fix: keep std.trace lazy during static optimization

[He-Pin]

Motivation

std.trace has an observable side effect: it writes a trace message and returns the rest value. The static optimizer previously considered it safe to fold, so it could execute traces while optimizing expressions that the Jsonnet runtime never forces.

That caused two correctness problems:

• unused values such as object fields could emit traces too early;
• trace output could be reordered compared with official Jsonnet runtime behavior.

Key Design Decision

Mark std.trace as not static-safe. This keeps the existing optimizer behavior for pure builtins while preventing the optimizer from executing this side-effecting builtin.

Modification

• Override staticSafe = false on the std.trace builtin.
• Add regression tests proving unused traces inside object fields and format RHS values stay silent.
• Keep a positive regression test proving directly used std.trace still logs and returns its rest value.
• Update the upstream trace.jsonnet.golden trace order to match runtime evaluation order.

Benchmark Results

This is a correctness/compatibility PR, not a performance PR. No benchmark change is expected.

Analysis

StaticOptimizer.tryStaticApply can execute static-safe builtins during optimization. That is correct for pure functions, but std.trace is intentionally observable, so executing it before runtime changes program behavior. Disabling static folding for this builtin preserves Jsonnet laziness and official trace ordering without changing the runtime implementation.

References

• Jsonnet stdlib: std.trace(str, rest) outputs str and returns rest.
• Local commit: e176c85b fix: keep std.trace lazy during static optimization.

Result

./mill --no-server --ticker false --color false -j 1 __.test passed with 438 tests. Formatting was applied with __.reformat, git diff --check passed, and three independent reviews reported no blockers.