own: a let given only fields borrows each until the next assignment; a let that only holds parameters owns nothing (TSLANG 56, 57) - #850

TSLANG_ISSUES 56 and 57, found by compiling powder_sandbox's -mm=own build (native_ts_graphics as its submodule) with the compiler from main. Write-ups are in TSLANG_ISSUES.md.

56: a let given only fields borrows each until the next assignment

let layout = this.computeLayout; ... if (this.fp16Enabled) { layout = this.computeLayoutFP16; ... } (ntg examples/nri/16bit_arithmetic_nri.ts) was refused: "borrows a field and cannot be assigned; -mm=own cannot prove that yet".

• Which locals: an owning let qualifies (borrowsOnly) when its declaration and every assignment give it a read of a place or a parameter.
• How the borrows are bounded: each value's borrow covers only the reads its own store reaches before another assignment, and the assignments end it (collectBorrowerUses, with the store as from).
• The assignment itself: its Store, and rc's Retain right before it, are taken as that read's borrower and bookkeeping (borrowingAssignmentOf, in checkPlaceRead).
• Now accepted: a field written once the let holds another field, in a loop too.
• Still refused:
  • the field the let currently holds written before a use, directly or through a call;
  • the let stored or returned;
  • a fresh value assigned to it (the camera's dir = rotateAround(...)).
• One sound case is still refused: in a loop that reads the let, reassigns it, then writes the first field, a read after the loop is taken to see the first field.

57: a let that only holds parameters owns nothing

let src = order; let dst = scratch; ... const swap = src; src = dst; dst = swap; (ntg core/octomap_common.ts, the exported sortDescending) was refused: "a parameter, kept here, but -mm=own cannot make the callers ... give it up". So was let s = a; return s.length;.

• The cause: MLIRGen makes every let an owning local, so storing a parameter into one counted as keeping the parameter.
• The fix: borrowParametersInLocals runs at the start of the signature pass, before the facts. It turns a let that only ever holds parameters, or another such let's value, into a plain local and drops rc's counting for it.
• Left as they were: Shared<T> handles, which own counts, and copyable values such as strings.
• Still refused: storing such a let, capturing it, or giving it a fresh value. These are refused as they would be for the parameter itself.
• Result: octomap_common.ts compiles under own, and with it examples/render_octomap.ts.

Effect on the sweep (276 sources, every facts object built)

• Sources failing: 143 → 141.
• Errors from borrow-only lets: all gone. What remains in donut_first_person_camera.ts is the camera's two, where a fresh array is assigned.
• ntg core/ modules that don't compile under own: 7 → 6.
• No file got worse, and powder_sandbox's own src/ compiles clean.

Also noted in TSLANG_ISSUES.md, for native_ts_graphics rather than the compiler: ray_tracing_extended_scene.ts's exported constructors need @keeps(...). I checked that @keeps works on a constructor.

Tests

• New tests: 00tslang_issue_56.ts and 00tslang_issue_57.ts, under every model, JIT and compiled. Both were also run as an exe by hand.
• New own error tests: own_err_let_borrows_reassigned_overwritten and own_err_parameter_let_stored.
• ctest (Windows Release): 7238/7238.
• DefaultLib main rebuilt with this compiler, release and debug, all four models. tests.ps1 for each model is running; results to follow in a comment.

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