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Multiple references to a single footnote is a part of GitHub Flavored
Markdown syntax (although not explicitly documented as well as regular
footnotes, it is implemented in GitHub's fork of CommonMark) and not
prohibited by rustdoc.
cf. <https://github.com/github/cmark-gfm/blob/587a12bb54d95ac37241377e6ddc93ea0e45439b/test/extensions.txt#L762-L780>
However, using it makes multiple "sup" elements with the same "id"
attribute, which is invalid per the HTML specification.
Still, not only this is a valid GitHub Flavored Markdown syntax, this is
helpful on certain cases and actually tested (accidentally) in
tests/rustdoc/footnote-reference-in-footnote-def.rs.
This commit keeps track of the number of references per footnote and gives
unique ID to each reference. It also emits *all* back links from a footnote
to its references as "↩" (return symbol) plus a numeric list in superscript.
As a known limitation, it assumes that all references to a footnote are
rendered (this is not always true if a dangling footnote has one or more
references but considered a reasonable compromise).
Also note that, this commit is designed so that no HTML changes will occur
unless multiple references to a single footnote is actually used.
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