Tracking Issue for future-incompatibility lint `macro_expanded_macro_exports_accessed_by_absolute_paths`

The macro_expanded_macro_exports_accessed_by_absolute_paths lint
detects macro-expanded macro_export macros from the current crate
that cannot be referred to by absolute paths.

Example

macro_rules! define_exported {
    () => {
        #[macro_export]
        macro_rules! exported {
            () => {};
        }
    };
}

define_exported!();

fn main() {
    crate::exported!();
}

Explanation

The intent is that all macros marked with the #[macro_export]
attribute are made available in the root of the crate. However, when a
macro_rules! definition is generated by another macro, the macro
expansion is unable to uphold this rule.
See https://github.com/rust-lang/rust/pull/52234#issuecomment-486819782 for more details.

Implementation history

• https://github.com/rust-lang/rust/pull/52234 introduced the restriction as a hard error
• https://github.com/rust-lang/rust/pull/53653 relaxed it to a deny-by-default deprecation lint
• https://github.com/rust-lang/rust/pull/143929 made the lint report-in-deps
• XXX turned the lint into a hard error

Start by reading the implementation history in PRs #52234, #53653, and #143929, then use the compile-fail example in this issue to understand the current behavior. The work is done when this future-incompatibility lint becomes a hard error, with the example and relevant compiler behavior updated accordingly.