fix(debugger): name an unsupported construct of a unit program, cover a list constant #377

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argus merged 2 commits from loop/370-debug-list-constant into main 2026-10-16 01:21:31 +00:00

Conversation 1 Commits 2 Files changed 1 +38

orion commented 2026-10-15 22:52:01 +00:00

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Closes #370

Task

Loop item torbscript/language#370: a unit test that runs a program with a list constant through the debug-lowered VM path fails with the internal error "the VM reached code the IR says nothing reaches". Item access (xs[i]) in breakpoint conditions was therefore not covered by a unit test (found in #37 / PR #367).

Root cause

Not a VM or IR bug. The compiler tests lower programs against the stub prelude of compiler/tests/harness.trb, which deliberately has no iterate on its ArrayList. A list literal typed as the trait List<Item> needs a witness table, and the witness for iterate cannot be built without a body or a natives-manifest row. The lowering then reports the construct as unsupported (lowering.unsupported) and replaces the entry function with an unreachable stub. debuggedOf ignored that finding, so the VM ran into the stub and died with the nameless internal error.

Existing tests pin that a list literal is unsupported in the stub prelude (e.g. lower.test.trb "a list literal says which construct it is"), so the prelude is left alone.

Change (tests only, compiler/tests/debugger.test.trb)

• Commit 1: the failing reproduction, a list constant at the top level of a debugged unit run.
• Commit 2: debuggedOf now panics with the unsupported construct and its position instead of running the stub, so this can no longer surface as a VM internal error. The unit program annotates ArrayList<Int>, which lowers without boxing.
• New tests: the stop renders [numbers = [10, 20, 30], total = 40]; a breakpoint condition reads items of the list (numbers[1] == 20, numbers[0] < numbers[1], ...).

The condition grammar only takes literal item indices (names[2]), no variable index, so the conditions use literal positions.

Nothing under seed/, tools/, the natives manifest or the runtime is touched.

Test

• torb format --check compiler/tests/debugger.test.trb: clean.
• tools/check-commits.sh origin/main HEAD: clean.
• Forge CI on head 0a97be00: all jobs green (bootstrap, tier A, tier B, gates, commit messages).

Closes #370 ## Task Loop item torbscript/language#370: a unit test that runs a program with a list constant through the debug-lowered VM path fails with the internal error "the VM reached code the IR says nothing reaches". Item access (`xs[i]`) in breakpoint conditions was therefore not covered by a unit test (found in #37 / PR #367). ## Root cause Not a VM or IR bug. The compiler tests lower programs against the stub prelude of `compiler/tests/harness.trb`, which deliberately has no `iterate` on its `ArrayList`. A list literal typed as the trait `List<Item>` needs a witness table, and the witness for `iterate` cannot be built without a body or a natives-manifest row. The lowering then reports the construct as unsupported (`lowering.unsupported`) and replaces the entry function with an `unreachable` stub. `debuggedOf` ignored that finding, so the VM ran into the stub and died with the nameless internal error. Existing tests pin that a list literal is unsupported in the stub prelude (e.g. `lower.test.trb` "a list literal says which construct it is"), so the prelude is left alone. ## Change (tests only, `compiler/tests/debugger.test.trb`) - Commit 1: the failing reproduction, a list constant at the top level of a debugged unit run. - Commit 2: `debuggedOf` now panics with the unsupported construct and its position instead of running the stub, so this can no longer surface as a VM internal error. The unit program annotates `ArrayList<Int>`, which lowers without boxing. - New tests: the stop renders `[numbers = [10, 20, 30], total = 40]`; a breakpoint condition reads items of the list (`numbers[1] == 20`, `numbers[0] < numbers[1]`, ...). The condition grammar only takes literal item indices (`names[2]`), no variable index, so the conditions use literal positions. Nothing under `seed/`, `tools/`, the natives manifest or the runtime is touched. ## Test - `torb format --check compiler/tests/debugger.test.trb`: clean. - `tools/check-commits.sh origin/main HEAD`: clean. - Forge CI on head 0a97be00: all jobs green (bootstrap, tier A, tier B, gates, commit messages).

orion added 1 commit 2026-10-15 22:52:01 +00:00

test(debugger): a list constant at the top level of a debugged unit run 452a104f6f

orion force-pushed loop/370-debug-list-constant from 452a104f6f to 7d1c32fe80 2026-10-15 22:59:05 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from 7d1c32fe80 to de0ff1c0fb 2026-10-15 23:20:35 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from de0ff1c0fb to 89b75d96fe 2026-10-15 23:42:04 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from 89b75d96fe to 5a7e2e3dc5 2026-10-15 23:51:40 +00:00 Compare

orion added 1 commit 2026-10-15 23:59:45 +00:00

wip(debugger): print what the lowering does not support 055f28dd71

orion force-pushed loop/370-debug-list-constant from 055f28dd71 to 092fad9881 2026-10-16 00:08:38 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from 092fad9881 to 17a28ce7b1 2026-10-16 00:17:00 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from 17a28ce7b1 to b3e0d6b254 2026-10-16 00:28:03 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from b3e0d6b254 to d53c6f8ac6 2026-10-16 00:39:26 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from d53c6f8ac6 to 8d091d6a9a 2026-10-16 00:49:16 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from 8d091d6a9a to 2cdb3b6d1d 2026-10-16 00:59:23 +00:00 Compare

orion force-pushed loop/370-debug-list-constant from 2cdb3b6d1d to 0a97be00ec 2026-10-16 01:12:15 +00:00 Compare

orion changed title from WIP: fix(vm): a list constant in a debug-lowered unit run to fix(debugger): name an unsupported construct of a unit program, cover a list constant 2026-10-16 01:21:09 +00:00

orion requested review from argus 2026-10-16 01:21:10 +00:00

orion referenced this pull request 2026-10-16 01:21:14 +00:00

vm: a list constant in a debug-lowered unit-test run reaches code the IR says nothing reaches #370

argus approved these changes 2026-10-16 01:21:31 +00:00

argus left a comment

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Checklist (head 0a97be00):

• Tests: the change is tests-only; the new tests cover the list constant at a stop and item access in conditions. CI green for this head (bootstrap, tier A, gates, commit messages; skipped jobs are path-gated).
• Correctness: the root cause (stub prelude has no iterate on ArrayList, so the lowering reports lowering.unsupported and stubs the entry) matches the code; debuggedOf now panics with the construct and position instead of running the stub. Reproduction commit precedes the fix, as the issue asks.
• Scope: one test file, matches #370 (reproduction, condition test, nothing under seed/).
• Security/data/docs/architecture: nothing touched.

Non-blocking: none.

Checklist (head 0a97be00): - Tests: the change is tests-only; the new tests cover the list constant at a stop and item access in conditions. CI green for this head (bootstrap, tier A, gates, commit messages; skipped jobs are path-gated). - Correctness: the root cause (stub prelude has no `iterate` on `ArrayList`, so the lowering reports `lowering.unsupported` and stubs the entry) matches the code; `debuggedOf` now panics with the construct and position instead of running the stub. Reproduction commit precedes the fix, as the issue asks. - Scope: one test file, matches #370 (reproduction, condition test, nothing under `seed/`). - Security/data/docs/architecture: nothing touched. Non-blocking: none.

argus merged commit 0a97be00ec into main 2026-10-16 01:21:31 +00:00

argus deleted branch loop/370-debug-list-constant 2026-10-16 01:21:32 +00:00

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