Fix when input redirection becomes visible.

With `&parse-at/from` we were updating the internal state on our
current position immediately, meaning they were visible already when
evaluating other attributes on the same field afterwards, which is
unexpected.

Closes #1842.

spicy/toolchain/src/compiler/codegen/parser-builder.cc

@@ -548,6 +548,7 @@ struct ProductionVisitor : public production::Visitor {
             auto etype = c->parseType()->type()->elementType();
             auto container_element = builder()->addTmp("elem", etype);
             pushDestination(container_element);
+            pb->saveParsePosition(); // need to update position for container elements in case input is redirected
         }
 
         else if ( ! meta.isFieldProduction() )
@@ -1430,7 +1431,6 @@ struct ProductionVisitor : public production::Visitor {
         pstate.cur = builder()->addTmp("parse_cur", builder()->typeStreamView(), builder()->deref(tmp));
         pstate.ncur = {};
         pushState(std::move(pstate));
-        pb->saveParsePosition();
     }
 
     // Redirects input to be read from given stream position next.
@@ -1446,7 +1446,6 @@ struct ProductionVisitor : public production::Visitor {
         pstate.cur = builder()->addTmp("parse_cur", cur);
         pstate.ncur = {};
         pushState(std::move(pstate));
-        pb->saveParsePosition();
     }
 
     // Start sync and trial mode.

tests/Baseline/spicy.types.unit.size-with-offset/output

@@ -0,0 +1,5 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+x, 3
+x, \x01\x02\x03
+z, 4
+z, 1234

tests/spicy/types/unit/size-with-offset.spicy

@@ -0,0 +1,30 @@
+# @TEST-EXEC: ${SCRIPTS}/printf '\x01\x02\x03\x04\0x05' | spicy-driver %INPUT >output
+# @TEST-EXEC: btest-diff output
+#
+# @TEST-EXED: Check that changing the input with `&parse-at` isn't visible to other attributes on same field; regression test for #1842.
+
+module Test;
+
+import spicy;
+
+public type X = unit {
+    a: uint8;
+    b: uint8;
+    c: uint8;
+
+    x: bytes &size=self.offset()  # yields 3
+             &parse-at=self.input() {
+        print "x", self.offset(); # yields still 3
+        print "x", $$;
+    }
+
+    y: uint8;
+
+    z: bytes &size=self.offset()  # yields 4
+             &parse-from=b"12345" {
+        print "z", self.offset(); # yields still 4
+        print "z", $$;
+    }
+
+    d: uint8;
+};