feat(lsp): flag owners assigned to reference fields

An attribute target whose field annotation starts with `&` only accepts `&value`, another reference or None; anything else is reported like the compiler does.

App/lib/lsp/LspChecks.vp

@@ -1020,6 +1020,16 @@ def _CheckAssignValue(pool: memhub.MemManager*, doc: LspDocuments.LspDocument*,
     _EmitTypeError(pool, doc, value, msg, errors)
 
 
+def _RefDetail(det: str) -> int:
+    """1 when an annotation text is a reference (`&T` / `&mut T`, optional trailing `?`)."""
+    if det is None: return 0
+    s: str = det
+    while string.strlen(s) > 0 and s[0] == ' ':
+        s += 1
+    if string.strlen(s) == 0: return 0
+    return 1 if s[0] == '&' else 0
+
+
 def _NullableDetail(det: str) -> int:
     """1 when an annotation text is the nullable `T?` form."""
     if det is None: return 0
@@ -1609,6 +1619,25 @@ def _WalkChecks(pool: memhub.MemManager*, doc: LspDocuments.LspDocument*,
                         continue
                     if tgt.kind() == ast.ASTKind.Attribute:
                         _WalkChecksExprs(pool, doc, a, ws, table_pool, ast.Attribute*(tgt).value, errors)
+                        # Reference field: the slot holds a borrow, not an owner.
+                        det_f: str = _SymDetailAt(pool, a, tgt)
+                        if _RefDetail(det_f) != 0:
+                            ok_f: int = 0
+                            rv_f: ast.AST* = asn.value
+                            if rv_f is not None:
+                                if rv_f.kind() == ast.ASTKind.UnaryOp:
+                                    if (ast.UnaryOp*)(rv_f).op == ast.OpKind.AddrOf: ok_f = 1
+                                elif rv_f.kind() == ast.ASTKind.Constant:
+                                    if (ast.Constant*)(rv_f).const_kind == ast.CONST_NONE: ok_f = 1
+                                elif _RefDetail(_SymDetailAt(pool, a, rv_f)) != 0:
+                                    ok_f = 1
+                            if ok_f == 0:
+                                at_f: ast.Attribute* = ast.Attribute*(tgt)
+                                msg_f: bytes = pool.alloc(sizet(320))
+                                if msg_f is not None and at_f.attr is not None:
+                                    stdio.snprintf(msg_f, sizet(320),
+                                        "field '.%s' is a reference: assign a borrow (`&value`), not an owner", at_f.attr)
+                                    _EmitTypeError(pool, doc, rv_f, str(msg_f), errors)
                         continue
                     if tgt.kind() == ast.ASTKind.Subscript:
                         sb_t: ast.Subscript* = ast.Subscript*(tgt)