[ASDAlexander77]

Summary

Under -mm=own, a function that returns this (copyWithin, reverse) gets a result that borrows this (__own_result_borrows). Spec §22.4 then kept every string read out of this from being copied, so two shapes failed with "borrows an element":

• this[t] = this[s]
• a swap's let v = this[i]

That fact is about what is returned. copyForRetain now makes a copy in two cases:

• A store into an element or a field: a read of the parameter the result borrows, stored through a ts.ElementRef or a ts.PropertyRef.
• A let that cannot be returned: a let initialised from such a read starts from a copy when mayBeReturned finds that no load of it, or of a local it is stored into, reaches a ts.ReturnInternal.

A read that is returned, directly or through a local, stays the borrow. A parameter the function keeps is unchanged.

Spec §41 is new and §22.4 points to it. docs/own-memory-model.md lists the remaining limit: sort and reverse over class instances, where a swap's local would be a second owner.

Tests

• New own_copy_within_returned_param, over strings built at run time with churn between steps, under own, rc, gc and none:

  • this[t] = this[s] in a function that returns this
  • a swap through a let in one
  • a read returned directly
  • a read returned through a local

  AOT under own: 1000, 200000 and 400000 iterations peak at 4.0, 4.0 and 4.4 MB, so a returned read copied by mistake would show as a leak.

• Full ctest -C Release: 5901 passed, 0 failed.

• DefaultLib suites, with the companion reverse/sort change (ASDAlexander77/TypeScriptCompilerDefaultLib, PR to follow):

  • rc, gc and none pass every test.
  • own goes from 145 to 150/170 (array_copyWithin, owning_copies, array_reverse, array_sort, string_localeCompare), with no new failures.

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