Wobbling ty constraint order

TY_CONSTRAINT_SET_ORDER perturbs both the builder-local TDD-variable order and the local typevar order used to orient typevar-to-typevar constraints. The setting is fixed for the lifetime of each test process.

• unset/0: normal ordering;
• reverse: reverse both orderings;
• an integer: XOR each local ID with that mask. Small masks immediately perturb dense arena IDs: 1 swaps adjacent IDs, 3 reverses blocks of four, and powers of two exchange neighboring blocks.

This deliberately changes internal TDD shape. Run mdtests only: graph-structure unit snapshots are expected to differ. Never enable snapshot updates for a wobble run, since updating would hide the failures being sought.

Run

From the Ruff root, first establish the normal baseline, then run the reversed and XOR-masked orders sequentially. Set TY_CONSTRAINT_ORDER_LOG_DIR to retain logs in a particular writable directory; otherwise mktemp chooses an appropriate temporary directory (respecting the environment's temporary-directory configuration).

set -u

if test -n "${TY_CONSTRAINT_ORDER_LOG_DIR:-}"; then
    log_dir="$TY_CONSTRAINT_ORDER_LOG_DIR"
    mkdir -p "$log_dir"
else
    log_dir="$(mktemp -d -t ty-constraint-order.XXXXXXXX)"
fi
printf '%s\n' "logs: $log_dir"

if cargo nextest --version >/dev/null 2>&1; then
    runner=nextest
else
    runner=test
fi
printf '%s\n' "runner: cargo $runner"

export CARGO_PROFILE_DEV_OPT_LEVEL=1
export CARGO_PROFILE_DEV_DEBUG=line-tables-only
export INSTA_UPDATE=no
export MDTEST_UPDATE_SNAPSHOTS=0
unset INSTA_FORCE_PASS || true

for order in normal reverse 1 2 3 4 7 8 15; do
    if test "$order" = normal; then
        unset TY_CONSTRAINT_SET_ORDER || true
    else
        export TY_CONSTRAINT_SET_ORDER="$order"
    fi

    log="$log_dir/ty-constraint-order-${order}.log"
    if test "$runner" = nextest; then
        cargo nextest run -p ty_python_semantic --test mdtest \
            --no-fail-fast --status-level fail --failure-output immediate-final \
            >"$log" 2>&1
    else
        cargo test -p ty_python_semantic --test mdtest >"$log" 2>&1
    fi
    status=$?

    printf '%-7s exit=%s\n' "$order" "$status"
    grep -E 'Summary \[|test result:' "$log" | tail -1 || true
    printf '%s\n' "  log: $log"
done

Read each failing log and report the mdtest file, section, line, expected result, and actual diagnostic/revealed type. A wobble failure is evidence that inference semantics or displayed solution types still depend on ordering; do not update the mdtest expectations merely to make the wobble run green.

The knob does not perturb hashing of Salsa-backed values. The Solution binding order follows constraint source order section of regression/constraint_set_ordering.md separately varies typevar declaration order to catch binding-order changes caused by draining an FxHashMap<BoundTypeVarInstance, ...>.