OpenSearch for standard library docs

[niklasmohrin]

Summary

Add OpenSearch description file for the standard library documentation. This means serving a new XML file and possibly introducing a new <link> in the documentation html.

Motivation

I would like to add the standard library docs as a search engine in my browser. This would allow for quick access to the docs with a custom search prefix in all modern browsers without having to install an extra plugin from the store.

Drawbacks

Extra configuration for the new file route would be needed. For an automatic detection of the search configuration, a <link> should be added to the html. This would differentiate the docs html at https://doc.rust-lang.org/std/index.html from what cargo doc would generate.

Rationale and alternatives

• https://github.com/huhu/rust-search-extension
• https://duckduckgo.com/ has custom search prefixes such as !rust.
• this could be autogenerated with cargo doc for all pieces of Rust documentation

Maintenance

I don't know. To me it seems that this doesn't need frequent attention, but I might be totally wrong.

Unresolved Questions

None so far.

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In any case, thank you all for your work and time :)