Auto merge of #64564 - jonas-schievink:cowardly-default, r=nikomatsakis

Deny specializing items not in the parent impl

Part of https://github.com/rust-lang/rust/issues/29661 (https://github.com/rust-lang/rfcs/pull/2532). At least sort of?

This was discussed in https://github.com/rust-lang/rust/pull/61812#discussion_r300504114 and is needed for that PR to make progress (fixing an unsoundness).

One annoyance with doing this is that it sometimes requires users to copy-paste a provided trait method into an impl just to mark it `default` (ie. there is no syntax to forward this impl method to the provided trait method).

cc @Centril and @arielb1

5 files changed, 144 insertions, 3 deletions

diff --git a/src/test/ui/specialization/auxiliary/cross_crates_defaults.rs b/src/test/ui/specialization/auxiliary/cross_crates_defaults.rs
index 5cf975b5752..1e5555355c3 100644
--- a/src/test/ui/specialization/auxiliary/cross_crates_defaults.rs
+++ b/src/test/ui/specialization/auxiliary/cross_crates_defaults.rs
@@ -22,7 +22,9 @@ pub trait Bar {
     fn bar(&self) -> i32 { 0 }
 }
 
-impl<T> Bar for T {} // use the provided method
+impl<T> Bar for T {
+    default fn bar(&self) -> i32 { 0 }
+}
 
 impl Bar for i32 {
     fn bar(&self) -> i32 { 1 }
diff --git a/src/test/ui/specialization/issue-36804.rs b/src/test/ui/specialization/issue-36804.rs
index 36cb939bc48..9546a5dd5f5 100644
--- a/src/test/ui/specialization/issue-36804.rs
+++ b/src/test/ui/specialization/issue-36804.rs
@@ -13,6 +13,10 @@ where
     fn next(&mut self) -> Option<T> {
         unimplemented!()
     }
+
+    default fn count(self) -> usize where Self: Sized {
+        self.fold(0, |cnt, _| cnt + 1)
+    }
 }
 
