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SetLength on a record string/array field via a var parameter fails codegen

Symptom

SetLength on a string (or dynamic-array) field reached through a var record parameter errors at compile time:

type TConn = record Buf: AnsiString; end;
procedure Grow(var c: TConn; n: Integer);
begin
  SetLength(c.Buf, Length(c.Buf) + n);   { error }
end;

→ error: SetLength expects an array variable in IR codegen

SetLength on a plain local string, and assignment to c.Buf (e.g. c.Buf := c.Buf + x), both work — only SetLength(c.Buf, ...) through the var-param record field is rejected.

Workaround in use

lib/rtl/http.pas HttpConnRecvMore builds a local string and concatenates (SetLength(chunk, n); Move(...); conn.Buf := conn.Buf + chunk;) instead of SetLength-ing the field in place. Correct, slightly more copying.

Fix

Make the SetLength lowering accept a field-of-var-param l-value (the same addressable-l-value set that direct assignment already accepts), not only a direct array/string variable.

Done when

Resolution (2026-06-25, Track A)

Fixed by generalizing the managed-string SetLength path to be address-based, matching what dyn-arrays already do. Added one IR op IR_SETLEN_STR (target slot-ADDRESS node + count) that calls the existing PXXStrSetLen(addr, n) RTL helper. ir.inc now diverts SetLength on any non-symbol managed-string lvalue (record/class field, var-param field, index, pointer deref) through it via IRLowerAddress — no symbol reconstruction, so a field works like a plain variable. Plain string vars (AN_IDENT) keep the existing symbol path, preserving x86-64's amortized in-place inline resize (no O(n^2) regression).

Per-backend handler is ~5 instructions (load addr, load n, call helper) on all 6 targets. New helper NodeIsManagedString guards against false positives: RecFieldType reports the element type for an array field, so array of string fields and multidim s[i] sub-arrays are excluded via dyn-array depth.

Tests: test/test_setlength_managed_field.pas (field / var-param field / pointer- deref field / plain var) in make test; verified x86-64 + i386/aarch64/arm32. Frozen AND managed self-host both byte-identical; full make test + all 3 cross gates green.

Follow-ups (not blocking): (1) Track B can drop the lib/rtl/http.pas HttpConnRecvMore local-string-concat workaround and SetLength the field in place. (2) Optional surface reduction: port PXXStrSetLen's geometric in-place growth so the plain-symbol path + x86-64 inline string resize can also be deleted.