[ASDAlexander77]

Summary

Under -mm=own, new Set([1, 2, 3]).values().take(2) failed with "borrows its generator's arguments and cannot be stored, returned or captured". The cause:

• values() is a generator whose state borrows the set (spec §19), so its result is bounded by the set.
• take says @keeps (spec §38), so passing that result to take counted as keeping it.

The temporary Set was never freed early; this rule alone refused the code.

The helper's state owns the bounded state, so it is bounded by the same arguments. When boundedUses meets the bounded result as a kept argument of a call that returns a @keeps generator's state (ownsKeptState), it now follows that call's result as it follows a local holding the bounded one. That result's uses are then checked against the ends of the original arguments:

• Allowed: walking the chain in place: a for...of, .next() in the same statement, a local holding it, or another helper given it (.drop(1).take(2)).
• Still refused: returning it, storing it, or using it after its source ends.

Nothing changes at release: the helper's state frees the bounded state, and that release gives back nothing it borrows.

Spec §39 is new. docs/own-memory-model.md no longer says s.values().take(2) is an error.

Tests

• New own_generator_keeps_bounded, run under own, rc, gc and none with churn between steps:

  • helpers over a generator that borrows a temporary
  • two helpers deep
  • one step in the statement that made it
  • a chain over a named source held in a local

  AOT under own: 200000 iterations peak at 4.4 MB, the same as 1000.

• own_err_generator_keeps_bounded from -mm=own: a generator that says @keeps owns the arguments its state holds #699 walked the chain in place, which is now allowed. It now returns the chain instead. New own_err_generator_keeps_bounded_outlives uses the chain after its source is overwritten.

• Full ctest -C Release: 5882 passed, 0 failed.

• DefaultLib suites, with ASDAlexander77/TypeScriptCompilerDefaultLib's take/drop limit made a number (separate PR):

  • rc, gc and none pass every test.
  • own goes from 140 to 142/170 (iterator_take, iterator_drop), with no new failures.

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