Allow impl on projection

compiler/rustc_hir_analysis/src/coherence/inherent_impls.rs

@@ -186,7 +186,13 @@ impl<'tcx> InherentCollect<'tcx> {
             return;
         };
 
-        let self_ty = self.tcx.type_of(item.owner_id);
+        let mut self_ty = self.tcx.type_of(item.owner_id);
+        if matches!(self_ty.kind(), ty::Alias(ty::AliasKind::Projection, _)) {
+            let param_env = self.tcx.param_env(item.owner_id);
+            if let Ok(new_ty) = self.tcx.try_normalize_erasing_regions(param_env, self_ty) {
+                self_ty = new_ty;
+            }
+        }
         match *self_ty.kind() {
             ty::Adt(def, _) => {
                 self.check_def_id(item, self_ty, def.did());

compiler/rustc_privacy/src/lib.rs

@@ -339,7 +339,15 @@ trait VisibilityLike: Sized {
         effective_visibilities: &EffectiveVisibilities,
     ) -> Self {
         let mut find = FindMin { tcx, effective_visibilities, min: Self::MAX };
-        find.visit(tcx.type_of(def_id));
+        let mut ty = tcx.type_of(def_id);
+        let param_env = tcx.param_env(def_id);
+        // We need this normalization when we check the visibility of a projection so it is applied
+        // to the item behind the projection and not to the projection itself.
+        ty = match tcx.try_normalize_erasing_regions(param_env, ty) {
+            Ok(new_ty) => new_ty,
+            Err(_) => ty,
+        };
+        find.visit(ty);
         if let Some(trait_ref) = tcx.impl_trait_ref(def_id) {
             find.visit_trait(trait_ref.subst_identity());
         }

tests/ui/privacy/private-in-public-ill-formed.rs

@@ -12,8 +12,10 @@ mod aliases_pub {
     }
 
     impl <Priv as PrivTr>::AssocAlias {
-        //~^ ERROR no nominal type found for inherent implementation
-        pub fn f(arg: Priv) {} // private type `aliases_pub::Priv` in public interface
+        pub fn f(arg: Priv) {}
+        //~^ ERROR private type `aliases_pub::Priv` in public interface
+        //~| ERROR private type `aliases_pub::Priv` in public interface
+        //~| ERROR private trait `aliases_pub::PrivTr` in public interface
     }
 }
 
@@ -29,7 +31,6 @@ mod aliases_priv {
     }
 
     impl <Priv as PrivTr>::AssocAlias {
-        //~^ ERROR no nominal type found for inherent implementation
         pub fn f(arg: Priv) {} // OK
     }
 }

tests/ui/privacy/private-in-public-ill-formed.stderr

@@ -1,19 +1,31 @@
-error[E0118]: no nominal type found for inherent implementation
-  --> $DIR/private-in-public-ill-formed.rs:14:10
+error[E0446]: private type `aliases_pub::Priv` in public interface
+  --> $DIR/private-in-public-ill-formed.rs:15:9
    |
-LL |     impl <Priv as PrivTr>::AssocAlias {
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl requires a nominal type
-   |
-   = note: either implement a trait on it or create a newtype to wrap it instead
+LL |     struct Priv;
+   |     ----------- `aliases_pub::Priv` declared as private
+...
+LL |         pub fn f(arg: Priv) {}
+   |         ^^^^^^^^^^^^^^^^^^^ can't leak private type
 
-error[E0118]: no nominal type found for inherent implementation
-  --> $DIR/private-in-public-ill-formed.rs:31:10
+error[E0445]: private trait `aliases_pub::PrivTr` in public interface
+  --> $DIR/private-in-public-ill-formed.rs:15:9
    |
-LL |     impl <Priv as PrivTr>::AssocAlias {
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl requires a nominal type
+LL |     trait PrivTr {
+   |     ------------ `aliases_pub::PrivTr` declared as private
+...
+LL |         pub fn f(arg: Priv) {}
+   |         ^^^^^^^^^^^^^^^^^^^ can't leak private trait
+
+error[E0446]: private type `aliases_pub::Priv` in public interface
+  --> $DIR/private-in-public-ill-formed.rs:15:9
    |
-   = note: either implement a trait on it or create a newtype to wrap it instead
+LL |     struct Priv;
+   |     ----------- `aliases_pub::Priv` declared as private
+...
+LL |         pub fn f(arg: Priv) {}
+   |         ^^^^^^^^^^^^^^^^^^^ can't leak private type
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0118`.
+Some errors have detailed explanations: E0445, E0446.
+For more information about an error, try `rustc --explain E0445`.

