Auto merge of rust-lang#127589 - notriddle:notriddle/search-sem-3, r=GuillaumeGomez

rustdoc-search: simplify rules for generics and type params

**Heads up!**: This PR is a follow-up that depends on rust-lang#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 rust-lang#109802

## Preview

* no results: [`Box<[A]> -> Vec<B>`](http://notriddle.com/rustdoc-html-demo-12/search-sem-3/std/index.html?search=Box%3C%5BA%5D%3E%20-%3E%20Vec%3CB%3E)
* results: [`Box<[A]> -> Vec<A>`](http://notriddle.com/rustdoc-html-demo-12/search-sem-3/std/index.html?search=Box%3C%5BA%5D%3E%20-%3E%20Vec%3CA%3E)
* [`T -> U`](http://notriddle.com/rustdoc-html-demo-12/search-sem-3/std/index.html?search=T%20-%3E%20U)
* [`Cx -> TyCtxt`](http://notriddle.com/rustdoc-html-demo-12/search-sem-3-compiler/rustdoc/index.html?search=Cx%20-%3E%20TyCtxt)

![image](https://github.com/user-attachments/assets/015ae28c-7469-4f7f-be03-157d28d7ec97)

## 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>
* <rust-lang#124544 (comment)>
* <https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/deciding.20on.20semantics.20of.20generics.20in.20rustdoc.20search>

Author: bors

alloc/src/boxed.rs

@@ -225,6 +225,7 @@ pub use thin::ThinBox;
 #[fundamental]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_insignificant_dtor]
+#[cfg_attr(not(bootstrap), doc(search_unbox))]
 // The declaration of the `Box` struct must be kept in sync with the
 // compiler or ICEs will happen.
 pub struct Box<

alloc/src/rc.rs

@@ -307,6 +307,7 @@ fn rc_inner_layout_for_value_layout(layout: Layout) -> Layout {
 /// `value.get_mut()`. This avoids conflicts with methods of the inner type `T`.
 ///
 /// [get_mut]: Rc::get_mut
+#[cfg_attr(not(bootstrap), doc(search_unbox))]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Rc")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_insignificant_dtor]

alloc/src/sync.rs

@@ -235,6 +235,7 @@ macro_rules! acquire {
 /// counting in general.
 ///
 /// [rc_examples]: crate::rc#examples
+#[cfg_attr(not(bootstrap), doc(search_unbox))]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_insignificant_dtor]

core/src/future/future.rs

@@ -25,6 +25,7 @@ use crate::task::{Context, Poll};
 /// [`async`]: ../../std/keyword.async.html
 /// [`Waker`]: crate::task::Waker
 #[doc(notable_trait)]
+#[cfg_attr(not(bootstrap), doc(search_unbox))]
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 #[stable(feature = "futures_api", since = "1.36.0")]
 #[lang = "future_trait"]

core/src/option.rs

@@ -563,6 +563,7 @@ use crate::pin::Pin;
 use crate::{cmp, convert, hint, mem, slice};
 
 /// The `Option` type. See [the module level documentation](self) for more.
+#[cfg_attr(not(bootstrap), doc(search_unbox))]
 #[derive(Copy, Eq, Debug, Hash)]
 #[rustc_diagnostic_item = "Option"]
 #[lang = "Option"]

core/src/result.rs

@@ -520,6 +520,7 @@ use crate::{convert, fmt, hint};
 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
 ///
 /// See the [module documentation](self) for details.
+#[cfg_attr(not(bootstrap), doc(search_unbox))]
 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
 #[rustc_diagnostic_item = "Result"]