[ASDAlexander77]

Summary

Under -mm=own, an array's a.keys() and a.values() failed with:

'this' is a parameter, kept here, but -mm=own cannot make the callers of '___Array.keys' give it up

Both are generators whose first parameter is this (function* keys<T>(this: T[])).

The cause is in the generator lowering. buildGeneratorWrapperDeclaration gives next the body's this through an alias, const .this = this, because next has a this of its own.

• For a method, that is the class's this, which the state holds by value.
• For a this parameter, the alias was a separate local initialised from the parameter and captured into the state's box as a cell. That cell is not the parameter's, so the state owned a copy of the caller's array.

The fix, in two parts:

• When this is a parameter, the alias is now a ConstRef, so the name is the parameter's storage, as for-in's .a is its array's.
• ConstRefs are normally never captured (setIgnoreCapturing, for .a); .this now is. So next captures the parameter's ts.Param itself, and the state borrows this like any other parameter (spec §19):
  • it is bounded by this
  • it is walked while the array lives
  • it can only be returned by a function that says @borrows("this")

Under the other models the alias was a second reference to the same array; now it is the parameter's own cell.

Spec §40 is new, and docs/own-memory-model.md is updated.

Tests

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

  • an extension generator over this: C[] in a for...of
  • a generic one held in a local
  • one returned by a @borrows("this") function
  • one over a temporary array in the statement that made it

  Without the change it fails: @borrows("this") is refused because nothing the function returns borrows. AOT under own: 200000 iterations peak at 4.4 MB, against 4.3 MB at 1000.

• New negative own_err_generator_this_param_returned: the generator returned by a function without @borrows("this").

• Full ctest -C Release: 5892 passed, 0 failed.

• DefaultLib suites, with the companion @borrows("this") on Array's iterator():

  • rc, gc and none pass every test.
  • own goes from 142 to 145/170 (array_keys, array_values, array_iterator), with no new failures.

Not here: entries(), whose yield [i, this[i]] puts a borrowed element into a tuple the caller owns.

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