tests/ui/traits/const-traits/macro-bare-trait-objects-const-trait-bounds.rs

// Ensure that we don't consider `const Trait` to
// match the macro fragment specifier `ty` as that would be a breaking
// change theoretically speaking. Syntactically trait object types can
// be "bare", i.e., lack the prefix `dyn`.
// By contrast, `?Trait` *does* match `ty` and therefore an arm like
// `?$Trait:path` would never be reached.
// See `parser/macro/mbe-bare-trait-object-maybe-trait-bound.rs`.
// `[const] Trait` is already an error for a `ty` fragment,
// so we do not need to prevent that.

macro_rules! check {
    ($Type:ty) => {
        compile_error!("ty");
    };
    (const $Trait:path) => {};
    ([const] $Trait:path) => { [const] Trait };
}

check! { const Trait }
check! { [const] Trait }
//~^ ERROR: expected identifier, found `]`
//~| ERROR: const trait impls are experimental

fn main() {}