-starkware]

@ilyalesokhin-starkware made 2 comments.
Reviewable status: 8 of 11 files reviewed, 4 unresolved discussions (waiting on orizi).

---

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 354 at r10 (raw file):

            elem_ty,
        );
    }

Done.

---

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 443 at r10 (raw file):

    let snapshot_member_id = members.iter().next().unwrap().1.id;
    let snapshot_array_ty = members.iter().next().unwrap().1.ty;
    let snapshot_array_var = graph.new_var(snapshot_array_ty, location);

Done.

[orizi]

@orizi reviewed 3 files and all commit messages, made 1 comment, and resolved 3 discussions.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on ilyalesokhin-starkware).

---

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 411 at r9 (raw file):

Previously, ilyalesokhin-starkware wrote…

"can open the type"?

i think i didn't understand the original question.

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 411 at r9 (raw file):

Previously, orizi wrote…

i think i didn't understand the original question.

I'm wondering if we are going to have a

 Span { snapshot: ... }

pattern that is not a catch all pattern.

In that case, I can't just break after reaching that pattern.

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 411 at r9 (raw file):

Previously, ilyalesokhin-starkware wrote…

I'm wondering if we are going to have a

 Span { snapshot: ... }

pattern that is not a catch all pattern.

In that case, I can't just break after reaching that pattern.

fox example:

if
Span { snapshot: inner } if inner.len() == 5 was supported, then the break is incorrect.

but I think the code to support it would be much more complicated.

[orizi]

@orizi made 1 comment.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on ilyalesokhin-starkware).

---

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 411 at r9 (raw file):

Previously, ilyalesokhin-starkware wrote…

fox example:

if
Span { snapshot: inner } if inner.len() == 5 was supported, then the break is incorrect.

but I think the code to support it would be much more complicated.

we must cover all patterns - either by explicit matching - or a final _ - same as rust

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 411 at r9 (raw file):

Previously, orizi wrote…

we must cover all patterns - either by explicit matching - or a final _ - same as rust

The question is not whether we cover everything.

It is whether the code can assume that a struct pattern on a slice is a catch-all pattern.

[orizi]

@orizi made 1 comment.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on ilyalesokhin-starkware).

---

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 411 at r9 (raw file):

Previously, ilyalesokhin-starkware wrote…

The question is not whether we cover everything.

It is whether the code can assume that a struct pattern on a slice is a catch-all pattern.

yes - match guards would change that - but since we don't have them currently - it is fine.

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 411 at r9 (raw file):

Previously, orizi wrote…

yes - match guards would change that - but since we don't have them currently - it is fine.

Done.

[orizi]

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

[orizi]

@orizi reviewed all commit messages and made 1 comment.
Reviewable status: 10 of 11 files reviewed, 1 unresolved discussion (waiting on ilyalesokhin-starkware).

---

crates/cairo-lang-lowering/src/lower/flow_control/test_data/match line 3077 at r13 (raw file):

//! > lowering_diagnostics
error[E3004]: Match is non-exhaustive: `(Span { snapshot: _ }, _)` not covered.

this seems like a bug - right?

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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^{Reviewed by Cursor Bugbot for commit 9be728f. Configure here.}

[cursor]

Struct pattern break drops subsequent reachable arms in tuples

High Severity

The break after processing a Span { snapshot: ... } struct pattern in create_slice_destructure_chain incorrectly drops all subsequent patterns. While Span { snapshot: inner } is a catch-all for the span dimension, it is NOT a catch-all when nested inside a tuple match — subsequent arms with different patterns on other tuple elements remain reachable. This causes valid exhaustive matches like match (s, val) { ..., (Span { snapshot: inner }, 5) => ..., _ => 0 } to produce a false Match is non-exhaustive compilation error, because the _ => 0 arm is never added to any filter. The test confirms this via the Missing node (node 5) and the spurious error diagnostic.

Additional Locations (1)

• crates/cairo-lang-lowering/src/lower/flow_control/test_data/match#L3066-L3067

 

^{Reviewed by Cursor Bugbot for commit 9be728f. Configure here.}

[ilyalesokhin-starkware]

fixed

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/test_data/match line 3077 at r13 (raw file):

Previously, orizi wrote…

this seems like a bug - right?

yes, it seems that the approch of breaking after a wildcard does not work.

[orizi]

@orizi made 2 comments and resolved 1 discussion.
Reviewable status: 9 of 11 files reviewed, 3 unresolved discussions (waiting on ilyalesokhin-starkware).

---

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 467 at r15 (raw file):

                    inner_patterns: vec![None; fallback_group.inner_patterns.len()],
                    inner_bindings: fallback_group.inner_bindings.clone(),
                });

Suggestion:

                let group = size_groups.entry(n).or_insert_with(|| fallback_group.clone());

---

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 549 at r15 (raw file):

    // callback. The path string is `"_"` because the match has no `[..]` syntax to name the
    // fallback case.
    let mut failure_node = {

Suggestion:

    let mut current_node = {

[ilyalesokhin-starkware]

fixed

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 467 at r15 (raw file):

                    inner_patterns: vec![None; fallback_group.inner_patterns.len()],
                    inner_bindings: fallback_group.inner_bindings.clone(),
                });

Done.

[ilyalesokhin-starkware]

crates/cairo-lang-lowering/src/lower/flow_control/create_graph/patterns.rs line 549 at r15 (raw file):

    // callback. The path string is `"_"` because the match has no `[..]` syntax to name the
    // fallback case.
    let mut failure_node = {

Done.

[orizi]

@orizi reviewed 3 files and all commit messages, made 1 comment, and resolved 2 discussions.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on ilyalesokhin-starkware).

[ilyalesokhin-starkware]

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

[ilyalesokhin-starkware]

@ilyalesokhin-starkware reviewed 11 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ilyalesokhin-starkware).