One of the last blocks that needs to become cachable is SystemBreadcrumbBlock.
Updated: Comment #0
Problem/Motivation
The breadcrumbs block is currently not cacheable at all.
Proposed resolution
Cache breadcrumbs on a per-URL, per-role, per-language basis. This, combined with the fact that proper cache tags should be set by the used breadcrumb builder (BreadcrumbBuilderInterface::build()), would ensure that we can cache the breadcrumb block forever, since any change that could affect the breadcrumbs to be rendered would invalidate the corresponding render cache entries.
Unfortunately, all of the breadcrumb classes don't even comply with the interface they're implementing: they're returning an array of rendered links, not a render array! Therefore, it is currently impossible to set cache tags!
Remaining tasks
Allow breadcrumb builders to bubble up cache tags.
User interface changes
None.
API changes
None.