Rollup merge of #124106 - compiler-errors:tait-lifetime-dedup, r=oli-obk

Don't repeatedly duplicate TAIT lifetimes for each subsequently nested TAIT

Make it so that nested TAITs inherit the lifetimes from their parent item, not their parent TAIT. This is because we don't need to re-duplicate lifetimes for nested TAITs over and over, since the only lifetimes they can capture are from the parent item anyways.

This mirrors how RPITs work. This is **not** a functional change that should be observable, since the whole point of duplicating lifetimes and marking the shadowed ones (and uncaptured ones) as bivariant is designed to *not* be observable.

r? oli-obk

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -195,16 +195,19 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 }
                 Some(fn_def_id.to_def_id())
             }
-            ItemKind::OpaqueTy(hir::OpaqueTy {
-                origin: hir::OpaqueTyOrigin::TyAlias { .. },
+            ItemKind::OpaqueTy(&hir::OpaqueTy {
+                origin: hir::OpaqueTyOrigin::TyAlias { parent, in_assoc_ty },
                 ..
             }) => {
-                let parent_id = tcx.hir().get_parent_item(hir_id);
-                assert_ne!(parent_id, hir::CRATE_OWNER_ID);
-                debug!("generics_of: parent of opaque ty {:?} is {:?}", def_id, parent_id);
+                if in_assoc_ty {
+                    assert!(matches!(tcx.def_kind(parent), DefKind::AssocTy));
+                } else {
+                    assert!(matches!(tcx.def_kind(parent), DefKind::TyAlias));
+                }
+                debug!("generics_of: parent of opaque ty {:?} is {:?}", def_id, parent);
                 // Opaque types are always nested within another item, and
                 // inherit the generics of the item.
-                Some(parent_id.to_def_id())
+                Some(parent.to_def_id())
             }
             _ => None,
         },

tests/ui/type-alias-impl-trait/variance.rs

@@ -52,4 +52,28 @@ impl<'i> Foo<'i> for () {
     //~^ ERROR: unconstrained opaque type
 }
 
+trait Nesting<'a> {
+    type Output;
+}
+impl<'a> Nesting<'a> for &'a () {
+    type Output = &'a ();
+}
+type NestedDeeply<'a> =
+    impl Nesting< //~ [*, o]
+        'a,
+        Output = impl Nesting< //~ [*, o]
+            'a,
+            Output = impl Nesting< //~ [*, o]
+                'a,
+                Output = impl Nesting< //~ [*, o]
+                    'a,
+                    Output = impl Nesting<'a> //~ [*, o]
+                >
+            >,
+        >,
+    >;
+fn test<'a>() -> NestedDeeply<'a> {
+    &()
+}
+
 fn main() {}

tests/ui/type-alias-impl-trait/variance.stderr

@@ -176,6 +176,60 @@ error: [*, *, o, o]
 LL |     type ExplicitCaptureFromGat<'a> = impl Sized + Captures<'a>;
    |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 24 previous errors
+error: [*, o]
+  --> $DIR/variance.rs:62:5
+   |
+LL | /     impl Nesting<
+LL | |         'a,
+LL | |         Output = impl Nesting<
+LL | |             'a,
+...  |
+LL | |         >,
+LL | |     >;
+   | |_____^
+
+error: [*, o]
+  --> $DIR/variance.rs:64:18
+   |
+LL |           Output = impl Nesting<
+   |  __________________^
+LL | |             'a,
+LL | |             Output = impl Nesting<
+LL | |                 'a,
+...  |
+LL | |             >,
+LL | |         >,
+   | |_________^
+
+error: [*, o]
+  --> $DIR/variance.rs:66:22
+   |
+LL |               Output = impl Nesting<
+   |  ______________________^
+LL | |                 'a,
+LL | |                 Output = impl Nesting<
+LL | |                     'a,
+LL | |                     Output = impl Nesting<'a>
+LL | |                 >
+LL | |             >,
+   | |_____________^
+
+error: [*, o]
+  --> $DIR/variance.rs:68:26
+   |
+LL |                   Output = impl Nesting<
+   |  __________________________^
+LL | |                     'a,
+LL | |                     Output = impl Nesting<'a>
+LL | |                 >
+   | |_________________^
+
+error: [*, o]
+  --> $DIR/variance.rs:70:30
+   |
+LL |                     Output = impl Nesting<'a>
+   |                              ^^^^^^^^^^^^^^^^
+
+error: aborting due to 29 previous errors
 
 For more information about this error, try `rustc --explain E0657`.