1629. gopy compile golden tests

This spec lives next to 1625 (the broad v0.5 testing strategy) but narrows to one thing: the disassembly-golden corpus that pins the output of the full compile pipeline (symtable -> codegen -> flowgraph -> assemble) for a curated panel of programs.

The marshal-byte parity gate against host CPython lands in v0.8 with the import system. Until then the disassembly text is the gate: it captures opcode names, opargs (after EXTENDED_ARG recombination), nested code objects, and emit order. A change in any of those flips the diff.

Why disassembly text, not marshal bytes

• Marshal-equivalent bytes need a code-object encoder that mirrors CPython's w_object PyCode arm. That arm depends on TYPE_LONG for co_firstlineno etc., on the REF de-dup table, on tuple/frozenset parity for co_consts, and on the line/exception tables already matching byte-for-byte. Each piece is a separate v0.8 follow-on.
• Disassembly text round-trips through compile.Disassemble, which is the same surface a future dis.dis will sit behind. Locking it now means we catch drift in opcode emission, oparg widening, and nested-code traversal before the marshal layer arrives.
• We have no parser. Every fixture builds its AST in Go, which keeps the corpus self-contained: a fixture that wants to exercise a syntax form spells the AST literally, no python -c round-trip.

Layout

v05test/
├── gate_test.go          # structural panel (unchanged)
├── golden_test.go        # the runner described here
└── testdata/
    └── golden/
        ├── empty_module.golden
        ├── simple_assign.golden
        ├── binary_add.golden
        ├── load_after_store.golden
        ├── if_pass.golden
        ├── while_pass.golden
        ├── def_add_one.golden
        ├── async_def_pass.golden
        ├── class_pass.golden
        └── type_alias.golden

Each .golden file is the literal output of compile.Disassemble on the fixture's compiled module. The runner reads the file at test time and compares with ==. A mismatch dumps the unified diff.

Refresh contract

go test ./v05test/ -update -run TestGolden rewrites every golden in place. The -update flag is a flag.Bool registered by the test file via flag.Bool("update", false, ...). There is no separate generator binary: the test is the generator.

The expected workflow is:

1. Edit codegen / flowgraph / assemble.
2. Run the gate. If a golden diffs, inspect the diff and decide: either the change is intended (re-run with -update) or unintended (fix the code).
3. Commit the .go change and the regenerated .golden in the same commit so future bisects line up.

Fixture rules

• Self-contained AST: each fixture is a Go function that returns a *ast.Module built from the helpers already in gate_test.go (module, nameLoad, nameStore, cnst, findInnerCode).
• No parser: never call out to the host CPython, never read a .py source file. The test must run on a hermetic builder.
• One concept per fixture: an if fixture exercises only if; a def fixture only def. Compounding hides regressions.
• Stable across cosmetic refactors: a fixture that assigns x = 1 does not also store the int constant 0 to __doc__ or pull in import machinery. The corpus minimises noise so a real bytecode delta shows up clean in the diff.

Panel

Every entry below is one .golden file. The leftmost column is the filename stem; the source column is the literal Python the AST spells.

Stem	Source	Pins
empty_module	(empty)	implicit LOAD_CONST None / RETURN_VALUE trailer
simple_assign	x = 1	LOAD_CONST + STORE_NAME
binary_add	a = 1 + 2	flowgraph int-int fold to constant 3
load_after_store	x = 1\nx	STORE_NAME then LOAD_NAME round-trip
if_pass	if x: pass	POP_JUMP_IF_FALSE shape
while_pass	while x: pass	back-edge JUMP threading
def_add_one	def f(x): return x + 1	nested code in Consts, MAKE_FUNCTION + LOAD_FAST
async_def_pass	async def f(): pass	CoCoroutine on the inner Code
class_pass	class C: pass	LOAD_BUILD_CLASS + CALL count
type_alias	type X = int	INTRINSIC_TYPEALIAS argument

Two fixtures are deferred behind the same gap that holds the structural panel back (TestGateTryExcept, TestGateComprehension): the linear stack-depth analyser cannot seed handler entry from an exception edge or comprehension back-edge. Both fixtures land here once the CFG-based analyser ships.

Test-runner shape

//go:generate go test -update -run TestGolden ./...

var update = flag.Bool("update", false,
    "rewrite v05test/testdata/golden/*.golden files in place")

type goldenCase struct {
    name string
    mod  *ast.Module
}

func goldenPanel() []goldenCase { /* the table above */ }

func TestGolden(t *testing.T) {
    for _, tc := range goldenPanel() {
        t.Run(tc.name, func(t *testing.T) {
            co, err := compile.Compile(tc.mod, "<gate>", 0)
            if err != nil {
                t.Fatalf("Compile: %v", err)
            }
            got := compile.Disassemble(co)
            path := filepath.Join("testdata", "golden", tc.name+".golden")
            if *update {
                if err := os.WriteFile(path, []byte(got), 0o644); err != nil {
                    t.Fatal(err)
                }
                return
            }
            want, err := os.ReadFile(path)
            if err != nil {
                t.Fatalf("read %s: %v (run with -update to create)", path, err)
            }
            if got != string(want) {
                t.Fatalf("disassembly diff for %s:\n--- want\n%s\n+++ got\n%s",
                    tc.name, want, got)
            }
        })
    }
}

Two things to note about the shape:

• One test, many subtests: t.Run per fixture gives readable per-fixture pass/fail without a per-fixture _test.go file.
• -update writes everything: a single command refreshes all goldens after a deliberate codegen change. No flag means strict compare; CI never has -update in its argv.

CI integration

The existing test job runs go test -race -count=1 ./... which includes the golden runner. CI does not pass -update, so any unmerged code change that flips bytecode flags the gate.

The lint job runs golangci-lint run. The runner has no special needs there beyond the rest of the codebase.

Gate

• v05test/golden_test.go runner with -update.
• All ten fixtures in the panel emit a checked-in .golden.
• go test ./v05test/ is green without -update.
• go test ./v05test/ -update rewrites every .golden in one go and the result diffs to nothing immediately after.

Out of scope for v0.5

• Marshal-byte parity against CPython. v0.8.
• Source-text fixtures. Pending the parser port.
• Negative goldens (compile errors): the panel is positive cases only. Error-message fixtures live alongside the validator panel in ast/validate_panel_test.go.
• Property tests over random ASTs. Would need a generator that respects symtable invariants; deferred.