Support weak alias types as self type of inherent impls

Author: fmease

Diff for: compiler/rustc_hir_analysis/src/coherence/inherent_impls.rs

@@ -144,6 +144,7 @@ impl<'tcx> InherentCollect<'tcx> {
         let id = id.owner_id.def_id;
         let item_span = self.tcx.def_span(id);
         let self_ty = self.tcx.type_of(id).instantiate_identity();
+        let self_ty = peel_off_weak_aliases(self.tcx, self_ty);
         match *self_ty.kind() {
             ty::Adt(def, _) => self.check_def_id(id, self_ty, def.did()),
             ty::Foreign(did) => self.check_def_id(id, self_ty, did),
@@ -166,14 +167,15 @@ impl<'tcx> InherentCollect<'tcx> {
             | ty::Never
             | ty::FnPtr(_)
             | ty::Tuple(..) => self.check_primitive_impl(id, self_ty),
-            ty::Alias(..) | ty::Param(_) => {
+            ty::Alias(ty::Projection | ty::Inherent | ty::Opaque, _) | ty::Param(_) => {
                 Err(self.tcx.dcx().emit_err(errors::InherentNominal { span: item_span }))
             }
             ty::FnDef(..)
             | ty::Closure(..)
             | ty::CoroutineClosure(..)
             | ty::Coroutine(..)
             | ty::CoroutineWitness(..)
+            | ty::Alias(ty::Weak, _)
             | ty::Bound(..)
             | ty::Placeholder(_)
             | ty::Infer(_) => {
@@ -184,3 +186,30 @@ impl<'tcx> InherentCollect<'tcx> {
         }
     }
 }
+
+/// Peel off all weak alias types in this type until there are none left.
+///
+/// <div class="warning">
+///
+/// This assumes that `ty` gets normalized later and that any overflows occurring
+/// during said normalization get reported.
+///
+/// </div>
+fn peel_off_weak_aliases<'tcx>(tcx: TyCtxt<'tcx>, mut ty: Ty<'tcx>) -> Ty<'tcx> {
+    let ty::Alias(ty::Weak, _) = ty.kind() else { return ty };
+
+    let limit = tcx.recursion_limit();
+    let mut depth = 0;
+
+    while let ty::Alias(ty::Weak, alias) = ty.kind() {
+        if !limit.value_within_limit(depth) {
+            let guar = tcx.dcx().delayed_bug("overflow expanding weak alias type");
+            return Ty::new_error(tcx, guar);
+        }
+
+        ty = tcx.type_of(alias.def_id).instantiate(tcx, alias.args);
+        depth += 1;
+    }
+
+    ty
+}

Diff for: tests/ui/lazy-type-alias/inherent-impls-conflicting.rs

@@ -0,0 +1,10 @@
+#![feature(lazy_type_alias)]
+#![allow(incomplete_features)]
+
+type Alias = Local;
+struct Local;
+
+impl Alias { fn method() {} } //~ ERROR duplicate definitions with name `method`
+impl Local { fn method() {} }
+
+fn main() {}

Diff for: tests/ui/lazy-type-alias/inherent-impls-conflicting.stderr

@@ -0,0 +1,11 @@
+error[E0592]: duplicate definitions with name `method`
+  --> $DIR/inherent-impls-conflicting.rs:7:14
+   |
+LL | impl Alias { fn method() {} }
+   |              ^^^^^^^^^^^ duplicate definitions for `method`
+LL | impl Local { fn method() {} }
+   |              ----------- other definition for `method`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0592`.

Diff for: tests/ui/lazy-type-alias/inherent-impls-not-nominal.rs

@@ -0,0 +1,12 @@
+#![feature(lazy_type_alias)]
+#![allow(incomplete_features)]
+
+type Alias = <() as Trait>::Out;
+
+trait Trait { type Out; }
+impl Trait for () { type Out = Local; }
+struct Local;
+
+impl Alias {} //~ ERROR no nominal type found for inherent implementation
+
+fn main() {}

Diff for: tests/ui/lazy-type-alias/inherent-impls-not-nominal.stderr

@@ -0,0 +1,11 @@
+error[E0118]: no nominal type found for inherent implementation
+  --> $DIR/inherent-impls-not-nominal.rs:10:1
+   |
+LL | impl Alias {}
+   | ^^^^^^^^^^ impl requires a nominal type
+   |
+   = note: either implement a trait on it or create a newtype to wrap it instead
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0118`.

