-mm=own: a let assigned a new value after its value moved is still treated as moved #528

Under -mm=own, once a let has given its value away, assigning it a new object does not make it an owner again. A later read is reported as a use after the move, or as a borrow outliving an overwrite. Rust accepts both shapes: assigning to a moved-from binding gives it a fresh value. Both programs print 1 2 under -mm=rc and -mm=gc.

Moved into a field

class Node { constructor(public value: number) {} }
class Holder { node: Node; }

function main() {
    const h = new Holder();
    let a = new Node(1);
    h.node = a;          // the first Node moves into h
    a = new Node(2);     // a owns a new Node
    print(h.node.value, a.value);
}

reassign.ts:9:25: error: 'this value' is used here after its value was moved
    print(h.node.value, a.value);
                        ^
reassign.ts:7:5: note: value moved here
    h.node = a;          // the first Node moves into h
    ^

Moved into another local

class Node { constructor(public value: number) {} }

function main() {
    let a = new Node(1);
    let b = a;           // the first Node moves into b
    a = new Node(2);     // a owns a new Node
    print(b.value, a.value);
}

reassign2.ts:7:11: error: 'this value' borrows 'this value' but is used here after 'this value' is released or overwritten
    print(b.value, a.value);
          ^
reassign2.ts:6:5: note: 'this value' is released or overwritten here

Here b borrows a instead of taking it over, because a is read again later. The overwrite then ends the borrow, even though the later read is of the new value.

Expected

Both programs compile and print 1 2. The assignment a = new Node(2) ends the old value's story for a. The moved-from slot holds nothing to release, and reads after the assignment see the new owner.

Notes

• The diagnostics name every place 'this value'. Naming the variable would help.
• Reproduced with tslang --emit=jit -mm=own --no-default-lib <file>.ts on a Release build of current main (after Default library binaries split by target triple, then build, then memory model #526).

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