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macro_rules! log {
( $ctx:expr, $( $args:expr),* ) => {
if $ctx.trace {
//~^ no field `trace` on type `&T`
println!( $( $args, )* );
}
}
}
// Create a structure.
struct Foo {
trace: bool,
}
// Generic wrapper calls log! with a structure.
fn wrap<T>(context: &T) -> ()
{
log!(context, "entered wrapper");
//~^ in this expansion of log!
}
fn main() {
// Create a structure.
let x = Foo { trace: true };
log!(x, "run started");
// Apply a closure which accesses internal fields.
wrap(&x);
log!(x, "run finished");
}
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