Function bind_expression_variables 

pub fn bind_expression_variables(
    expression: &Expr,
    resolver: &mut dyn VariableResolver,
    conflict: VariableConflict,
    resolve_sites: VariableSiteResolver<'_>,
) -> Result<Expr>

Bind the variables of an embedded expression, which PostgreSQL analyzes as SELECT expression: only a subquery lets a name of the expression meet a column, so an expression without one binds every variable it names.