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>
* <#124544 (comment)>
* <https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/deciding.20on.20semantics.20of.20generics.20in.20rustdoc.20search/near/476841363>

Author: notriddle

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
 }
 );
 }

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 =

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::{
-selfas hir,self,CRATE_HIR_ID,CRATE_OWNER_ID,FnSig,ForeignItem,HirId,Item,ItemKind,
-MethodKind,Safety,Target,TraitItem,
+selfas 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;
@@ -940,6 +940,23 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
 }
 }
 
+fncheck_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
@@ -1152,6 +1169,12 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
 }
 }
 
+ sym::search_unbox => {
+ifself.check_attr_not_crate_level(meta, hir_id,"fake_variadic"){
+self.check_doc_search_unbox(meta, hir_id);
+}
+}
+
  sym::test => {
 ifself.check_attr_crate_level(attr, meta, hir_id){
 self.check_test_attr(meta, hir_id);

compiler/rustc_passes/src/errors.rs

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

compiler/rustc_span/src/symbol.rs

@@ -1762,6 +1762,7 @@ symbols! {
  saturating_add,
  saturating_div,
  saturating_sub,
+ search_unbox,
  select_unpredictable,
  self_in_typedefs,
  self_struct_ctor,

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.
 pubstructBox<

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]

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]

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"]

library/core/src/option.rs

@@ -563,6 +563,7 @@ use crate::pin::Pin;
 usecrate::{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"]