Tracking Issue for `$$crate`

This is a tracking issue for a subset of the RFC "3086" (https://github.com/rust-lang/rfcs/pull/3086), specifically the $$crate meta-variable expression.
The feature gate for the issue is #![feature(macro_metavar_expr_dollar_dollar_crate)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement original $$crate metavariable expression: https://github.com/rust-lang/rust/pull/94368/changes
• Change how the span of $$crate resolves

Unresolved Questions

• https://github.com/rust-lang/rust/issues/151175

See also https://github.com/rust-lang/rust/issues/99035

Implementation history

• Stabilization of $$ was attempted in: https://github.com/rust-lang/rust/pull/95860.
• It was later reverted: https://github.com/rust-lang/rust/pull/99435, because we don't know how $$crate should be handled.
• A PR was made to forbid $$crate: https://github.com/rust-lang/rust/pull/99193
• Which was then closed because the author lost motivation: https://github.com/rust-lang/rust/pull/99193#issuecomment-1193955362
• The new PR for extracting $$crate into a separate feature gate is: https://github.com/rust-lang/rust/pull/155107

Start by reading RFC 3086 and the macro_metavar_expr_dollar_dollar_crate feature context. Review the linked implementation history, unresolved issue #151175, and the remaining step about how the $$crate span resolves. Done means resolving that design question and completing the required implementation and stabilization work.