tests/ui/projection/impl-foreign.rs

@@ -0,0 +1,13 @@
+#![crate_type = "lib"]
+
+pub trait Identity {
+    type Identity: ?Sized;
+}
+
+impl<T: ?Sized> Identity for T {
+    type Identity = Self;
+}
+
+impl <String as Identity>::Identity { //~ ERROR
+    pub fn foo(&self) {}
+}

tests/ui/projection/impl-foreign.stderr

@@ -0,0 +1,13 @@
+error[E0116]: cannot define inherent `impl` for a type outside of the crate where the type is defined
+  --> $DIR/impl-foreign.rs:11:1
+   |
+LL | / impl <String as Identity>::Identity {
+LL | |     pub fn foo(&self) {}
+LL | | }
+   | |_^ impl for type defined outside of crate.
+   |
+   = note: define and implement a trait or new type instead
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0116`.

tests/ui/projection/impl-on-assoc-const.rs

@@ -0,0 +1,15 @@
+#![crate_type = "lib"]
+
+pub trait Identity {
+    const Identity: u32;
+}
+
+impl<T: ?Sized> Identity for T {
+    const Identity: u32 = 0;
+}
+
+pub struct S;
+
+impl <S as Identity>::Identity { //~ ERROR
+    pub fn foo(&self) {}
+}

tests/ui/projection/impl-on-assoc-const.stderr

@@ -0,0 +1,9 @@
+error[E0575]: expected associated type, found associated constant `Identity::Identity`
+  --> $DIR/impl-on-assoc-const.rs:13:6
+   |
+LL | impl <S as Identity>::Identity {
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^ not a associated type
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0575`.

tests/ui/projection/impl-on-weird-projection.rs

@@ -0,0 +1,16 @@
+//~ ERROR
+#![crate_type = "lib"]
+
+pub trait Identity {
+    type Identity: ?Sized;
+}
+
+impl<T: ?Sized> Identity for T {
+    type Identity = Self;
+}
+
+pub struct I8<const F: i8>;
+
+impl <I8<{i8::MIN}> as Identity>::Identity {
+    pub fn foo(&self) {}
+}

tests/ui/projection/impl-on-weird-projection.stderr

@@ -0,0 +1,76 @@
+error[E0391]: cycle detected when finding all inherent impls defined in crate
+   |
+   = note: ...which requires normalizing `<I8<{i8::MIN}> as Identity>::Identity`...
+note: ...which requires evaluating type-level constant...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires const-evaluating + checking `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires const-evaluating + checking `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires caching MIR for CTFE of the const argument `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires elaborating drops for `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires borrow-checking the const argument`<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires processing MIR for the const argument `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires const checking the const argument `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires preparing the const argument `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}` for borrow checking...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires unsafety-checking the const argument `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires building MIR for `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires building THIR for `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+note: ...which requires type-checking the const argument `<impl at $DIR/impl-on-weird-projection.rs:14:1: 14:43>::{constant#0}`...
+  --> $DIR/impl-on-weird-projection.rs:14:10
+   |
+LL | impl <I8<{i8::MIN}> as Identity>::Identity {
+   |          ^^^^^^^^^
+   = note: ...which requires collecting all inherent impls for `IntSimplifiedType(I8)`...
+   = note: ...which requires collecting all impls for a type in a crate...
+   = note: ...which again requires finding all inherent impls defined in crate, completing the cycle
+   = note: cycle used when running analysis passes on this crate
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.

tests/ui/projection/impl-projection-on-primitive.rs

@@ -0,0 +1,15 @@
+#![crate_type = "lib"]
+
+pub trait Identity {
+    type Identity: ?Sized;
+}
+
+impl<T: ?Sized> Identity for T {
+    type Identity = ();
+}
+
+pub struct S;
+
+impl <S as Identity>::Identity { //~ ERROR
+    pub fn foo(&self) {}
+}

tests/ui/projection/impl-projection-on-primitive.stderr

@@ -0,0 +1,11 @@
+error[E0390]: cannot define inherent `impl` for primitive types
+  --> $DIR/impl-projection-on-primitive.rs:13:6
+   |
+LL | impl <S as Identity>::Identity {
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using an extension trait instead
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0390`.

tests/ui/projection/impl-projection.rs

@@ -0,0 +1,23 @@
+// check-pass
+
+#![crate_type = "lib"]
+
+pub trait Identity {
+    type Identity: ?Sized;
+}
+
+impl<T: ?Sized> Identity for T {
+    type Identity = Self;
+}
+
+pub struct S;
+
+impl <S as Identity>::Identity {
+    pub fn foo(&self) {}
+}
+
+impl S {
+    pub fn bar(&self) {
+        self.foo();
+    }
+}

tests/ui/projection/impl-projection2.rs

@@ -0,0 +1,24 @@
+// check-pass
+
+#![crate_type = "lib"]
+
+pub trait Identity {
+    type Identity: ?Sized;
+}
+
+impl<T: ?Sized> Identity for T {
+    type Identity = S;
+}
+
+pub struct S;
+pub struct Bar;
+
+impl <Bar as Identity>::Identity {
+    pub fn foo(&self) {}
+}
+
+impl S {
+    pub fn bar(&self) {
+        self.foo();
+    }
+}