[ASDAlexander77]

-mm=own: strings as values (spec §22)

Under -mm=own, a second reference to a string that own cannot prove to be a move or a borrow
becomes a copy (ts.StringCopy). Copies are made only in a function the analysis rejects, and only
for the string retains a dry run of the analysis could not erase, so a function that compiles on
main compiles to the same IR.

• ts.StringCopy and its lowering, under every model (null stays null; an optional is copied only
  when set).
• isCopyableString: string, a string literal type, or an optional or a tag-free union of one
  with null/undefined.
• The inference pass's dry run: the whole analysis, reporting and changing nothing.
• makeStringCopies / copyForRetain: a string retain whose single use is in its block becomes a
  copy for that use; no copy for a parameter whose callers know it is kept or borrowed, for one
  operand taken twice, or for a use in another block.

Corpus (test/tester/tests, --emit=llvm -mm=own --no-default-lib)

	main	this branch	gained	lost
plain	417 / 593	453 / 593	36	0
--opt	416 / 593	451 / 593	35	0

first error	main	this branch
takes a second reference (the catch-all)	25 (27)	19 (22)
used after its value was moved	23 (25)	24 (25)
borrows a field and cannot be stored, returned or captured	20	11
returns a borrow of its argument on some paths and another value on others	17	1
borrows an element and cannot be stored, returned or captured	10	3
borrows a field but is used here after it may be released or overwritten	6	13
borrows an element but is used here after it may be released or overwritten	0	2
merged from several branches	8 (7)	5 (4)
borrows the global and cannot be stored, returned or captured	8	7
the rest	59 (57)	55 (54)

(--opt in brackets where it differs.) No error is new: every error this branch reports, plain,
--opt and --di, main reports too.

• Gated: of the 35 files gained under --opt, 30 pass test-runner -mm=own AOT and JIT, with the
  default flags and with -noopt. The other 5 are halves of four import/export pairs. Three pairs
  pass the way the suite runs them. import_type_alias is still rejected by an error main has too.
• Unchanged where it compiles: the 417 files that compile on main give identical LLVM IR from both
  binaries, once the names that change from run to run are normalised.
• Measured (AOT, MB, a loop of r.s = "v" + i; r.t = r.s): own 4.4 / 4.4 at 20000 / 200000
  iterations, rc 4.4 / 4.4, none 6.6 / 25.9, gc 5.9 / 5.9.

Tests

• Positive: own_string_copy_field, _after_move, _nullable, _exported, _mixed_return,
  _branch (own, rc, none, gc; AOT and JIT), and own_string_copy_written (own only).
• test-own-string-copy-in-store-block: the copy for a store in a branch is made in the branch's
  block, not the entry block (a copy there would leak on the other path; test-runner cannot see it).
• Negative: own_err_string_kept_param_known, own_err_string_call_keeps_twice,
  own_err_string_pushed_twice.
• Four string negatives that compile now use a class; own_err_unbox_string is deleted.
• Windows: ctest -C Release -R own 490/490. Linux (GCC, WSL): own-err|own-no-counting|own-verify|own_string_copy|own_err_string 191/191.

Full results, teeth and the shapes outside the rule: spec §22.7.

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