Rollup merge of #114566 - fmease:type-alias-laziness-is-crate-specific, r=oli-obk

Store the laziness of type aliases in their `DefKind`

Previously, we would treat paths referring to type aliases as *lazy* type aliases if the current crate had lazy type aliases enabled independently of whether the crate which the alias was defined in had the feature enabled or not.

With this PR, the laziness of a type alias depends on the crate it is defined in. This generally makes more sense to me especially if / once lazy type aliases become the default in a new edition and we need to think about *edition interoperability*:

Consider the hypothetical case where the dependency crate has an older edition (and thus eager type aliases), it exports a type alias with bounds & a where-clause (which are void but technically valid), the dependent crate has the latest edition (and thus lazy type aliases) and it uses that type alias. Arguably, the bounds should *not* be checked since at any time, the dependency crate should be allowed to change the bounds at will with a *non*-major version bump & without negatively affecting downstream crates.

As for the reverse case (dependency: lazy type aliases, dependent: eager type aliases), I guess it rules out anything from slight confusion to mild annoyance from upstream crate authors that would be caused by the compiler ignoring the bounds of their type aliases in downstream crates with older editions.

---

This fixes #114468 since before, my assumption that the type alias associated with a given weak projection was lazy (and therefore had its variances computed) did not necessarily hold in cross-crate scenarios (which [I kinda had a hunch about](https://github.com/rust-lang/rust/pull/114253#discussion_r1278608099)) as outlined above. Now it does hold.

`@rustbot` label F-lazy_type_alias
r? `@oli-obk`

8 files changed, 31 insertions, 12 deletions

diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index c31d104f8cb..384010034e6 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -79,7 +79,7 @@ pub(crate) fn try_inline(
             build_impls(cx, did, attrs_without_docs, &mut ret);
             clean::UnionItem(build_union(cx, did))
         }
-        Res::Def(DefKind::TyAlias, did) => {
+        Res::Def(DefKind::TyAlias { .. }, did) => {
             record_extern_fqn(cx, did, ItemType::Typedef);
             build_impls(cx, did, attrs_without_docs, &mut ret);
             clean::TypedefItem(build_type_alias(cx, did))
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index a5efcc989f6..b6ba4c853d4 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -1706,7 +1706,7 @@ fn maybe_expand_private_type_alias<'tcx>(
     cx: &mut DocContext<'tcx>,
     path: &hir::Path<'tcx>,
 ) -> Option<Type> {
-    let Res::Def(DefKind::TyAlias, def_id) = path.res else { return None };
+    let Res::Def(DefKind::TyAlias { .. }, def_id) = path.res else { return None };
     // Substitute private type aliases
     let def_id = def_id.as_local()?;
     let alias = if !cx.cache.effective_visibilities.is_exported(cx.tcx, def_id.to_def_id())
@@ -1970,7 +1970,7 @@ impl<'tcx> ContainerTy<'tcx> {
                 let (DefKind::Struct
                 | DefKind::Union
                 | DefKind::Enum
-                | DefKind::TyAlias
+                | DefKind::TyAlias { .. }
                 | DefKind::Trait
                 | DefKind::AssocTy
                 | DefKind::Variant) = tcx.def_kind(container)
@@ -2709,7 +2709,7 @@ fn clean_impl<'tcx>(
     let for_ = clean_ty(impl_.self_ty, cx);
     let type_alias =
         for_.def_id(&cx.cache).and_then(|alias_def_id: DefId| match tcx.def_kind(alias_def_id) {
-            DefKind::TyAlias => Some(clean_middle_ty(
+            DefKind::TyAlias { .. } => Some(clean_middle_ty(
                 ty::Binder::dummy(tcx.type_of(def_id).instantiate_identity()),
                 cx,
                 Some(def_id.to_def_id()),
diff --git a/src/librustdoc/clean/utils.rs b/src/librustdoc/clean/utils.rs
index 944d0145de2..3c79ce57782 100644
--- a/src/librustdoc/clean/utils.rs
+++ b/src/librustdoc/clean/utils.rs
@@ -504,8 +504,22 @@ pub(crate) fn register_res(cx: &mut DocContext<'_>, res: Res) -> DefId {
 
     let (kind, did) = match res {
         Res::Def(
-            kind @ (AssocTy | AssocFn | AssocConst | Variant | Fn | TyAlias | Enum | Trait | Struct
-            | Union | Mod | ForeignTy | Const | Static(_) | Macro(..) | TraitAlias),
+            kind @ (AssocTy
+            | AssocFn
+            | AssocConst
+            | Variant
+            | Fn
+            | TyAlias { .. }
+            | Enum
+            | Trait
+            | Struct
+            | Union
+            | Mod
+            | ForeignTy
+            | Const
+            | Static(_)
+            | Macro(..)
+            | TraitAlias),
             did,
         ) => (kind.into(), did),
 
