Function emit_non_null_assert_on_stack 

pub fn emit_non_null_assert_on_stack(
    emitter: &mut FunctionEmitter<'_>,
    ctx: &CodegenCtx<'_>,
    source_ty: &Type,
    target_ty: &Type,
) -> Result<(), CompilerFailure>

! applied to a value already on the stack at source_ty’s slot — the optional-chain form, where the operand is the step before rather than an expression this can emit itself.