Previous version leaned on chess-specific terminology (GlyphCache, render,
frame arena) and made the rule read like a project memo. Replaced with a
generic `LongLived` example, a two-question test for when to apply, and
no incident-specific narrative. The "field name is by convention" line
removes the implicit prescription of `parent_allocator` so projects can
follow their own naming.
Also drops the explicit cross-reference list of existing examples — those
already drift with the code; the principle is enough to recognise the
shape when it appears.
The chess panel-text regression (text vanished after the first move on
macOS) had a single root cause: GlyphCache's entries List, hash table,
and shaped_buf grew through `context.allocator` — which during render
is the per-frame arena. On the next arena reset the backing died, and
subsequent glyph lookups read garbage / wrote into freshly-allocated
view-tree memory.
Fix is shaped as the user proposed: `List(T)`'s mutations take an
optional trailing `alloc: Allocator = context.allocator` argument. No
allocator stored on the container, no init ceremony, every existing
`list.append(item)` callsite keeps working unchanged. Long-lived
owners now write `list.append(item, self.parent_allocator)` and the
arena-leak bug becomes impossible to write accidentally.
Default-arg substitution previously only fired for identifier callees
(`expandCallDefaults` at lower.zig:7978). Extended to the generic
struct-method dispatch path (`list.append(...)` lands here) via a new
`appendDefaultArgs` helper that lowers fd.params[i].default_expr in
the caller's scope and appends to the lowered args slice.
Long-lived owners updated to capture `parent_allocator: Allocator` at
init and use it for every internal growth:
- GlyphCache (the chess bug) — entries, shaped_buf, hash_keys,
hash_vals, atlas bitmap.
- DockInteraction — drops the existing `push Context` workaround in
`ensure_capacity` for the explicit-arg form.
- StateStore — entries list + per-entry data buffer.
- Gles3Gpu, MetalGPU — shaders, buffers, textures (atlas-grow during
render would otherwise leak resources into the frame arena).
Also kept: an operator-precedence fix in pipeline.sx
(`(self.frame_index & 1) == 0` instead of
`self.frame_index & 1 == 0`, which parses as
`self.frame_index & (1 == 0)` = always 0). That was a stealth
single-arena-only bug that masked the GlyphCache one for a long time.
Docs:
- specs.md §11 documents `param: T = expr` default parameter values.
The parser already supported it — formalised in the spec now.
- current/CHECKPOINT-MEM.md logs the change.
- CLAUDE.md REJECTED PATTERNS gains a "Long-lived containers growing
through context.allocator" section with the `parent_allocator`
capture template and the list of existing examples to mirror.
155/155 example tests pass — zero-diff against snapshots since every
existing callsite still resolves to `context.allocator`.
Sibling to the silent-fallback-defaults rule. Catch-all `else`
branches that pass a value through unchanged, write a default width,
or swallow errors into a zero-init are the same class of bug — a
case the implementer didn't think of corrupts data silently.
Both bites this session:
- `storeAtRawPtr` writing 8 bytes regardless of IR type (fixed by
threading val_ty through inst.Store).
- `.deref` else-arm returning val unchanged (now errors loudly for
raw pointers).
- comptime init catch swallowing the error into `.void_val`.
Preferred order: implement the arm in the same step. If plumbing is
out of scope, bail loudly with `bailDetail(comptime msg)` and leave a
one-line comment about what's needed. Width/type/layout info that's
ambiguous from the Value tag belongs in the IR op struct, not in a
"this case is usually 8 bytes" shortcut.