diff --git a/src/librustdoc/formats/item_type.rs b/src/librustdoc/formats/item_type.rs
index a3017201cb1..a788935581f 100644
--- a/src/librustdoc/formats/item_type.rs
+++ b/src/librustdoc/formats/item_type.rs
@@ -115,7 +115,7 @@ impl From<DefKind> for ItemType {
             DefKind::Struct => Self::Struct,
             DefKind::Union => Self::Union,
             DefKind::Trait => Self::Trait,
-            DefKind::TyAlias => Self::Typedef,
+            DefKind::TyAlias { .. } => Self::Typedef,
             DefKind::TraitAlias => Self::TraitAlias,
             DefKind::Macro(kind) => match kind {
                 MacroKind::Bang => ItemType::Macro,
diff --git a/src/librustdoc/passes/collect_intra_doc_links.rs b/src/librustdoc/passes/collect_intra_doc_links.rs
index 4de30e4ed9d..1b8d999024c 100644
--- a/src/librustdoc/passes/collect_intra_doc_links.rs
+++ b/src/librustdoc/passes/collect_intra_doc_links.rs
@@ -592,7 +592,7 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
                         .unwrap_or(Vec::new())
                 }
             }
-            Res::Def(DefKind::TyAlias, did) => {
+            Res::Def(DefKind::TyAlias { .. }, did) => {
                 // Resolve the link on the type the alias points to.
                 // FIXME: if the associated item is defined directly on the type alias,
                 // it will show up on its documentation page, we should link there instead.
@@ -1865,7 +1865,12 @@ fn resolution_failure(
                                 }
                                 return;
                             }
-                            Trait | TyAlias | ForeignTy | OpaqueTy | TraitAlias | TyParam
+                            Trait
+                            | TyAlias { .. }
+                            | ForeignTy
+                            | OpaqueTy
+                            | TraitAlias
+                            | TyParam
                             | Static(_) => "associated item",
                             Impl { .. } | GlobalAsm => unreachable!("not a path"),
                         }
diff --git a/src/tools/clippy/clippy_lints/src/init_numbered_fields.rs b/src/tools/clippy/clippy_lints/src/init_numbered_fields.rs
index f95d2c2edb1..b00fa104f98 100644
--- a/src/tools/clippy/clippy_lints/src/init_numbered_fields.rs
+++ b/src/tools/clippy/clippy_lints/src/init_numbered_fields.rs
@@ -50,7 +50,7 @@ impl<'tcx> LateLintPass<'tcx> for NumberedFields {
                 && fields
                     .iter()
                     .all(|f| f.ident.as_str().as_bytes().iter().all(u8::is_ascii_digit))
-                && !matches!(cx.qpath_res(path, e.hir_id), Res::Def(DefKind::TyAlias, ..))
+                && !matches!(cx.qpath_res(path, e.hir_id), Res::Def(DefKind::TyAlias { .. }, ..))
             {
                 let expr_spans = fields
                     .iter()
diff --git a/src/tools/clippy/clippy_utils/src/lib.rs b/src/tools/clippy/clippy_utils/src/lib.rs
index 4cd8a8c3325..171b7faf219 100644
--- a/src/tools/clippy/clippy_utils/src/lib.rs
+++ b/src/tools/clippy/clippy_utils/src/lib.rs
@@ -286,7 +286,7 @@ pub fn is_wild(pat: &Pat<'_>) -> bool {
 /// Checks if the given `QPath` belongs to a type alias.
 pub fn is_ty_alias(qpath: &QPath<'_>) -> bool {
     match *qpath {
-        QPath::Resolved(_, path) => matches!(path.res, Res::Def(DefKind::TyAlias | DefKind::AssocTy, ..)),
+        QPath::Resolved(_, path) => matches!(path.res, Res::Def(DefKind::TyAlias { .. } | DefKind::AssocTy, ..)),
         QPath::TypeRelative(ty, _) if let TyKind::Path(qpath) = ty.kind => { is_ty_alias(&qpath) },
         _ => false,
     }
diff --git a/src/tools/clippy/clippy_utils/src/ty/type_certainty/mod.rs b/src/tools/clippy/clippy_utils/src/ty/type_certainty/mod.rs
index 0669ea72eb3..06fd9529044 100644
--- a/src/tools/clippy/clippy_utils/src/ty/type_certainty/mod.rs
+++ b/src/tools/clippy/clippy_utils/src/ty/type_certainty/mod.rs
@@ -219,7 +219,7 @@ fn path_segment_certainty(
                 // See the comment preceding `qpath_certainty`. `def_id` could refer to a type or a value.
                 let certainty = lhs.join_clearing_def_ids(rhs);
                 if resolves_to_type {
-                    if cx.tcx.def_kind(def_id) == DefKind::TyAlias {
+                    if let DefKind::TyAlias { .. } = cx.tcx.def_kind(def_id) {
                         adt_def_id(cx.tcx.type_of(def_id).instantiate_identity())
                             .map_or(certainty, |def_id| certainty.with_def_id(def_id))
                     } else {