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Borrow Diagnostics

This is the cheat sheet for the borrow-checker diagnostic codes. Every borrow rejection from the v0.25.x compiler maps to one of these.

ARIA-023 — conflicting borrow

You tried to introduce a loan that overlaps an existing live loan on the same host (or a covering path).

Common shapes:

$$i int32:r = a;
$$m int32:m = a;             // ARIA-023: $$m while $$i is live

$$m int32:r1 = pair.a;
$$m int32:r2 = pair.a;       // ARIA-023: same path twice as $$m

$$m Leaf:rl = box.leaf;
$$m int32:rx = box.leaf.x;   // ARIA-023: rl covers box.leaf.x

$$m int32:a = arr[i];
$$m int32:b = arr[j];        // ARIA-023: arr[*] vs arr[*]

Fix: narrow the path so the two loans become disjoint, scope-release the older loan, or convert one of the $$ms to $$i if read-only access is enough.

ARIA-026 — assignment through borrowed path

You tried to assign directly to a host (or path) that has a live loan:

$$m int32:r = pair.a;
pair.a = 99;                 // ARIA-026

$$m int32:r = arr[0];
arr[0] = 99;                 // ARIA-026

Fix: write through the borrow (r = 99), or release the loan first by letting the borrowing binding leave scope.

ARIA-022 — borrow of moved value

The host was moved (e.g. into a callee that takes ownership) and is no longer available to borrow.

some_take(pair);             // pair moved
$$i Pair:r = pair;           // ARIA-022

Fix: clone the value before the move, or have the callee take a borrow instead of ownership.

ARIA-028 — stack escape (return/pass of borrow into a local)

You tried to return, pass, or fail a borrow whose host is a binding local to the current function (or to an inner block) — its stack frame is destroyed at the function (or block) boundary, so the reference would dangle:

func:bad = $$m int32() {
    stack int32:tmp = 7i32;
    $$m int32:r = tmp;
    pass r;                  // ARIA-028 — tmp's stack frame ends here
};

The same diagnostic also fires when the host is a gc binding: even though the underlying object survives on the heap, the named binding is local to the function, so the borrow path goes out of scope.

Fix: return by value, take ownership in the caller and pass a borrow in, or accept a borrow parameter and return that.

Note (v0.26.6): Earlier compiler versions emitted these errors as ARIA-017 and the docs cited ARIA-027; both are unified under ARIA-028 STACK_ESCAPE from v0.26.6 on. The message text now mentions the host's stack frame and includes a fix hint.

Reading the messages

The compiler prints the path of the conflicting loan, the source location of both loans, and the kind of each loan ($$i / $$m). The first line is the new (rejected) loan; the "previous loan here" line is the existing one. If the path printed contains [*], you are in dynamic-index territory — see arrays.md.

Where to dig deeper