rustdoc: search should prioritize matching capitalization

[joshtriplett]

It's good that rustdoc's search is case-insensitive, to make it easier for people to find what they're looking for. However, if the user types in capitalization that exactly matches (or more closely matches) a given type, rustdoc's search results should prioritize that type over things that don't match in case.

For example, https://doc.rust-lang.org/std/?search=Copy (note the capital C) should list std::marker::Copy at the top, not std::fs::copy.

[camelid]

It might also make sense to make this based on whether there were capital letters in the user's input. Vim has a similar feature that can be enabled, where searching with all lowercase is case-insensitive, but mixed-case is sensitive. In rustdoc's case, we could do something similar, except make it case-preferenced rather than exclude non-matching capitalization altogether.

[tgross35]

The Vim style is smartcase (ripgrep has a --smart-case flag), which matches case insensitive unless there are any uppercase characters, in which case it does a case-sensitive match.

However, that is better for when you have binary results (either it matches or it doesn't), giving precedence to exact-case matches is better when you can rank the results. This is what e.g. https://github.com/kentcdodds/match-sorter does.

(I would have liked this for adding a PATH alias in #119748)

[lolbinarycat]

@rustbot claim