Diff for: tests/ui/lazy-type-alias/inherent-impls-overflow.classic.stderr

@@ -0,0 +1,43 @@
+error[E0275]: overflow evaluating the requirement `Loop`
+  --> $DIR/inherent-impls-overflow.rs:7:13
+   |
+LL | type Loop = Loop;
+   |             ^^^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error[E0275]: overflow evaluating the requirement `Loop`
+  --> $DIR/inherent-impls-overflow.rs:9:1
+   |
+LL | impl Loop {}
+   | ^^^^^^^^^^^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error[E0275]: overflow evaluating the requirement `Poly0<((((((...,),),),),),)>`
+  --> $DIR/inherent-impls-overflow.rs:11:17
+   |
+LL | type Poly0<T> = Poly1<(T,)>;
+   |                 ^^^^^^^^^^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error[E0275]: overflow evaluating the requirement `Poly1<((((((...,),),),),),)>`
+  --> $DIR/inherent-impls-overflow.rs:14:17
+   |
+LL | type Poly1<T> = Poly0<(T,)>;
+   |                 ^^^^^^^^^^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error[E0275]: overflow evaluating the requirement `Poly1<((((((...,),),),),),)>`
+  --> $DIR/inherent-impls-overflow.rs:18:1
+   |
+LL | impl Poly0<()> {}
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = note: in case this is a recursive type alias, consider using a struct, enum, or union instead
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0275`.

Diff for: tests/ui/lazy-type-alias/inherent-impls-overflow.next.stderr

@@ -0,0 +1,38 @@
+error[E0275]: overflow evaluating the requirement `Loop == _`
+  --> $DIR/inherent-impls-overflow.rs:9:6
+   |
+LL | impl Loop {}
+   |      ^^^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`inherent_impls_overflow`)
+
+error[E0392]: type parameter `T` is never used
+  --> $DIR/inherent-impls-overflow.rs:11:12
+   |
+LL | type Poly0<T> = Poly1<(T,)>;
+   |            ^ unused type parameter
+   |
+   = help: consider removing `T` or referring to it in the body of the type alias
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
+
+error[E0392]: type parameter `T` is never used
+  --> $DIR/inherent-impls-overflow.rs:14:12
+   |
+LL | type Poly1<T> = Poly0<(T,)>;
+   |            ^ unused type parameter
+   |
+   = help: consider removing `T` or referring to it in the body of the type alias
+   = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
+
+error[E0275]: overflow evaluating the requirement `Poly0<()> == _`
+  --> $DIR/inherent-impls-overflow.rs:18:6
+   |
+LL | impl Poly0<()> {}
+   |      ^^^^^^^^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`inherent_impls_overflow`)
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0275, E0392.
+For more information about an error, try `rustc --explain E0275`.

Diff for: tests/ui/lazy-type-alias/inherent-impls-overflow.rs

@@ -0,0 +1,20 @@
+//@ revisions: classic next
+//@[next] compile-flags: -Znext-solver
+
+#![feature(lazy_type_alias)]
+#![allow(incomplete_features)]
+
+type Loop = Loop; //[classic]~ ERROR overflow evaluating the requirement
+
+impl Loop {} //~ ERROR overflow evaluating the requirement
+
+type Poly0<T> = Poly1<(T,)>;
+//[classic]~^ ERROR overflow evaluating the requirement
+//[next]~^^ ERROR type parameter `T` is never used
+type Poly1<T> = Poly0<(T,)>;
+//[classic]~^ ERROR overflow evaluating the requirement
+//[next]~^^ ERROR type parameter `T` is never used
+
+impl Poly0<()> {} //~ ERROR overflow evaluating the requirement
+
+fn main() {}

Diff for: tests/ui/lazy-type-alias/inherent-impls.rs

@@ -0,0 +1,18 @@
+//@ check-pass
+
+#![feature(lazy_type_alias)]
+#![allow(incomplete_features)]
+
+type Alias = Local;
+struct Local;
+
+impl Alias {
+    fn method(self) {}
+}
+
+fn main() {
+    let _ = Local.method();
+    let _ = Local::method;
+    let _ = Alias {}.method();
+    let _ = Alias::method;
+}