rustdoc-search: simplify rules for generics and type params

[notriddle]

Heads up!: This PR is a follow-up that depends on #124544. It adds 12dc24f, a change to the filtering behavior, and 9900ea4, a minor ranking tweak.

Part of rust-lang/rust-project-goals#112

This PR overturns #109802

Preview

• no results: Box<[A]> -> Vec<B>
• results: Box<[A]> -> Vec<A>
• T -> U
• Cx -> TyCtxt

Description

This commit is a response to feedback on the displayed type signatures results, by making generics act stricter.

• Order within generics is significant. This means Vec<Allocator> now matches only with a true vector of allocators, instead of matching the second type param. It also makes unboxing within generics stricter, so Result<A, B> only matches if B is in the error type and A is in the success type. The top level of the function search is unaffected.
• Generics are only "unboxed" if a type is explicitly opted into it. References and tuples are hardcoded to allow unboxing, and Box, Rc, Arc, Option, Result, and Future are opted in with an unstable attribute. Search result unboxing is the process that allows you to search for i32 -> str and get back a function with the type signature &Future<i32> -> Box<str>.
• Instead of ranking by set overlap, it ranks by the number of items in the type signature. This makes it easier to find single type signatures like transmute.

Find the discussion on

• https://rust-lang.zulipchat.com/#narrow/stream/393423-t-rustdoc.2Fmeetings/topic/meeting.202024-07-08/near/449965149
• rustdoc-search: show type signature on type-driven SERP #124544 (comment)
• https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/deciding.20on.20semantics.20of.20generics.20in.20rustdoc.20search