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

Author: bors

Diff for: compiler/rustc_ast_passes/src/feature_gate.rs

@@ -204,6 +204,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                     "meant for internal use only" {
                         keyword => rustdoc_internals
                         fake_variadic => rustdoc_internals
+                        search_unbox => rustdoc_internals
                     }
                 );
             }

Diff for: compiler/rustc_passes/messages.ftl

@@ -234,6 +234,9 @@ passes_doc_masked_only_extern_crate =
 passes_doc_rust_logo =
     the `#[doc(rust_logo)]` attribute is used for Rust branding
 
+passes_doc_search_unbox_invalid =
+    `#[doc(search_unbox)]` should be used on generic structs and enums
+
 passes_doc_test_literal = `#![doc(test(...)]` does not take a literal
 
 passes_doc_test_takes_list =

Diff for: compiler/rustc_passes/src/check_attr.rs

@@ -16,8 +16,8 @@ use rustc_feature::{AttributeDuplicates, AttributeType, BUILTIN_ATTRIBUTE_MAP, B
 use rustc_hir::def_id::LocalModDefId;
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{
-    self as hir, self, CRATE_HIR_ID, CRATE_OWNER_ID, FnSig, ForeignItem, HirId, Item, ItemKind,
-    MethodKind, Safety, Target, TraitItem,
+    self as hir, self, AssocItemKind, CRATE_HIR_ID, CRATE_OWNER_ID, FnSig, ForeignItem, HirId,
+    Item, ItemKind, MethodKind, Safety, Target, TraitItem,
 };
 use rustc_macros::LintDiagnostic;
 use rustc_middle::hir::nested_filter;
@@ -937,6 +937,23 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
+    fn check_doc_search_unbox(&self, meta: &MetaItemInner, hir_id: HirId) {
+        let hir::Node::Item(item) = self.tcx.hir_node(hir_id) else {
+            self.dcx().emit_err(errors::DocSearchUnboxInvalid { span: meta.span() });
+            return;
+        };
+        match item.kind {
+            ItemKind::Enum(_, generics) | ItemKind::Struct(_, generics)
+                if generics.params.len() != 0 => {}
+            ItemKind::Trait(_, _, generics, _, items)
+                if generics.params.len() != 0
+                    || items.iter().any(|item| matches!(item.kind, AssocItemKind::Type)) => {}
+            _ => {
+                self.dcx().emit_err(errors::DocSearchUnboxInvalid { span: meta.span() });
+            }
+        }
+    }
+
     /// Checks `#[doc(inline)]`/`#[doc(no_inline)]` attributes.
     ///
     /// A doc inlining attribute is invalid if it is applied to a non-`use` item, or
@@ -1149,6 +1166,12 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                             }
                         }
 
+                        sym::search_unbox => {
+                            if self.check_attr_not_crate_level(meta, hir_id, "fake_variadic") {
+                                self.check_doc_search_unbox(meta, hir_id);
+                            }
+                        }
+
                         sym::test => {
                             if self.check_attr_crate_level(attr, meta, hir_id) {
                                 self.check_test_attr(meta, hir_id);

Diff for: compiler/rustc_passes/src/errors.rs

@@ -244,6 +244,13 @@ pub(crate) struct DocKeywordOnlyImpl {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_doc_search_unbox_invalid)]
+pub(crate) struct DocSearchUnboxInvalid {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_doc_inline_conflict)]
 #[help]

Diff for: compiler/rustc_span/src/symbol.rs

@@ -1761,6 +1761,7 @@ symbols! {
         saturating_add,
         saturating_div,
         saturating_sub,
+        search_unbox,
         select_unpredictable,
         self_in_typedefs,
         self_struct_ctor,

Diff for: library/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<

Diff for: library/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]

Diff for: library/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]

Diff for: library/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"]

Diff for: library/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"]

Diff for: library/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"]

Diff for: src/doc/rustdoc/src/read-documentation/search.md

@@ -130,29 +130,31 @@ pub trait MyTrait {
 /// This function can be found using the following search queries:
 ///
 ///     MyTrait<First=u8, Second=u32> -> bool
-///     MyTrait<u32, First=u8> -> bool
 ///     MyTrait<Second=u32> -> bool
-///     MyTrait<u32, u8> -> bool
 ///
 /// The following queries, however, will *not* match it:
 ///
 ///     MyTrait<First=u32> -> bool
 ///     MyTrait<u32, u32> -> bool
+///     MyTrait<u32, First=u8> -> bool
+///     MyTrait<u32, u8> -> bool
 pub fn my_fn(x: impl MyTrait<First=u8, Second=u32>) -> bool { true }
 ```
 
-Generics and function parameters are order-agnostic, but sensitive to nesting
+Function parameters are order-agnostic, but sensitive to nesting
 and number of matches. For example, a function with the signature
 `fn read_all(&mut self: impl Read) -> Result<Vec<u8>, Error>`
 will match these queries:
 
 * `&mut Read -> Result<Vec<u8>, Error>`
 * `Read -> Result<Vec<u8>, Error>`
-* `Read -> Result<Error, Vec>`
 * `Read -> Result<Vec<u8>>`
 * `Read -> u8`
 
-But it *does not* match `Result<Vec, u8>` or `Result<u8<Vec>>`.
+But it *does not* match `Result<Vec, u8>` or `Result<u8<Vec>>`,
+because those are nested incorrectly, and it does not match
+`Result<Error, Vec<u8>>` or `Result<Error>`, because those are
+in the wrong order.
 
 To search for a function that accepts a function as a parameter,
 like `Iterator::all`, wrap the nested signature in parenthesis,

Diff for: src/librustdoc/html/render/mod.rs

@@ -204,6 +204,7 @@ pub(crate) struct IndexItemFunctionType {
     inputs: Vec<RenderType>,
     output: Vec<RenderType>,
     where_clause: Vec<Vec<RenderType>>,
+    param_names: Vec<Symbol>,
 }
 
 impl IndexItemFunctionType {