 impl<'a, I, T: 'a> Iterator for Cloned<I>
diff --git a/src/test/ui/specialization/non-defaulted-item-fail.rs b/src/test/ui/specialization/non-defaulted-item-fail.rs
new file mode 100644
index 00000000000..403f718d7dd
--- /dev/null
+++ b/src/test/ui/specialization/non-defaulted-item-fail.rs
@@ -0,0 +1,53 @@
+#![feature(specialization, associated_type_defaults)]
+
+// Test that attempting to override a non-default method or one not in the
+// parent impl causes an error.
+
+trait Foo {
+    type Ty = ();
+    const CONST: u8 = 123;
+    fn foo(&self) -> bool { true }
+}
+
+// Specialization tree for Foo:
+//
+//       Box<T>              Vec<T>
+//        / \                 / \
+// Box<i32>  Box<i64>   Vec<()>  Vec<bool>
+
+impl<T> Foo for Box<T> {
+    type Ty = bool;
+    const CONST: u8 = 0;
+    fn foo(&self) -> bool { false }
+}
+
+// Allowed
+impl Foo for Box<i32> {}
+
+// Can't override a non-`default` fn
+impl Foo for Box<i64> {
+    type Ty = Vec<()>;
+//~^ error: `Ty` specializes an item from a parent `impl`, but that item is not marked `default`
+    const CONST: u8 = 42;
+//~^ error: `CONST` specializes an item from a parent `impl`, but that item is not marked `default`
+    fn foo(&self) -> bool { true }
+//~^ error: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
+}
+
+
+// Doesn't mention the item = provided body/value is used and the method is final.
+impl<T> Foo for Vec<T> {}
+
+// Allowed
+impl Foo for Vec<()> {}
+
+impl Foo for Vec<bool> {
+    type Ty = Vec<()>;
+//~^ error: `Ty` specializes an item from a parent `impl`, but that item is not marked `default`
+    const CONST: u8 = 42;
+//~^ error: `CONST` specializes an item from a parent `impl`, but that item is not marked `default`
+    fn foo(&self) -> bool { true }
+//~^ error: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
+}
+
+fn main() {}
diff --git a/src/test/ui/specialization/non-defaulted-item-fail.stderr b/src/test/ui/specialization/non-defaulted-item-fail.stderr
new file mode 100644
index 00000000000..e6c5fc1441b
--- /dev/null
+++ b/src/test/ui/specialization/non-defaulted-item-fail.stderr
@@ -0,0 +1,81 @@
+error[E0520]: `Ty` specializes an item from a parent `impl`, but that item is not marked `default`
+  --> $DIR/non-defaulted-item-fail.rs:29:5
+   |
+LL | / impl<T> Foo for Box<T> {
+LL | |     type Ty = bool;
+LL | |     const CONST: u8 = 0;
+LL | |     fn foo(&self) -> bool { false }
+LL | | }
+   | |_- parent `impl` is here
+...
+LL |       type Ty = Vec<()>;
+   |       ^^^^^^^^^^^^^^^^^^ cannot specialize default item `Ty`
+   |
+   = note: to specialize, `Ty` in the parent `impl` must be marked `default`
+
+error[E0520]: `CONST` specializes an item from a parent `impl`, but that item is not marked `default`
+  --> $DIR/non-defaulted-item-fail.rs:31:5
+   |
+LL | / impl<T> Foo for Box<T> {
+LL | |     type Ty = bool;
+LL | |     const CONST: u8 = 0;
+LL | |     fn foo(&self) -> bool { false }
+LL | | }
+   | |_- parent `impl` is here
+...
+LL |       const CONST: u8 = 42;
+   |       ^^^^^^^^^^^^^^^^^^^^^ cannot specialize default item `CONST`
+   |
+   = note: to specialize, `CONST` in the parent `impl` must be marked `default`
+
+error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
+  --> $DIR/non-defaulted-item-fail.rs:33:5
+   |
+LL | / impl<T> Foo for Box<T> {
+LL | |     type Ty = bool;
+LL | |     const CONST: u8 = 0;
+LL | |     fn foo(&self) -> bool { false }
+LL | | }
+   | |_- parent `impl` is here
+...
+LL |       fn foo(&self) -> bool { true }
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot specialize default item `foo`
+   |
+   = note: to specialize, `foo` in the parent `impl` must be marked `default`
+
+error[E0520]: `Ty` specializes an item from a parent `impl`, but that item is not marked `default`
+  --> $DIR/non-defaulted-item-fail.rs:45:5
+   |
+LL | impl<T> Foo for Vec<T> {}
+   | ------------------------- parent `impl` is here
+...
+LL |     type Ty = Vec<()>;
+   |     ^^^^^^^^^^^^^^^^^^ cannot specialize default item `Ty`
+   |
+   = note: to specialize, `Ty` in the parent `impl` must be marked `default`
+
+error[E0520]: `CONST` specializes an item from a parent `impl`, but that item is not marked `default`
+  --> $DIR/non-defaulted-item-fail.rs:47:5
+   |
+LL | impl<T> Foo for Vec<T> {}
+   | ------------------------- parent `impl` is here
+...
+LL |     const CONST: u8 = 42;
+   |     ^^^^^^^^^^^^^^^^^^^^^ cannot specialize default item `CONST`
+   |
+   = note: to specialize, `CONST` in the parent `impl` must be marked `default`
+
+error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
+  --> $DIR/non-defaulted-item-fail.rs:49:5
+   |
+LL | impl<T> Foo for Vec<T> {}
+   | ------------------------- parent `impl` is here
+...
+LL |     fn foo(&self) -> bool { true }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot specialize default item `foo`
+   |
+   = note: to specialize, `foo` in the parent `impl` must be marked `default`
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0520`.
diff --git a/src/test/ui/specialization/specialization-default-methods.rs b/src/test/ui/specialization/specialization-default-methods.rs
index 5d65a0457e7..9ae3d1e9f39 100644
--- a/src/test/ui/specialization/specialization-default-methods.rs
+++ b/src/test/ui/specialization/specialization-default-methods.rs
@@ -55,8 +55,9 @@ trait Bar {
 //                   /  \
 //            Vec<i32>  $Vec<i64>
 
-// use the provided method
-impl<T> Bar for T {}
+impl<T> Bar for T {
+    default fn bar(&self) -> i32 { 0 }
+}
 
 impl Bar for i32 {
     fn bar(&self) -> i32 { 1 }