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`

9 files changed, 15 insertions, 13 deletions

diff --git a/compiler/rustc_middle/src/hir/map/mod.rs b/compiler/rustc_middle/src/hir/map/mod.rs
index 2aebb8f541f..fbc32263874 100644
--- a/compiler/rustc_middle/src/hir/map/mod.rs
+++ b/compiler/rustc_middle/src/hir/map/mod.rs
@@ -196,7 +196,9 @@ impl<'hir> Map<'hir> {
                 ItemKind::Macro(_, macro_kind) => DefKind::Macro(macro_kind),
                 ItemKind::Mod(..) => DefKind::Mod,
                 ItemKind::OpaqueTy(..) => DefKind::OpaqueTy,
-                ItemKind::TyAlias(..) => DefKind::TyAlias,
+                ItemKind::TyAlias(..) => {
+                    DefKind::TyAlias { lazy: self.tcx.features().lazy_type_alias }
+                }
                 ItemKind::Enum(..) => DefKind::Enum,
                 ItemKind::Struct(..) => DefKind::Struct,
                 ItemKind::Union(..) => DefKind::Union,
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 533c3b66cab..52a18c99edb 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -231,7 +231,7 @@ rustc_queries! {
             action = {
                 use rustc_hir::def::DefKind;
                 match tcx.def_kind(key) {
-                    DefKind::TyAlias => "expanding type alias",
+                    DefKind::TyAlias { .. } => "expanding type alias",
                     DefKind::TraitAlias => "expanding trait alias",
                     _ => "computing type of",
                 }
diff --git a/compiler/rustc_middle/src/ty/adt.rs b/compiler/rustc_middle/src/ty/adt.rs
index 6ba13a76563..b4c6e0d970a 100644
--- a/compiler/rustc_middle/src/ty/adt.rs
+++ b/compiler/rustc_middle/src/ty/adt.rs
@@ -448,7 +448,7 @@ impl<'tcx> AdtDef<'tcx> {
             Res::Def(DefKind::Ctor(..), cid) => self.variant_with_ctor_id(cid),
             Res::Def(DefKind::Struct, _)
             | Res::Def(DefKind::Union, _)
-            | Res::Def(DefKind::TyAlias, _)
+            | Res::Def(DefKind::TyAlias { .. }, _)
             | Res::Def(DefKind::AssocTy, _)
             | Res::SelfTyParam { .. }
             | Res::SelfTyAlias { .. }
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 5eef3e45f32..0b2d95506bf 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -1062,7 +1062,7 @@ impl<'tcx> TyCtxt<'tcx> {
         if let Some(hir::FnDecl { output: hir::FnRetTy::Return(hir_output), .. }) = self.hir().fn_decl_by_hir_id(hir_id)
             && let hir::TyKind::Path(hir::QPath::Resolved(
                 None,
-                hir::Path { res: hir::def::Res::Def(DefKind::TyAlias, def_id), .. }, )) = hir_output.kind
+                hir::Path { res: hir::def::Res::Def(DefKind::TyAlias { .. }, def_id), .. }, )) = hir_output.kind
             && let Some(local_id) = def_id.as_local()
             && let Some(alias_ty) = self.hir().get_by_def_id(local_id).alias_ty() // it is type alias
             && let Some(alias_generics) = self.hir().get_by_def_id(local_id).generics()
diff --git a/compiler/rustc_middle/src/ty/diagnostics.rs b/compiler/rustc_middle/src/ty/diagnostics.rs
index 8570f83dcc6..e71482326da 100644
--- a/compiler/rustc_middle/src/ty/diagnostics.rs
+++ b/compiler/rustc_middle/src/ty/diagnostics.rs
@@ -492,7 +492,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IsSuggestableVisitor<'tcx> {
             Alias(Opaque, AliasTy { def_id, .. }) => {
                 let parent = self.tcx.parent(def_id);
                 let parent_ty = self.tcx.type_of(parent).instantiate_identity();
-                if let DefKind::TyAlias | DefKind::AssocTy = self.tcx.def_kind(parent)
+                if let DefKind::TyAlias { .. } | DefKind::AssocTy = self.tcx.def_kind(parent)
                     && let Alias(Opaque, AliasTy { def_id: parent_opaque_def_id, .. }) = *parent_ty.kind()
                     && parent_opaque_def_id == def_id
                 {
@@ -576,7 +576,7 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for MakeSuggestableFolder<'tcx> {
             Alias(Opaque, AliasTy { def_id, .. }) => {
                 let parent = self.tcx.parent(def_id);
                 let parent_ty = self.tcx.type_of(parent).instantiate_identity();
-                if let hir::def::DefKind::TyAlias | hir::def::DefKind::AssocTy = self.tcx.def_kind(parent)
+                if let hir::def::DefKind::TyAlias { .. } | hir::def::DefKind::AssocTy = self.tcx.def_kind(parent)
                     && let Alias(Opaque, AliasTy { def_id: parent_opaque_def_id, .. }) = *parent_ty.kind()
                     && parent_opaque_def_id == def_id
                 {
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index d3fd49150ba..27ade16739d 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -364,7 +364,7 @@ pub trait PrettyPrinter<'tcx>:
                 self.write_str(get_local_name(&self, symbol, parent, parent_key).as_str())?;
                 self.write_str("::")?;
             } else if let DefKind::Struct | DefKind::Union | DefKind::Enum | DefKind::Trait
-                | DefKind::TyAlias | DefKind::Fn | DefKind::Const | DefKind::Static(_) = kind
+                | DefKind::TyAlias { .. } | DefKind::Fn | DefKind::Const | DefKind::Static(_) = kind
             {
             } else {
                 // If not covered above, like for example items out of `impl` blocks, fallback.
@@ -766,7 +766,7 @@ pub trait PrettyPrinter<'tcx>:
 
