rustdoc-search: simplify rules for generics and type params

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

Generics are tightened by making order 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.

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/near/476841363>

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"]