Add T: PartialEq bounds to derived StructuralPartialEq impls.

Fixes <#147714>.

Author: kpreid

compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs

@@ -22,7 +22,10 @@ pub(crate) fn expand_deriving_partial_eq(
         path: path_std!(marker::StructuralPartialEq),
         skip_path_as_bound: true, // crucial!
         needs_copy_as_bound_if_packed: false,
-        additional_bounds: Vec::new(),
+        // The `StructuralPartialEq` impl must have the *same* bounds as the `PartialEq` impl,
+        // or it will apply in situations where it should not, such as in the bug
+        // <https://github.com/rust-lang/rust/issues/147714>.
+        additional_bounds: vec![ty::Ty::Path(path_std!(cmp::PartialEq))],
         // We really don't support unions, but that's already checked by the impl generated below;
         // a second check here would lead to redundant error messages.
         supports_unions: true,

tests/ui/consts/const_in_pattern/derive-and-manual-partialeq-issue-147714.rs

@@ -0,0 +1,31 @@
+// `derive(PartialEq)`, which also implements `StructuralPartialEq`, must not allow the latter impl
+// to be used with other non-derived implementations of `PartialEq`.
+//
+// Test case by theemathas from <https://github.com/rust-lang/rust/issues/147714>.
+
+#[allow(dead_code)]
+#[derive(PartialEq)]
+enum Thing<T> {
+    A(T),
+    B,
+}
+
+struct Incomparable;
+
+// This impl does not obey StructuralPartialEq's rules.
+impl PartialEq for Thing<Incomparable> {
+    fn eq(&self, _: &Self) -> bool {
+        panic!()
+    }
+}
+
+// This constant does not obey StructuralPartialEq's rules, so it should not
+// implement StructuralPartialEq.
+const X: Thing<Incomparable> = Thing::B;
+
+fn main() {
+    if let X = X {
+        //~^ ERROR constant of non-structural type `Thing<Incomparable>` in a pattern
+        println!("equal");
+    }
+}

tests/ui/consts/const_in_pattern/derive-and-manual-partialeq-issue-147714.stderr

@@ -0,0 +1,20 @@
+error: constant of non-structural type `Thing<Incomparable>` in a pattern
+  --> $DIR/issue-147714.rs:27:12
+   |
+LL | enum Thing<T> {
+   | ------------- `Thing<Incomparable>` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
+...
+LL | const X: Thing<Incomparable> = Thing::B;
+   | ---------------------------- constant defined here
+...
+LL |     if let X = X {
+   |            ^ constant of non-structural type
+   |
+help: if `Thing<Incomparable>` manually implemented `PartialEq`, you could check for equality instead of pattern matching
+   |
+LL -     if let X = X {
+LL +     if X == X {
+   |
+
+error: aborting due to 1 previous error
+

tests/ui/consts/const_in_pattern/issue-65466.rs

@@ -11,7 +11,7 @@ const C: &[O<B>] = &[O::None];
 fn main() {
     let x = O::None;
     match &[x][..] {
-        C => (), //~ ERROR constant of non-structural type `&[O<B>]` in a pattern
+        C => (), //~ ERROR constant of non-structural type `O<B>` in a pattern
         _ => (),
     }
 }

tests/ui/consts/const_in_pattern/issue-65466.stderr

@@ -1,16 +1,20 @@
-error: constant of non-structural type `&[O<B>]` in a pattern
+error: constant of non-structural type `O<B>` in a pattern
   --> $DIR/issue-65466.rs:14:9
    |
-LL | struct B;
-   | -------- must be annotated with `#[derive(PartialEq)]` to be usable in patterns
-LL |
+LL | enum O<T> {
+   | --------- `O<B>` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
+...
 LL | const C: &[O<B>] = &[O::None];
    | ---------------- constant defined here
 ...
 LL |         C => (),
    |         ^ constant of non-structural type
    |
-   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
+help: if `O<B>` manually implemented `PartialEq`, you could add a condition to the match arm checking for equality
+   |
+LL -         C => (),
+LL +         binding if binding == C => (),
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/derives/deriving-all-codegen.stdout

@@ -836,7 +836,10 @@ impl<T: ::core::hash::Hash + Trait, U: ::core::hash::Hash> ::core::hash::Hash
     }
 }
 #[automatically_derived]
-impl<T: Trait, U> ::core::marker::StructuralPartialEq for Generic<T, U> { }
+impl<T: ::core::cmp::PartialEq + Trait, U: ::core::cmp::PartialEq>
+    ::core::marker::StructuralPartialEq for Generic<T, U> where
+    T::A: ::core::cmp::PartialEq {
+}
 #[automatically_derived]
 impl<T: ::core::cmp::PartialEq + Trait, U: ::core::cmp::PartialEq>
     ::core::cmp::PartialEq for Generic<T, U> where
@@ -953,8 +956,9 @@ impl<T: ::core::hash::Hash + ::core::marker::Copy + Trait,
     }
 }
 #[automatically_derived]
-impl<T: Trait, U> ::core::marker::StructuralPartialEq for PackedGeneric<T, U>
-    {
+impl<T: ::core::cmp::PartialEq + Trait, U: ::core::cmp::PartialEq>
+    ::core::marker::StructuralPartialEq for PackedGeneric<T, U> where
+    T::A: ::core::cmp::PartialEq {
 }
 #[automatically_derived]
 impl<T: ::core::cmp::PartialEq + ::core::marker::Copy + Trait,
@@ -1678,7 +1682,9 @@ impl<T: ::core::hash::Hash, U: ::core::hash::Hash> ::core::hash::Hash for
     }
 }
 #[automatically_derived]
-impl<T, U> ::core::marker::StructuralPartialEq for EnumGeneric<T, U> { }
+impl<T: ::core::cmp::PartialEq, U: ::core::cmp::PartialEq>
+    ::core::marker::StructuralPartialEq for EnumGeneric<T, U> {
+}
 #[automatically_derived]
 impl<T: ::core::cmp::PartialEq, U: ::core::cmp::PartialEq>
     ::core::cmp::PartialEq for EnumGeneric<T, U> {

tests/ui/match/issue-72896-non-partial-eq-const.stderr

@@ -1,16 +1,20 @@
 error: constant of non-structural type `EnumSet<Enum8>` in a pattern
   --> $DIR/issue-72896-non-partial-eq-const.rs:19:9
    |
-LL | enum Enum8 { }
-   | ---------- must be annotated with `#[derive(PartialEq)]` to be usable in patterns
+LL | struct EnumSet<T: EnumSetType> {
+   | ------------------------------ `EnumSet<Enum8>` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
 ...
 LL | const CONST_SET: EnumSet<Enum8> = EnumSet { __enumset_underlying: 3 };
    | ------------------------------- constant defined here
 ...
 LL |         CONST_SET => { /* ok */ }
    |         ^^^^^^^^^ constant of non-structural type
    |
-   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
+help: if `EnumSet<Enum8>` manually implemented `PartialEq`, you could add a condition to the match arm checking for equality
+   |
+LL -         CONST_SET => { /* ok */ }
+LL +         binding if binding == CONST_SET => { /* ok */ }
+   |
 
 error: aborting due to 1 previous error