                 let parent = self.tcx().parent(def_id);
                 match self.tcx().def_kind(parent) {
-                    DefKind::TyAlias | DefKind::AssocTy => {
+                    DefKind::TyAlias { .. } | DefKind::AssocTy => {
                         // NOTE: I know we should check for NO_QUERIES here, but it's alright.
                         // `type_of` on a type alias or assoc type should never cause a cycle.
                         if let ty::Alias(ty::Opaque, ty::AliasTy { def_id: d, .. }) =
@@ -2983,7 +2983,7 @@ fn for_each_def(tcx: TyCtxt<'_>, mut collect_fn: impl for<'b> FnMut(&'b Ident, N
 
             match child.res {
                 def::Res::Def(DefKind::AssocTy, _) => {}
-                def::Res::Def(DefKind::TyAlias, _) => {}
+                def::Res::Def(DefKind::TyAlias { .. }, _) => {}
                 def::Res::Def(defkind, def_id) => {
                     if let Some(ns) = defkind.ns() {
                         collect_fn(&child.ident, ns, def_id);
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index a695febf087..e6baa624205 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1223,7 +1223,7 @@ impl<'tcx> AliasTy<'tcx> {
             DefKind::AssocTy if let DefKind::Impl { of_trait: false } = tcx.def_kind(tcx.parent(self.def_id)) => ty::Inherent,
             DefKind::AssocTy => ty::Projection,
             DefKind::OpaqueTy => ty::Opaque,
-            DefKind::TyAlias => ty::Weak,
+            DefKind::TyAlias { .. } => ty::Weak,
             kind => bug!("unexpected DefKind in AliasTy: {kind:?}"),
         }
     }
@@ -1945,7 +1945,7 @@ impl<'tcx> Ty<'tcx> {
             (kind, tcx.def_kind(alias_ty.def_id)),
             (ty::Opaque, DefKind::OpaqueTy)
                 | (ty::Projection | ty::Inherent, DefKind::AssocTy)
-                | (ty::Weak, DefKind::TyAlias)
+                | (ty::Weak, DefKind::TyAlias { .. })
         );
         Ty::new(tcx, Alias(kind, alias_ty))
     }
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
index 90ecc8aa857..564f982f842 100644
--- a/compiler/rustc_middle/src/ty/util.rs
+++ b/compiler/rustc_middle/src/ty/util.rs
@@ -156,7 +156,7 @@ impl<'tcx> TyCtxt<'tcx> {
                 | DefKind::Enum
                 | DefKind::Trait
                 | DefKind::OpaqueTy
-                | DefKind::TyAlias
+                | DefKind::TyAlias { .. }
                 | DefKind::ForeignTy
                 | DefKind::TraitAlias
                 | DefKind::AssocTy
diff --git a/compiler/rustc_middle/src/values.rs b/compiler/rustc_middle/src/values.rs
index b0961d91787..384a368434a 100644
--- a/compiler/rustc_middle/src/values.rs
+++ b/compiler/rustc_middle/src/values.rs
@@ -209,7 +209,7 @@ fn find_item_ty_spans(
     match ty.kind {
         hir::TyKind::Path(hir::QPath::Resolved(_, path)) => {
             if let Res::Def(kind, def_id) = path.res
-                && kind != DefKind::TyAlias {
+                && !matches!(kind, DefKind::TyAlias { .. }) {
                 let check_params = def_id.as_local().map_or(true, |def_id| {
                     if def_id == needle {
                         spans.push(ty.span);