Add indexed Array.set and fixed-size array constructor

[ATX24]

The error

A mutable random-write array update was not expressible. Before this change, the smallest .set repro failed:

function main() -> int {
  let dp = [0, 0, 0, 0, 0, 0];
  dp.set(5, 9);
  dp.at(5) ?? 0
}

Old compiler output:

error: type `int[]` has no member `set`
   ╭─[ main.baml:2:31 ]
   │
 2 │ ╭─▶   let dp = [0, 0, 0, 0, 0, 0];
 3 │ ├─▶   dp.set(5, 9);
   │ │                     
   │ ╰───────────────────── type `int[]` has no member `set`
   │     
   │     Note: Error code: E0007
───╯

The requested fixed-size constructor syntax also failed before the change:

function main() -> int {
  let dp = int[](6, 0);
  dp.set(5, 9);
  dp.at(5) ?? 0
}

Old compiler output:

error: unresolved name: int
   ╭─[ main.baml:2:12 ]
   │
 2 │   let dp = int[](6, 0);
   │            ─┬─  
   │             ╰─── unresolved name: int
   │ 
   │ Note: Error code: E0003
───╯

error: Expected expression, found ']'
   ╭─[ main.baml:2:16 ]
   │
 2 │   let dp = int[](6, 0);
   │                ┬  
   │                ╰── Expected expression, found ']'
   │ 
   │ Note: Error code: E0010
───╯

error: Expected ']', found ';'
   ╭─[ main.baml:2:23 ]
   │
 2 │   let dp = int[](6, 0);
   │                       ┬  
   │                       ╰── Expected ']', found ';'
   │ 
   │ Note: Error code: E0010
───╯

Root cause

• crates/baml_builtins2/baml_std/baml/containers.baml defined Array<T> methods such as at, push, and splice, but did not define an indexed set method or any fixed-size constructor.
• crates/baml_compiler_parser/src/parser.rs only treated [] in expression position as array literals or indexing, so int[](6, 0) was parsed as a value path int followed by an invalid empty index expression rather than as an array constructor.
• crates/baml_compiler2_tir/src/builder.rs::try_container_method_call only had evolving-container special handling for push/append, so unannotated arrays could not establish or validate element type through .set.
• crates/bex_vm/src/package_baml/array.rs had no native runtime implementation for either operation.

The fix

• Added Array<T>.set(index: int, value: T) -> null throws baml.errors.InvalidArgument to the stdlib and implemented it in the VM array package with negative/out-of-bounds validation.
• Added Array<T>.new(size: int, default: T) -> T[] throws baml.errors.InvalidArgument and parser/AST lowering for T[](size, default) as sugar for baml.Array.new<T>(size, default).
• Extended TIR evolving-array mutation handling so let xs = []; xs.set(i, v) type-checks consistently with push, including index validation as int and value validation against the element type.
• Added focused engine tests for successful preallocation + indexed write, out-of-bounds set, and negative constructor size.
• Updated stdlib/compiler snapshots and compiler2 mutability docs.

Verification

Reproduction after the change:

PATH="$HOME/.local/bin:$PATH" RUSTFLAGS="-C link-arg=-lnode" \
  cargo run -q -p baml_cli --manifest-path /workspace/baml_language/Cargo.toml -- \
  check --from /tmp/baml-array-project

Output:

warning: using the internal BAML toolchain binary directly is not recommended. Use `baml` instead.
     Loading /tmp/baml-array-project/baml_src/main.baml
    Checking 1 file(s)
   Compiling 1 file(s)
    Finished checked 1 file(s) in 4s

PATH="$HOME/.local/bin:$PATH" RUSTFLAGS="-C link-arg=-lnode" \
  cargo run -q -p baml_cli --manifest-path /workspace/baml_language/Cargo.toml -- \
  run main --from /tmp/baml-array-project

Output:

warning: using the internal BAML toolchain binary directly is not recommended. Use `baml` instead.
     Loading /tmp/baml-array-project/baml_src/main.baml
    Checking 1 file(s)
   Compiling 1 file(s)
warning: Your code is unformatted — run `baml fmt` to format it. Continuing.
     Running main
9
    Finished main in 4s

Test commands run from baml_language/:

cargo test -p bex_engine --test array_set_constructor
# test result: ok. 3 passed; 0 failed; 0 ignored

PATH="$HOME/.local/bin:$PATH" RUSTFLAGS="-C link-arg=-lnode" cargo test --lib
# passed

PATH="$HOME/.local/bin:$PATH" RUSTFLAGS="-C link-arg=-lnode" cargo test --workspace --exclude sdk_test_typescript_node
# exit_code: 0

CI=true PATH="$HOME/.local/bin:$PATH" ./sdk_tests/crates/typescript_node/setup.sh
PATH="$HOME/.local/bin:$PATH" CARGO_MANIFEST_DIR="/workspace/baml_language/sdk_tests/crates/typescript_node" \
  ./target/debug/deps/sdk_test_typescript_node-0df1538f20582eac
# test result: ok. 15 passed; 0 failed; 0 ignored

Note: sdk_test_typescript_node is run with its documented setup split because sdk_tests/README.md states plain cargo test regenerates the TypeScript fixtures without running their setup script.

  

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