[Feature] Implement array slicing and concatenation.

[d0cd]

Motivation

This PR adds array slicing and concatenation support to Leo, enabling more expressive and convenient array manipulation.

Array Slicing allows extracting a contiguous portion of an array using Rust-like syntax:

let arr: [u32; 8] = [0u32, 1u32, 2u32, 3u32, 4u32, 5u32, 6u32, 7u32];
let a: [u32; 3] = arr[2..5];    // [2, 3, 4] - exclusive end
let b: [u32; 4] = arr[2..=5];   // [2, 3, 4, 5] - inclusive end
let c: [u32; 5] = arr[3..];     // [3, 4, 5, 6, 7] - slice to end
let d: [u32; 4] = arr[..4];     // [0, 1, 2, 3] - slice from start
let e: [u32; 8] = arr[..];      // full copy

Array Concatenation allows combining two arrays with matching element types using the + operator:

let a: [u32; 2] = [1u32, 2u32];
let b: [u32; 2] = [3u32, 4u32];
let c: [u32; 4] = a + b;        // [1, 2, 3, 4]
let d: [u32; 6] = a + b + a;    // chained concatenation
let e: [u32; 4] = a[0..1] + b;  // slice then concatenate

Both features are resolved at compile time - slices and concatenations are expanded to individual element accesses, ensuring no runtime overhead.

Test Plan

Slicing Tests:

Test	Description
slice_basic.leo	Basic arr[2..5] exclusive end slice
slice_inclusive.leo	Inclusive arr[2..=5] slice
slice_from_start.leo	Open-start arr[..5] slice
slice_to_end.leo	Open-end arr[3..] slice
slice_full.leo	Full slice arr[..]
slice_with_variables.leo	Const variable bounds
slice_out_of_bounds_fail.leo	Error: bounds exceed array length
slice_invalid_range_fail.leo	Error: start > end

Concatenation Tests:

Test	Description
array_concat.leo	Basic a + b, nested arrays, different sizes
array_concat_chained.leo	Chained a + b + c
array_concat_with_slice.leo	Combined a[0..2] + b[2..4]
array_concat_type_mismatch_fail.leo	Error: mismatched element types

Related PRs

• [Feature] Array slicing and concatenation. #28869 - Original array slicing implementation (rebased and incorporated)
• add the ability to slice arrays #28872 - Alternative design reference (concatenation approach adopted)

[d0cd]

Self-Review Finding

Found one bug that should be fixed:

BUG: Empty slices pass type checking but fail at runtime

Location: compiler/passes/src/type_checking/ast.rs (around line 536)

Empty slices like arr[5..5] pass type checking but fail in the interpreter due to inconsistent boundary checks:

Component	Check	Allows start == end?
Type checker	end < start	Yes (produces 0-length slice)
Interpreter	end <= start	No (returns None, causing runtime error)

Fix: Change the type checker to reject start == end:

// Before
if end < start {
    self.emit_err(TypeCheckerError::slice_range_invalid(start, end, input.span()));

// After  
if end <= start {
    self.emit_err(TypeCheckerError::slice_range_invalid(start, end, input.span()));

Consider adding a test case like slice_empty_fail.leo with arr[5..5] to verify the fix.